Toxicology – Miscellaneous Antimicrobials

Core Concept

This older grouping combines several unrelated antimicrobial classes, so there is no single toxidrome. The clinically useful approach is to consider each drug separately.

Major agents include:

• Clindamycin and lincomycin
• Chloramphenicol
• Fluoroquinolones — ciprofloxacin, levofloxacin, moxifloxacin, ofloxacin and others
• Vancomycin

Most isolated oral overdoses cause relatively mild GI symptoms. Important toxicity more often occurs from therapeutic exposure, drug interactions, excessive parenteral administration, or impaired clearance.

Characteristic problems are:

• Clindamycin → diarrhea and C. difficile infection
• Chloramphenicol → bone-marrow toxicity; neonatal gray syndrome
• Fluoroquinolones → CNS effects, dysglycemia, QT effects, tendinopathy and peripheral neuropathy
• Vancomycin → nephrotoxicity and infusion reaction

There is no universal antidote.

1. Clindamycin and Lincomycin

Clindamycin is a lincosamide antibiotic that inhibits bacterial protein synthesis through binding to the 50S ribosomal subunit.

Acute oral overdose is generally mild.

Possible effects include:

• Nausea
• Vomiting
• Abdominal discomfort
• Diarrhea

Severe toxicity is uncommon after an isolated ingestion.

Clindamycin and C. difficile

The most clinically important complication is disruption of normal intestinal flora followed by Clostridioides difficile infection (CDI).

This may develop:

• During therapy
• Shortly after therapy
• Even weeks after antibiotic exposure

Symptoms can include:

• Watery diarrhea
• Abdominal pain
• Fever
• Leukocytosis

Severe disease may progress to:

• Ileus
• Toxic megacolon
• Sepsis
• Shock

Modern CDI Diagnosis

Testing should be performed in an appropriate symptomatic patient rather than indiscriminately screening asymptomatic individuals.

Modern testing may involve:

• Stool toxin assays
• NAAT/PCR-based testing
• Multistep diagnostic algorithms

The historical use of routine stool occult blood testing, barium enema, or proctosigmoidoscopy is no longer the standard diagnostic approach.

Modern CDI Treatment

The old statement that antibiotic-associated colitis is simply treated with oral vancomycin needs refinement.

Modern treatment depends on:

• Severity
• Initial vs recurrent infection
• Patient risk factors
• Current infectious-disease guidelines

Fidaxomicin or oral vancomycin are commonly used depending on the clinical setting.

Antimotility drugs are generally avoided in severe or fulminant CDI, particularly when ileus or toxic megacolon is a concern.

Clindamycin – Other Toxicity

Less common effects include:

• Rash
• Hypersensitivity
• Hepatic injury
• Cytopenias
• Rare renal injury

Rapid IV administration has historically been associated with severe cardiovascular reactions, including:

• Hypotension
• Dysrhythmia
• Cardiovascular collapse

These are administration-related events rather than the usual presentation of oral overdose.

2. Chloramphenicol

Chloramphenicol inhibits bacterial protein synthesis at the 50S ribosomal subunit.

Systemic use is now limited in many settings because of serious hematologic toxicity.

The two major toxicologic concepts are:

1. Dose-related reversible bone-marrow suppression
2. Rare idiosyncratic aplastic anemia

These are distinct phenomena.

Dose-Related Bone-Marrow Suppression

High systemic exposure can suppress marrow function.

Possible findings include:

• Anemia
• Leukopenia
• Thrombocytopenia
• Pancytopenia

This form is generally related to dose/exposure and is usually reversible after discontinuation.

Idiosyncratic Aplastic Anemia

A separate and much more concerning reaction is rare aplastic anemia.

Important features:

• Not reliably dose dependent
• Can appear after therapy has stopped
• May be irreversible
• Can be fatal

This toxicity cannot be predicted simply from a chloramphenicol serum concentration.

Chloramphenicol – Gray Baby Syndrome

Neonates have limited capacity to metabolize and eliminate chloramphenicol.

Excessive systemic exposure can therefore produce gray baby syndrome.

Manifestations may include:

• Poor feeding
• Vomiting
• Abdominal distension
• Hypothermia
• Irregular respiration
• Cyanotic or gray discoloration
• Metabolic acidosis
• Hypotension
• Cardiovascular collapse

This is principally a neonatal pharmacokinetic toxicity rather than the typical manifestation of overdose in older children or adults.

Why Neonates Are Vulnerable

Neonates, especially premature infants, have immature:

• Hepatic glucuronidation
• Renal elimination

Therefore:

Reduced clearance → chloramphenicol accumulation → mitochondrial/cardiovascular toxicity

The historical term “gray baby syndrome” should not be generalized to toddlers.

Chloramphenicol Neurologic Toxicity

Prolonged exposure can rarely produce:

• Peripheral neuropathy
• Optic neuropathy
• Visual impairment

The old recommendation to treat chloramphenicol optic neuritis simply with large doses of B vitamins is not an established antidotal strategy.

The essential intervention is recognition and discontinuation of the offending drug, followed by ophthalmologic/neurologic assessment.

Chloramphenicol Concentrations

Serum concentrations can be useful during systemic therapy or suspected major parenteral dosing error.

However, a single historical threshold should not be treated as a universal predictor of toxicity.

Interpret concentrations according to:

• Age
• Dose
• Timing
• Hepatic function
• Renal function
• Clinical condition

3. Fluoroquinolones

Important modern fluoroquinolones include:

• Ciprofloxacin
• Levofloxacin
• Moxifloxacin
• Ofloxacin

Several agents listed in older references are now rarely used or unavailable in many countries.

Fluoroquinolones inhibit bacterial:

• DNA gyrase
• Topoisomerase IV

Acute overdose is usually mild, but this class has several important therapeutic adverse effects.

Fluoroquinolone GI Effects

Common effects include:

• Nausea
• Vomiting
• Abdominal discomfort
• Diarrhea

Antibiotic-associated diarrhea and CDI are also possible.

Fluoroquinolone CNS Toxicity

Neurologic and psychiatric effects can include:

• Headache
• Dizziness
• Insomnia
• Agitation
• Confusion
• Hallucinations
• Tremor
• Rare seizures

Risk may be increased by:

• Older age
• Renal impairment
• CNS disease
• High exposure
• Other medications that lower seizure threshold

Fluoroquinolone Seizures

If a toxicologic seizure occurs:

Benzodiazepines are first-line.

Persistent seizures are managed using standard toxicologic status-epilepticus principles.

Phenytoin is generally not preferred as the routine next treatment for medication-induced seizures.

Fluoroquinolone QT Prolongation

Some fluoroquinolones can delay cardiac repolarization.

The degree varies considerably by agent.

Moxifloxacin has particularly recognized QT-prolonging potential.

Risk increases with:

• Congenital long-QT syndrome
• Hypokalemia
• Hypomagnesemia
• Bradycardia
• Structural cardiac disease
• Other QT-prolonging medications

Torsades

For clinically important QT prolongation or torsades:

• Stop QT-prolonging medications
• Correct potassium
• Correct magnesium
• Give IV magnesium for torsades
• Use electrical treatment when unstable

Recurrent pause-dependent torsades may require selected heart-rate acceleration.

Fluoroquinolone Dysglycemia

Fluoroquinolones can disturb glucose regulation.

Both:

• Hypoglycemia
• Hyperglycemia

have been reported.

Risk is particularly relevant in patients with diabetes or those receiving glucose-lowering medication.

Therefore, altered mental status during fluoroquinolone treatment should include a bedside glucose check.

Tendinopathy and Tendon Rupture

A major modern safety issue is:

• Tendinitis
• Tendon degeneration
• Tendon rupture

The Achilles tendon is commonly involved, although other tendons can be affected.

Risk increases with factors such as:

• Older age
• Corticosteroid treatment
• Transplant status
• Renal dysfunction

This is primarily an adverse effect of therapeutic exposure rather than acute overdose.

Peripheral Neuropathy

Fluoroquinolones can rarely cause peripheral nerve injury.

Symptoms may include:

• Burning pain
• Tingling
• Numbness
• Altered sensation
• Weakness

Symptoms can occasionally persist after the drug has been stopped.

Other Important Fluoroquinolone Adverse Effects

Depending on the individual patient and agent, recognized concerns include:

• Photosensitivity
• Hepatic injury
• Renal injury/crystalluria with selected agents
• Severe hypersensitivity
• C. difficile infection
• Exacerbation of myasthenia gravis

Certain patients also require careful assessment of vascular/aortic risk during therapeutic prescribing.

These complications should not be interpreted as the expected findings after a single accidental ingestion.

Fluoroquinolones in Children

The historical blanket statement that fluoroquinolones must be avoided in everyone aged 17 years or younger is outdated.

They are not universally contraindicated in children.

Pediatric use may be appropriate for selected infections when:

• The organism or disease warrants therapy
• Suitable alternatives are unavailable
• Benefits outweigh musculoskeletal and other risks

Modern prescribing is indication-specific.

Fluoroquinolone Drug Interactions

Ciprofloxacin is an important inhibitor of CYP1A2.

It can increase concentrations of certain medications, particularly:

• Theophylline
• Caffeine
• Some other CYP1A2 substrates

Clinically important theophylline accumulation may cause:

• Vomiting
• Tachycardia
• Tremor
• Agitation
• Seizures
• Dysrhythmias

This should be recognized as an interaction-mediated toxicity.

Chelation in the GI Tract

Oral fluoroquinolone absorption can be substantially reduced by multivalent cations such as:

• Aluminum
• Magnesium
• Calcium
• Iron

This is an important therapeutic interaction but is not a treatment strategy for overdose.

4. Vancomycin

Vancomycin is a glycopeptide antibiotic used for serious gram-positive infections, including many infections caused by resistant organisms.

Its major toxicologic concerns are:

• Nephrotoxicity
• Infusion-related reaction
• Less commonly hematologic or auditory effects

Vancomycin Nephrotoxicity

Vancomycin-associated AKI is clinically important.

Risk increases with:

• High systemic exposure
• Prolonged therapy
• Critical illness
• Preexisting renal dysfunction
• Other nephrotoxic medications

Renal function should be monitored during significant systemic treatment.

Modern Vancomycin Monitoring

Older practice often focused heavily on trough concentrations.

For serious systemic infections, modern therapeutic monitoring increasingly uses AUC-guided exposure assessment rather than relying solely on a trough concentration.

The objective is to balance:

Adequate antimicrobial exposure ↔ reduced nephrotoxicity

This applies to therapeutic monitoring rather than routine evaluation of a small accidental ingestion.

Vancomycin Infusion Reaction

The historical term “red man syndrome” is increasingly replaced by:

Vancomycin infusion reaction

It is caused by non-IgE-mediated histamine release, usually associated with excessively rapid IV administration.

Manifestations include:

• Flushing
• Erythema
• Pruritus
• Warmth
• Upper-body rash
• Occasionally hypotension

Vancomycin Infusion Reaction vs Anaphylaxis

These are not the same process.

Vancomycin infusion reaction

Usually rate-related histamine release.

Anaphylaxis

IgE-mediated or otherwise mast-cell-mediated systemic hypersensitivity with potentially:

• Airway edema
• Bronchospasm
• Hypotension
• Generalized urticaria

True anaphylaxis requires standard emergency management with epinephrine as first-line therapy.

Management of Vancomycin Infusion Reaction

Management generally involves:

• Stop or slow the infusion
• Assess airway and hemodynamics
• Symptomatic antihistamine therapy when appropriate
• Resume more slowly if clinically necessary after symptoms resolve

Severe hypotension requires standard circulatory support.

Vancomycin Ototoxicity

Ototoxicity has historically been associated with vancomycin, but clinically important isolated vancomycin-induced hearing toxicity appears much less common than older literature suggested.

Risk may be greater with:

• Excessive exposure
• Other ototoxic drugs
• Preexisting hearing impairment

New tinnitus or hearing loss warrants evaluation.

Vancomycin Hematologic Effects

Prolonged exposure may rarely cause:

• Neutropenia
• Thrombocytopenia
• Immune-mediated cytopenias

These are generally therapeutic adverse reactions rather than acute overdose findings.

Oral Vancomycin

Oral vancomycin normally has minimal systemic absorption.

Therefore, its toxicity profile differs substantially from IV vancomycin.

Systemic absorption may increase in selected patients with:

• Severe intestinal inflammation
• Renal dysfunction
• Prolonged/high enteral exposure

This distinction is important when evaluating an exposure.

Diagnosis

Always identify:

• Exact antimicrobial
• Route
• Formulation
• Amount
• Timing
• Acute vs prolonged exposure
• Renal/hepatic function
• Coingestants
• Interacting medications

Because these drugs are unrelated pharmacologically, identifying the specific antibiotic is essential.

Laboratory Evaluation

Testing should be drug- and symptom-specific.

Chloramphenicol

Consider:

• CBC
• Electrolytes
• Renal function
• Acid–base assessment in severe toxicity
• Serum concentration when clinically appropriate

Clindamycin

With significant illness consider:

• CBC
• Electrolytes
• Renal/liver function
• Appropriate CDI testing for clinically significant diarrhea

Fluoroquinolones

Consider:

• Glucose
• Electrolytes
• Renal function
• ECG when QT risk exists
• Liver tests when indicated

Vancomycin

Consider:

• Creatinine
• Renal function trend
• Drug exposure monitoring during systemic therapy
• CBC when prolonged therapy or cytopenia is suspected

GI Decontamination

Do not induce vomiting.

Ipecac is obsolete.

Routine gastric lavage is also obsolete.

Activated charcoal may occasionally be considered following a substantial recent ingestion when:

• The agent is adsorbable
• The airway is safe
• Aspiration risk is acceptable
• The expected benefit justifies treatment

Most minor antibiotic ingestions do not require GI decontamination.

Hypotension

Significant hypotension should prompt assessment for:

• Volume depletion
• Infusion reaction
• Anaphylaxis
• Dysrhythmia
• Sepsis
• Coingestants

Appropriate isotonic crystalloid can be used when indicated.

Persistent vasodilatory shock generally favors norepinephrine rather than routine dopamine-first therapy.

Trendelenburg positioning is outdated.

Enhanced Elimination

There is no universal extracorporeal strategy for this group.

Dialysis usefulness depends on the individual antimicrobial’s:

• Protein binding
• Volume of distribution
• Molecular characteristics
• Renal clearance

It should therefore not be summarized simply as “dialysis works” or “dialysis does not work” for miscellaneous antibiotics.

Renal replacement therapy may also be required for conventional indications arising from severe AKI.

No Universal Antidote

There is no specific antidote for:

• Clindamycin
• Chloramphenicol
• Fluoroquinolones
• Vancomycin

Treatment is predominantly:

Stop exposure + supportive care + treat the specific complication

Observation

A fixed observation period is inappropriate for this diverse group.

Disposition depends on:

• Exact antibiotic
• Route
• Amount
• Symptoms
• ECG
• Renal/hepatic function
• Drug interactions
• Acute vs cumulative exposure
• Clinical trajectory

Some important adverse effects, such as aplastic anemia, tendinopathy, peripheral neuropathy, or CDI, cannot be excluded by a few hours of ED observation.

Admission

Hospitalization may be appropriate for:

• Persistent severe vomiting/dehydration
• Significant altered mental status
• Seizure
• Important QT prolongation or dysrhythmia
• Severe hypoglycemia
• Significant AKI
• Severe hepatic injury
• Gray baby syndrome
• Major infusion reaction
• Anaphylaxis
• Severe CDI
• Respiratory compromise

ICU care is appropriate for shock, status epilepticus, malignant dysrhythmia, respiratory failure, or severe neonatal chloramphenicol toxicity.

Pregnancy and Breastfeeding

The historical FDA pregnancy letter categories are obsolete.

Antimicrobial selection during pregnancy or breastfeeding should consider:

• Specific drug
• Infection severity
• Maternal condition
• Gestational/infant age
• Drug transfer
• Available alternatives
• Consequences of untreated infection

The historical blanket statement that fluoroquinolones must always be avoided during breastfeeding is too broad; decisions should be drug- and situation-specific using current lactation guidance.

Safeguarding

Rigid historical age thresholds for assuming neglect, abuse, or intentional poisoning are inappropriate.

Pediatric exposures should instead be evaluated according to:

• Developmental capability
• Medication accessibility
• Exposure circumstances
• Consistency of history
• Recurrent unexplained events
• Broader safeguarding concerns

Prognosis

Most isolated acute oral exposures have a favorable outcome.

However, important exceptions involve:

Chloramphenicol

• Severe neonatal accumulation
• Aplastic anemia

Clindamycin

• Severe C. difficile infection

Fluoroquinolones

• Severe CNS toxicity
• QT-related dysrhythmia
• Dysglycemia
• Persistent tendon or peripheral nerve complications

Vancomycin

• Significant AKI
• Severe infusion reaction or true anaphylaxis

Important Modernization of the Older Source

• “Miscellaneous antimicrobials” contains pharmacologically unrelated drugs and should be approached agent by agent.
• Clindamycin is strongly associated with C. difficile infection, which can occur during or after therapy.
• Modern CDI diagnosis relies on appropriate stool testing algorithms rather than routine occult blood testing, barium studies, or proctosigmoidoscopy.
• Modern CDI treatment commonly involves fidaxomicin or oral vancomycin, depending on the clinical scenario.
• Chloramphenicol causes both dose-related reversible marrow suppression and a separate rare idiosyncratic aplastic anemia.
• Gray baby syndrome is primarily a toxicity of neonates, especially premature infants, because of immature drug clearance.
• B vitamins are not an established antidote for chloramphenicol optic neuropathy.
• Fluoroquinolones can cause CNS effects, dysglycemia, QT prolongation, tendinopathy/tendon rupture, peripheral neuropathy, and exacerbation of myasthenia gravis.
• Fluoroquinolones are not absolutely prohibited in all children; selected pediatric indications exist.
• Ciprofloxacin can cause clinically important CYP1A2 interactions, particularly with theophylline.
• Vancomycin toxicity centers mainly on AKI and infusion-related reactions.
• “Red man syndrome” is better termed vancomycin infusion reaction.
• Vancomycin infusion reaction is not synonymous with anaphylaxis.
• Modern systemic vancomycin monitoring increasingly emphasizes AUC-guided exposure rather than trough concentration alone.
• Oral vancomycin usually has little systemic absorption.
• Ipecac and routine gastric lavage are obsolete.
• Trendelenburg and routine dopamine-first shock therapy are outdated.
• There is no universal observation period or extracorporeal treatment strategy for this diverse group.
• Historical FDA pregnancy categories are obsolete.

Key Points

• Clindamycin → GI effects and C. difficile infection.
• Chloramphenicol → reversible dose-related marrow suppression + rare idiosyncratic aplastic anemia + neonatal gray syndrome.
• Fluoroquinolones → CNS toxicity, dysglycemia, QT effects, tendinopathy, and peripheral neuropathy.
• Vancomycin → nephrotoxicity and rate-related infusion reaction.
• Severe acute overdose is uncommon with most of these antibiotics.
• Always distinguish acute overdose from adverse effects arising during prolonged therapeutic use.
• Drug interactions and renal dysfunction can substantially alter toxicity.
• There is no single antidote for this group.
• Management is primarily agent-specific supportive care and treatment of complications.

Toxicology – Macrolide Antibiotics

Core Concept

Macrolides are antimicrobial agents used for a variety of bacterial infections. Important modern examples include:

• Azithromycin
• Clarithromycin
• Erythromycin

Older agents such as dirithromycin and troleandomycin are now rarely used or unavailable in many regions.

Isolated acute overdose is usually mild and predominantly gastrointestinal. More important toxicity often occurs during therapeutic use because of:

• QT prolongation and ventricular dysrhythmia
• Drug–drug interactions
• Hepatotoxicity
• Ototoxicity
• Exacerbation of myasthenia gravis

There is no specific antidote.

Mechanism

Macrolides inhibit bacterial protein synthesis by binding primarily to the 50S ribosomal subunit.

Toxic effects are not simply an extension of this antimicrobial mechanism.

Clinically important adverse effects involve:

• GI motility
• Cardiac repolarization
• Hepatic metabolism
• Hearing
• Neuromuscular transmission

Acute Overdose

Most isolated acute macrolide overdoses cause:

• Nausea
• Vomiting
• Abdominal pain
• Diarrhea

Severe poisoning from a single ingestion is uncommon.

Risk assessment should nevertheless consider:

• Exact macrolide
• Amount
• Symptoms
• QT-risk factors
• Electrolyte abnormalities
• Coingestants
• Drug interactions
• Underlying cardiac disease

1. Gastrointestinal Toxicity

GI effects are the most common manifestations.

Possible symptoms include:

• Nausea
• Vomiting
• Abdominal cramping
• Diarrhea

Erythromycin is particularly associated with GI effects because it can stimulate motilin receptors, increasing gastrointestinal motility.

Large fluid losses can occasionally contribute to:

• Dehydration
• Tachycardia
• Hypotension
• Electrolyte abnormalities

2. Cardiotoxicity

Several macrolides can interfere with cardiac repolarization.

The major concern is:

Potassium-channel effects → delayed ventricular repolarization → QT prolongation → torsades de pointes

This is much more important clinically than the older generic description of “ventricular dysrhythmias.”

QT Prolongation

Risk varies among individual macrolides.

Erythromycin and clarithromycin have well-recognized QT-prolonging potential.

Azithromycin can also affect cardiac repolarization, although its interaction profile differs because it has much less CYP3A4 inhibition than erythromycin or clarithromycin.

Risk Factors for Torsades

Risk increases with:

• Baseline prolonged QT
• Congenital long-QT syndrome
• Hypokalemia
• Hypomagnesemia
• Bradycardia
• Structural heart disease
• Older age
• Other QT-prolonging medications
• High systemic exposure
• Drug interactions that increase macrolide concentrations

The presence of multiple risk factors is often more important than the antibiotic alone.

Torsades de Pointes

If macrolide-associated torsades occurs:

• Stop QT-prolonging drugs
• Correct potassium
• Correct magnesium
• Correct other relevant electrolyte abnormalities
• Give IV magnesium
• Treat unstable ventricular dysrhythmia electrically

Recurrent pause-dependent torsades may require heart-rate acceleration, such as selected overdrive pacing.

Avoid adding unnecessary QT-prolonging antiarrhythmics.

ECG Assessment

Obtain an ECG when there is:

• Syncope
• Palpitations
• Significant overdose
• Known long-QT syndrome
• Significant electrolyte disturbance
• Concomitant QT-prolonging drugs
• Cardiovascular instability

Evaluate:

• Rhythm
• Heart rate
• QRS
• QT/QTc
• Ventricular ectopy

Routine prolonged monitoring is unnecessary after every minor asymptomatic ingestion.

3. Drug Interactions

Drug interactions are one of the most clinically important aspects of macrolide toxicity.

However, the older statement that all macrolides broadly inhibit hepatic metabolism is too simplistic.

Erythromycin and clarithromycin

Can significantly inhibit CYP3A4 and alter concentrations of susceptible medications.

Azithromycin

Has substantially less CYP3A4 inhibition and therefore generally causes fewer metabolism-based interactions.

Clinically Important Interactions

Depending on the specific macrolide, interactions may involve:

• Certain statins
• Some calcium-channel blockers
• Calcineurin inhibitors
• Certain benzodiazepines
• Some antiarrhythmics
• Warfarin and other anticoagulant-related therapies
• Digoxin
• Other QT-prolonging medications

The exact interaction must be checked for the individual macrolide rather than assumed to be a class effect.

Macrolides and Statins

CYP3A4-inhibiting macrolides can increase concentrations of susceptible statins.

This can increase the risk of:

• Myopathy
• Rhabdomyolysis
• Acute kidney injury secondary to severe muscle injury

Azithromycin generally has a lower interaction potential than clarithromycin or erythromycin.

Macrolides and Calcium-Channel Blockers

Some macrolides can increase exposure to CYP3A4-metabolized calcium-channel blockers.

Potential consequences include:

• Hypotension
• Bradycardia
• Acute kidney injury secondary to hemodynamic compromise

This is primarily an interaction problem rather than direct macrolide poisoning.

Macrolides and Digoxin

Some macrolides may increase digoxin exposure through mechanisms including alterations in transport and intestinal flora.

If digoxin toxicity develops, manifestations may include:

• Nausea
• Bradyarrhythmia
• AV block
• Ventricular dysrhythmia
• Hyperkalemia in severe acute poisoning

The resulting syndrome should be treated as digoxin toxicity, not simply as macrolide overdose.

4. Hepatotoxicity

Macrolides can cause liver injury.

Possible manifestations include:

• Transaminase elevation
• Cholestatic hepatitis
• Jaundice
• Rare severe hepatic dysfunction

Certain erythromycin formulations, historically especially erythromycin estolate, have been strongly associated with cholestatic liver injury.

Hepatic Evaluation

Consider:

• AST/ALT
• Bilirubin
• Alkaline phosphatase
• Coagulation studies in severe liver dysfunction

Testing is most relevant when the patient develops:

• Jaundice
• Pruritus
• Persistent vomiting
• Right-upper-quadrant symptoms
• Unexplained systemic illness

Routine liver testing is unnecessary after every small acute ingestion.

5. Ototoxicity

Macrolides can occasionally produce sensorineural hearing impairment, particularly with high systemic exposure or prolonged treatment.

Possible symptoms include:

• Tinnitus
• Reduced hearing
• Hearing distortion

Risk may increase with:

• High doses
• Renal or hepatic dysfunction
• Prolonged therapy
• Other ototoxic medications

Macrolide-associated hearing impairment is often reversible after withdrawal, although persistent deficits have occasionally been reported.

6. Myasthenia Gravis

Macrolides may impair neuromuscular transmission and can exacerbate myasthenia gravis.

Possible manifestations include:

• Ptosis
• Diplopia
• Dysphagia
• Dysarthria
• Generalized weakness
• Respiratory muscle weakness

A patient with myasthenia gravis who develops worsening weakness after starting a macrolide requires prompt assessment.

Severe respiratory weakness may require ventilatory support.

7. Infusion-Related Effects

IV erythromycin can cause:

• Local pain
• Venous irritation
• Thrombophlebitis

This is mainly a complication of parenteral therapeutic administration rather than oral overdose.

Antibiotic-Associated Diarrhea

Macrolides can alter intestinal flora.

Diarrhea during or following antibiotic treatment may simply be medication related, but persistent or severe diarrhea should raise concern for Clostridioides difficile infection.

This is an antibiotic complication rather than a direct overdose effect.

Diagnosis

Diagnosis requires identification of:

• Exact macrolide
• Formulation
• Route
• Amount
• Timing
• Symptoms
• Coingestants
• Other medications

A medication review is particularly important because a serious presentation may result from a drug interaction rather than the macrolide concentration alone.

Laboratory Evaluation

No laboratory testing may be necessary after a small, uncomplicated exposure.

For significant illness, consider:

• Glucose
• Electrolytes
• Potassium
• Magnesium
• Creatinine
• Liver tests

Additional testing depends on suspected interactions.

For example:

• CK if rhabdomyolysis is suspected
• Digoxin concentration if digoxin toxicity is suspected
• Coagulation testing when a clinically important anticoagulant interaction is suspected

Serum Macrolide Concentrations

Routine serum erythromycin, clarithromycin, or azithromycin concentrations are not useful in acute poisoning.

Clinical findings, ECG, electrolytes, organ function, and interacting medications are more useful.

Occult Coingestion

Intentional overdose warrants evaluation for clinically important coingestants.

Acetaminophen testing is often appropriate because early toxicity may be asymptomatic.

Other testing should be directed by the history and clinical presentation.

Initial Management

General priorities are:

Airway/breathing → circulation → identify exact macrolide → review interacting medications → ECG when indicated → correct electrolytes → supportive care

Most isolated acute overdoses require only symptomatic treatment.

GI Decontamination

Do not induce vomiting.

Routine gastric lavage is obsolete.

Activated charcoal may occasionally be considered after a substantial recent ingestion when:

• The exposure is potentially clinically important
• The drug is adsorbable
• The airway is safe
• Aspiration risk is low

Because most isolated macrolide overdoses are relatively benign, charcoal is unnecessary in many cases.

Vomiting and Dehydration

Persistent GI losses should be treated with:

• Appropriate oral or IV fluid replacement
• Correction of electrolyte abnormalities
• Symptomatic care

When selecting an antiemetic, consider whether it also prolongs QT in a patient already at risk for macrolide-associated QT prolongation.

Hypotension

Significant hypotension should prompt evaluation for:

• Severe dehydration
• Dysrhythmia
• Drug interaction
• Anaphylaxis
• Coingestant
• Sepsis or underlying illness

Management includes appropriate isotonic crystalloid and treatment of the underlying cause.

Persistent vasodilatory shock generally favors norepinephrine rather than the historical dopamine-first approach.

Trendelenburg positioning is obsolete.

Anaphylaxis

Although uncommon, immediate hypersensitivity can occur.

For true anaphylaxis:

Epinephrine is first-line therapy.

Airway, breathing, and circulatory support should follow standard anaphylaxis management.

Enhanced Elimination

Hemodialysis and hemoperfusion are not routinely useful for macrolide overdose.

These drugs generally have pharmacokinetic characteristics that limit meaningful extracorporeal removal.

Management remains predominantly supportive.

Monitoring

Monitoring depends on the presentation.

Minor uncomplicated exposure

Usually requires:

• Vital signs
• Symptom assessment

Significant exposure or cardiac risk

Consider:

• Continuous ECG
• Potassium
• Magnesium
• Renal function
• QT trend

Suspected interaction

Monitor the toxicity of the affected interacting drug.

Observation and Disposition

There is no universal observation period for all macrolide exposures.

Disposition depends on:

• Exact drug
• Amount
• Symptoms
• ECG
• Electrolytes
• Drug interactions
• Coingestants
• Clinical trajectory

An asymptomatic minor accidental ingestion generally does not require prolonged monitoring.

Admission

Hospitalization may be appropriate for:

• Persistent severe vomiting/dehydration
• Significant electrolyte disturbance
• Syncope with abnormal ECG
• Marked QT prolongation
• Ventricular dysrhythmia
• Significant hypotension
• Severe hepatic injury
• Severe rhabdomyolysis from a drug interaction
• Myasthenic deterioration
• Respiratory compromise

ICU care is appropriate for torsades, cardiovascular collapse, severe respiratory weakness, or other major organ dysfunction.

Pregnancy

The old FDA pregnancy letter categories are obsolete.

Antibiotic selection during pregnancy should consider:

• Specific macrolide
• Infection being treated
• Gestational stage
• Available alternatives
• Maternal and fetal risks of untreated infection

Erythromycin and azithromycin have substantial clinical experience, while individual macrolides should still be assessed separately.

Safeguarding

Rigid historical age thresholds for assuming neglect, abuse, or intentional poisoning are outdated.

Pediatric exposures should instead be evaluated according to:

• Developmental capability
• Medication accessibility
• Circumstances of exposure
• Consistency of history
• Recurrent unexplained events
• Broader safeguarding concerns

Prognosis

Most isolated acute macrolide overdoses have a favorable outcome.

Prognosis becomes more concerning when toxicity involves:

• Torsades or other ventricular dysrhythmia
• Severe electrolyte abnormalities
• Major drug interactions
• Significant hepatic injury
• Severe myasthenic weakness
• Complications of prolonged hypoperfusion or hypoxia

Ototoxicity is frequently reversible but should still be formally evaluated when clinically significant.

Important Modernization of the Older Source

• Acute isolated macrolide overdose is usually GI-predominant and relatively benign.
• QT prolongation and torsades are the major direct cardiac concerns.
• Erythromycin and clarithromycin have greater CYP3A4 interaction potential than azithromycin.
• It is inaccurate to treat all macrolides as equally potent inhibitors of hepatic drug metabolism.
• Serious toxicity frequently reflects a drug interaction rather than the antibiotic overdose itself.
• Macrolide–statin interactions can cause myopathy and rhabdomyolysis.
• Macrolide–calcium-channel blocker interactions can contribute to hypotension and AKI.
• Macrolides can exacerbate myasthenia gravis.
• Macrolide-associated hearing loss is usually reversible but can occasionally persist.
• Persistent antibiotic-associated diarrhea should raise consideration of C. difficile infection.
• Routine serum macrolide concentrations are not clinically useful.
• Ipecac and routine gastric lavage are obsolete.
• Activated charcoal is only selectively useful.
• Hemodialysis and hemoperfusion are not routine treatments.
• Trendelenburg and routine dopamine-first shock management are outdated.
• Monitoring and disposition should be based on symptoms, ECG, electrolytes, interactions, and clinical trajectory rather than a fixed observation interval.
• Historical FDA pregnancy letter categories are obsolete.

Key Points

• Macrolide overdose usually causes nausea, vomiting, abdominal pain, and diarrhea.
• Erythromycin and clarithromycin can prolong QT and cause torsades.
• Correct hypokalemia and hypomagnesemia when QT prolongation is present.
• IV magnesium is central to treatment of torsades.
• Erythromycin and clarithromycin can cause important CYP3A4-mediated interactions.
• Azithromycin generally has fewer CYP3A4 interactions but is not completely free of cardiac risk.
• Macrolides may cause cholestatic/hepatic injury, reversible ototoxicity, and worsening of myasthenia gravis.
• There is no specific antidote.
• Treatment is primarily supportive, ECG-directed, and interaction-focused.

Toxicology – Antituberculous Agents

Core Concept

Antituberculous medications have very different toxicity profiles, so poisoning should be approached agent by agent rather than as a single toxidrome.

This section focuses on:

• Ethambutol
• Cycloserine
• Capreomycin
• Para-aminosalicylic acid (PAS)

Isoniazid and rifampin are best considered separately because they have distinctive toxicology.

Modern tuberculosis therapy has also expanded to include agents such as bedaquiline, pretomanid, linezolid, and delamanid, especially in drug-resistant disease.

Major toxicities in this group include:

• Ethambutol → optic neuropathy
• Cycloserine → CNS and psychiatric toxicity, including seizures
• Capreomycin → renal, electrolyte, auditory, and vestibular toxicity
• PAS → GI, hepatic, hematologic, and hypersensitivity reactions

There is no single antidote for the entire group.

1. Ethambutol

Ethambutol inhibits mycobacterial cell-wall synthesis by interfering with arabinosyl transferases involved in arabinogalactan production.

Its most characteristic toxicity is:

Optic neuropathy

This usually develops during therapeutic exposure rather than following a single acute ingestion.

Ethambutol Optic Neuropathy

Clinical manifestations may include:

• Reduced visual acuity
• Blurred vision
• Central or cecocentral visual defects
• Impaired color discrimination
• Reduced contrast sensitivity

Classically, red-green color discrimination may be affected, but color abnormalities are not restricted to one specific pattern.

Risk Factors for Ethambutol Eye Toxicity

Risk increases with:

• Higher exposure
• Longer duration of treatment
• Renal impairment
• Older age
• Preexisting ophthalmic disease

Because ethambutol is substantially cleared by the kidneys, impaired renal function can increase systemic exposure.

Reversibility of Visual Toxicity

Visual function often improves after early recognition and discontinuation, but recovery may be:

• Slow
• Incomplete
• Occasionally poor

Permanent visual impairment can occur.

Therefore, new visual symptoms during ethambutol treatment require prompt assessment.

Ethambutol Monitoring

Patients should be educated to report:

• Blurred vision
• Reduced acuity
• Altered color perception
• New central visual disturbance

Depending on risk and treatment duration, monitoring may include:

• Visual acuity
• Color vision
• Formal ophthalmologic evaluation

Renal function should also be considered because it influences clearance.

Other Ethambutol Effects

Less characteristic adverse effects include:

• GI symptoms
• Rash
• Hyperuricemia
• Peripheral neuropathy
• Rare hematologic abnormalities
• Rare hepatic injury

The older source overemphasizes bone-marrow and liver toxicity relative to ethambutol’s defining clinical problem: optic neuropathy.

2. Cycloserine

Cycloserine is a second-line antituberculous drug that interferes with bacterial cell-wall synthesis.

Its toxicology is dominated by the central nervous system.

Important manifestations include:

• Irritability
• Headache
• Tremor
• Confusion
• Depression
• Anxiety
• Psychosis
• Seizures
• Encephalopathy
• Coma in severe poisoning

Neuropsychiatric toxicity may develop during therapeutic treatment as well as excessive exposure.

Cycloserine – Psychiatric Effects

Cycloserine is particularly notable for psychiatric adverse effects.

Possible manifestations include:

• Mood disturbance
• Anxiety
• Behavioral changes
• Hallucinations
• Psychosis
• Suicidal thoughts or behavior

New psychiatric symptoms during treatment should therefore be taken seriously and assessed clinically rather than automatically attributed to the underlying illness.

Cycloserine and Seizures

Cycloserine can interfere with pathways involving GABA, lowering the seizure threshold.

Risk increases with:

• Excessive systemic concentrations
• Renal impairment
• CNS disease
• Other proconvulsant drugs
• Alcohol use or withdrawal
• Electrolyte disturbances

Cycloserine and Renal Function

Cycloserine is substantially eliminated by the kidneys.

Therefore:

Renal impairment → reduced clearance → accumulation → greater neurotoxicity

Dose adjustment and clinical monitoring are particularly important when kidney function declines.

Cycloserine Concentrations

Unlike many poisoning scenarios, serum cycloserine concentrations can sometimes help evaluate accumulation during therapy.

However, a rigid historical concentration threshold should not replace clinical assessment.

Management should be guided by:

• Neurologic findings
• Renal function
• Exposure history
• Clinical trajectory

Cycloserine and Pyridoxine

Pyridoxine (vitamin B6) supplementation is commonly used during cycloserine therapy to reduce neurologic adverse effects.

Pyridoxine may also be considered as an adjunct in significant cycloserine neurotoxicity.

However, the evidence and role are less definitive than in isoniazid poisoning, where pyridoxine is a central antidotal treatment.

Cycloserine Seizure Treatment

For acute toxicologic seizures:

Benzodiazepines are first-line.

Persistent seizures may require:

• Additional benzodiazepines
• Phenobarbital
• Appropriate anesthetic management for refractory status epilepticus

Pyridoxine can be considered as an adjunct when cycloserine toxicity is strongly suspected.

Physostigmine – Important Correction

The historical recommendation of physostigmine for cycloserine-associated neuromuscular blockade is not standard modern toxicologic management.

Physostigmine should not be treated as a cycloserine antidote.

Severe weakness or respiratory failure is managed primarily with:

• Airway support
• Ventilation
• Correction of contributing abnormalities
• Discontinuation of the causative drug

Hemodialysis and Cycloserine

Cycloserine has pharmacokinetic characteristics that allow extracorporeal removal.

Hemodialysis may therefore be considered in severe poisoning or marked accumulation, particularly when:

• Renal failure is present
• Severe neurologic toxicity persists
• Recurrent seizures occur
• Clearance is substantially impaired

Dialysis decisions should be individualized with toxicology/nephrology input.

3. Capreomycin

Capreomycin is an older injectable antimycobacterial agent historically used particularly for resistant tuberculosis.

Its contemporary use has declined substantially as treatment strategies for drug-resistant TB have changed.

Its toxicity resembles aminoglycoside toxicity in several respects.

Major targets are:

• Kidneys
• Electrolytes
• Cochlear/vestibular system

Capreomycin Nephrotoxicity

Capreomycin can cause renal tubular injury.

Possible findings include:

• Rising creatinine
• Azotemia
• Tubular dysfunction
• Acute kidney injury

Risk increases with:

• Prolonged therapy
• Renal impairment
• Dehydration
• Other nephrotoxic drugs

Capreomycin Electrolyte Disturbances

Renal tubular effects can cause significant electrolyte losses, particularly:

• Hypokalemia
• Hypomagnesemia

Other electrolyte and acid–base disturbances can occur.

These abnormalities may contribute to:

• Weakness
• ECG abnormalities
• Dysrhythmia risk

Capreomycin Ototoxicity

Capreomycin may produce:

• Tinnitus
• Hearing loss
• Vestibular dysfunction
• Disequilibrium
• Vertigo

As with aminoglycosides, ototoxicity can sometimes be persistent or irreversible.

Capreomycin Neuromuscular Effects

Neuromuscular blockade has been reported.

Severe blockade could theoretically produce:

• Weakness
• Hypoventilation
• Respiratory failure

Management is primarily supportive, including mechanical ventilation when necessary.

4. Para-Aminosalicylic Acid – PAS

PAS is an older antituberculous medication now used mainly in selected resistant-TB regimens.

Its adverse-effect profile is dominated by:

• GI intolerance
• Hypersensitivity
• Hepatotoxicity

Acute overdose experience is limited.

PAS Gastrointestinal Toxicity

Common adverse effects include:

• Nausea
• Vomiting
• Abdominal discomfort
• Diarrhea

GI intolerance can become clinically important during prolonged treatment.

PAS Hypersensitivity

Hypersensitivity reactions may produce:

• Fever
• Rash
• Systemic symptoms

More serious immune-mediated effects have occasionally been described, including hepatic or hematologic involvement.

PAS Hepatic Toxicity

PAS may cause:

• Transaminase elevation
• Hepatitis
• Rare clinically significant hepatic dysfunction

Patients developing systemic hypersensitivity symptoms plus hepatic abnormalities require prompt assessment.

PAS Hematologic Effects

Rare abnormalities can include:

• Leukopenia
• Agranulocytosis
• Thrombocytopenia
• Hemolytic anemia

These are primarily complications of therapeutic exposure rather than the expected manifestation of a single acute ingestion.

PAS and Thyroid Function

A useful modern addition is that prolonged PAS therapy can contribute to hypothyroidism, especially when combined with other drugs that impair thyroid function.

This is mainly a chronic-treatment issue rather than acute poisoning.

Acute Oral Overdose

For several of these agents, acute single-ingestion data are limited.

The absence of extensive overdose reports does not mean toxicity is impossible, but many important complications occur through:

Repeated dosing + accumulation + organ dysfunction

rather than immediately after one ingestion.

Diagnosis

Identify the exact antituberculous agent.

Then assess the organ system characteristic of that drug:

Ethambutol → eyes

Cycloserine → CNS/psychiatric system

Capreomycin → kidney/electrolytes/hearing

PAS → GI/liver/hypersensitivity/blood

This agent-specific approach is more useful than treating them as a single toxicologic syndrome.

Laboratory Evaluation

Testing should be directed by the drug and symptoms.

Ethambutol

Consider:

• Renal function
• Ophthalmologic evaluation when visual symptoms occur
• Additional testing based on clinical presentation

Cycloserine

Consider:

• Glucose
• Electrolytes
• Renal function
• Acid–base status after severe seizures
• Drug concentration in selected circumstances

Capreomycin

Consider:

• Creatinine/BUN
• Potassium
• Magnesium
• Calcium
• Bicarbonate
• Other electrolytes
• ECG when significant electrolyte disturbance is present
• Hearing/vestibular assessment when symptomatic

PAS

Consider:

• Liver tests
• CBC when hematologic toxicity is suspected
• Electrolytes/renal function with significant GI losses

ECG

Routine continuous ECG monitoring is not necessary for every minor exposure.

It is appropriate when there is:

• Significant electrolyte abnormality
• Severe systemic illness
• Hypotension
• Altered consciousness
• Significant coingestion
• Dysrhythmia

Occult Coingestion

In an intentional overdose, evaluate for clinically important coingestants.

Acetaminophen testing is often useful because early poisoning can be clinically silent.

Additional testing should be guided by history and presentation rather than routine indiscriminate drug screening.

Initial Management

General priorities are:

Airway/breathing → circulation → identify exact drug → treat seizures → evaluate characteristic target organs → correct metabolic abnormalities

There is no universal antidote for this group.

GI Decontamination

Do not induce vomiting.

Ipecac is obsolete.

Routine gastric lavage is also obsolete.

Activated charcoal may occasionally be considered after a substantial recent ingestion when:

• The substance is adsorbable
• The airway is safe
• Aspiration risk is acceptable
• Expected benefit justifies treatment

Routine decontamination is unnecessary for many minor exposures.

Hypotension

If hypotension develops:

• Assess volume status
• Give appropriate isotonic crystalloid when indicated
• Correct contributing metabolic abnormalities
• Treat anaphylaxis when present
• Use vasopressor support for persistent shock

Norepinephrine is generally favored for persistent vasodilatory shock.

Trendelenburg positioning and automatic dopamine-first therapy are outdated.

Anaphylaxis

Any of these drugs can potentially cause hypersensitivity.

When true anaphylaxis occurs:

Epinephrine is first-line therapy.

Airway, oxygenation, circulation, and additional supportive treatment should follow standard anaphylaxis management.

Renal Injury

For capreomycin-associated renal toxicity:

• Stop further exposure
• Optimize volume status
• Avoid additional nephrotoxins where possible
• Monitor creatinine
• Monitor urine output
• Correct potassium and magnesium abnormalities

Renal replacement therapy is reserved for appropriate clinical indications.

Visual Toxicity

Suspected ethambutol optic neuropathy requires:

• Prompt recognition
• Discontinuation/reassessment of ethambutol therapy
• Ophthalmologic evaluation
• Review of renal function

Do not wait for profound visual loss before investigating symptoms.

Monitoring

Monitoring should match the agent.

Ethambutol

• Vision
• Renal function

Cycloserine

• Mental status
• Psychiatric symptoms
• Seizures
• Renal function

Capreomycin

• Renal function
• Potassium
• Magnesium
• Hearing
• Vestibular symptoms

PAS

• GI tolerance
• Liver function
• CBC when indicated
• Thyroid function during prolonged therapy when clinically appropriate

Observation

The historical universal 4–6-hour observation period is too simplistic.

Disposition depends on:

• Exact drug
• Route
• Amount
• Acute vs cumulative exposure
• Renal function
• Symptoms
• Laboratory abnormalities
• Coingestants
• Clinical trajectory

Several important toxicities—particularly ethambutol optic neuropathy and cumulative capreomycin toxicity—cannot be excluded by a few hours of emergency-department observation.

Admission

Hospitalization may be appropriate for:

• Seizures
• Severe altered mental status
• Psychosis with safety concerns
• Coma
• Respiratory failure
• Significant AKI
• Major electrolyte disturbances
• Severe hypersensitivity/anaphylaxis
• Significant hepatic injury
• Severe coingestion

ICU care is appropriate for refractory seizures, coma, respiratory failure, shock, or other major organ dysfunction.

Pregnancy

The old FDA pregnancy letter categories are obsolete.

Tuberculosis during pregnancy itself poses important maternal and fetal risks, so treatment decisions require balancing:

• Severity and drug susceptibility of TB
• Specific medication
• Maternal condition
• Gestational stage
• Available alternatives

Potential drug toxicity should therefore be considered within the context of maintaining effective tuberculosis treatment.

Pediatric Use – Important Correction

The historical statement that ethambutol, cycloserine, and capreomycin are simply “not recommended for pediatric use” is outdated as a blanket rule.

Modern pediatric TB treatment is based on:

• Drug susceptibility
• Disease severity
• Age
• Ability to monitor toxicity
• Contemporary TB guidelines

Ethambutol, in particular, is used in children when clinically indicated.

Safeguarding

Rigid age cutoffs for assuming neglect, abuse, or intentional poisoning are outdated.

Pediatric exposures should instead be assessed according to:

• Developmental capability
• Medication access
• Exposure circumstances
• Consistency of history
• Recurrent unexplained events
• Broader safeguarding concerns

Prognosis

Ethambutol

Visual toxicity may improve after discontinuation but can be permanent.

Cycloserine

Most acute CNS toxicity is potentially reversible with drug withdrawal and appropriate supportive treatment.

Capreomycin

Renal dysfunction may improve, whereas auditory or vestibular injury may persist.

PAS

GI, hepatic, hypersensitivity, and hematologic effects generally improve after recognition and withdrawal, although severe reactions can require prolonged care.

Important Modernization of the Older Source

• Antituberculous agents have agent-specific toxicities rather than one common poisoning syndrome.
• Ethambutol → optic neuropathy is the defining toxicity.
• Ethambutol toxicity is more likely with prolonged/high exposure and renal impairment.
• Ethambutol is used in modern pediatric TB care when clinically indicated; the historical blanket prohibition is outdated.
• Cycloserine → neuropsychiatric toxicity and seizures.
• Renal impairment increases cycloserine accumulation and CNS toxicity.
• Pyridoxine may be used as an adjunct with cycloserine but does not have the same established antidotal role as in isoniazid poisoning.
• Physostigmine is not a standard cycloserine antidote.
• Hemodialysis can enhance cycloserine elimination in selected severe poisoning.
• Capreomycin → nephrotoxicity + electrolyte wasting + ototoxicity and is now much less prominent in contemporary resistant-TB treatment.
• PAS → GI intolerance + hypersensitivity + hepatic/hematologic effects, with hypothyroidism relevant during chronic therapy.
• Ipecac and routine gastric lavage are obsolete.
• Trendelenburg and routine dopamine-first shock management are outdated.
• Fixed 4–6-hour observation cannot exclude delayed or cumulative toxicities.
• Historical FDA pregnancy letter categories are obsolete.

Key Points

• Ethambutol → optic neuropathy and color/central vision abnormalities.
• Cycloserine → psychiatric changes, tremor, confusion, seizures, and coma.
• Capreomycin → renal injury, hypokalemia/hypomagnesemia, and cochlear/vestibular toxicity.
• PAS → GI, hypersensitivity, hepatic, and hematologic toxicity.
• Renal dysfunction can substantially increase toxicity of renally eliminated TB medications.
• Benzodiazepines are first-line for cycloserine-associated toxicologic seizures.
• Pyridoxine may be considered as an adjunct for cycloserine neurotoxicity.
• Severe cycloserine accumulation may be amenable to hemodialysis.
• New visual symptoms during ethambutol therapy require prompt evaluation.
• There is no single antidote or universal monitoring strategy for this group.

Toxicology – Aminoglycoside Antimicrobials

Core Concept

Aminoglycosides are bactericidal antibiotics used mainly for serious infections caused by susceptible aerobic gram-negative organisms, often as part of combination therapy.

Important agents include:

• Gentamicin
• Tobramycin
• Amikacin
• Streptomycin
• Plazomicin
• Neomycin — mainly topical/oral nonabsorbed use
• Kanamycin — now rarely used in many settings

The major toxicities are:

• Nephrotoxicity
• Ototoxicity
• Rare neuromuscular blockade

Unlike many acute poisonings, clinically important aminoglycoside toxicity usually reflects repeated exposure or impaired clearance, rather than a single isolated dose.

Mechanism of Antimicrobial Action

Aminoglycosides enter susceptible bacteria and bind primarily to the 30S ribosomal subunit.

This disrupts protein synthesis through mechanisms including:

• Interference with initiation
• Misreading of mRNA
• Production of abnormal proteins

Their antibacterial activity is concentration dependent.

Toxicokinetics

Aminoglycosides are:

• Highly water soluble
• Poorly absorbed from the normal GI tract
• Distributed mainly in extracellular fluid
• Minimally metabolized
• Eliminated predominantly by the kidneys

Therefore:

Reduced renal function → decreased clearance → accumulation → increased toxicity

This is fundamental to aminoglycoside toxicology.

Acute vs Cumulative Toxicity

A single accidental dose, even if excessive, often does not produce the characteristic renal or auditory toxicity when baseline renal function is normal.

Greater concern arises with:

• Repeated excessive dosing
• Prolonged therapy
• Renal impairment
• Dehydration
• Critical illness
• Older age
• Concomitant nephrotoxins
• Persistently excessive systemic exposure

Thus, cumulative exposure is generally more informative than a single historical “toxic dose.”

1. Nephrotoxicity

Aminoglycosides accumulate within proximal renal tubular cells.

Intracellular accumulation causes cellular injury and can produce:

Proximal tubular injury → impaired renal function → acute kidney injury

The injury is usually nonoliguric initially, although more severe renal dysfunction can occur.

Clinical Features of Renal Toxicity

Possible findings include:

• Rising serum creatinine
• Reduced GFR
• Tubular dysfunction
• Electrolyte abnormalities

Importantly, renal injury usually develops after several days of exposure, rather than immediately after a single dose.

Reversibility

Aminoglycoside nephrotoxicity is often at least partially reversible after the drug is discontinued because tubular epithelium can recover.

However, severe AKI can require temporary renal replacement therapy.

Risk Factors for Nephrotoxicity

Important risk factors include:

• Preexisting kidney disease
• Prolonged therapy
• Excessive systemic exposure
• Older age
• Dehydration or hypovolemia
• Critical illness
• Sepsis
• Concurrent nephrotoxic medications

Nephrotoxic combinations deserve particular attention.

Other Nephrotoxic Drugs

Potentially important interacting exposures include:

• Vancomycin
• Amphotericin B
• Cisplatin
• Calcineurin inhibitors
• Other nephrotoxic medications

The total clinical context matters more than memorizing a single historical drug combination.

2. Ototoxicity

Aminoglycosides can damage sensory structures of the inner ear.

Toxicity may involve:

Cochlear system

→ hearing impairment

Vestibular system

→ disequilibrium/vertigo

Different aminoglycosides have somewhat different tendencies toward cochlear versus vestibular toxicity.

Cochlear Toxicity

Possible manifestations include:

• Tinnitus
• Reduced hearing
• Difficulty hearing high-frequency sounds
• Progressive sensorineural hearing loss

High-frequency hearing loss may occur before the patient recognizes impairment in ordinary conversation.

Vestibular Toxicity

Possible findings include:

• Dizziness
• Vertigo
• Disequilibrium
• Oscillopsia
• Gait instability

Severe bilateral vestibular injury may cause substantial chronic disability even without dramatic spinning vertigo.

Ototoxicity May Be Permanent

This distinguishes aminoglycoside ototoxicity from much of their renal toxicity.

Renal injury frequently improves after drug withdrawal.

In contrast:

Cochlear or vestibular damage may be irreversible.

Therefore, early recognition is important.

Delayed Ototoxicity

Auditory or vestibular injury may continue to become apparent even after therapy has stopped.

A normal bedside hearing assessment immediately after an exposure does not completely exclude evolving ototoxicity after significant cumulative treatment.

Genetic Susceptibility

Certain mitochondrial genetic variants, particularly involving MT-RNR1, can markedly increase susceptibility to aminoglycoside-associated hearing loss.

In susceptible individuals, significant ototoxicity may occur even with otherwise conventional therapeutic exposure.

This genetic susceptibility was underrecognized in older toxicology references.

3. Neuromuscular Blockade

Aminoglycosides can interfere with neuromuscular transmission.

Mechanisms include impaired presynaptic acetylcholine release and reduced neuromuscular transmission.

Severe toxicity may produce:

• Generalized weakness
• Reduced respiratory muscle strength
• Respiratory depression
• Apnea

This complication is uncommon but potentially life-threatening.

Risk Factors for Neuromuscular Blockade

Risk may increase with:

• High systemic concentrations
• Rapid parenteral administration
• Neuromuscular blocking drugs
• Myasthenia gravis
• Other neuromuscular disorders
• Electrolyte abnormalities
• Anesthesia

Aminoglycosides can potentiate pharmacologic neuromuscular blockade.

Neuromuscular Blockade – Management

The priority is:

Airway protection + ventilation when required + discontinue the offending drug

Modern management should not rely on physostigmine as a specific antidote.

The historical recommendation for physostigmine is not established contemporary treatment for aminoglycoside-induced neuromuscular blockade.

Specialist management may include correction of contributing electrolyte abnormalities and selected pharmacologic measures, but respiratory support is the critical intervention.

4. Oral Exposure

Most aminoglycosides are poorly absorbed from an intact gastrointestinal tract.

Therefore, an isolated accidental oral ingestion usually produces little systemic toxicity.

This is especially relevant to agents such as neomycin.

However, systemic absorption may become more significant when:

• GI mucosa is severely damaged
• Large or prolonged enteral exposure occurs
• Renal function is impaired

Topical Exposure

Topical aminoglycosides generally produce limited systemic absorption.

However, absorption can increase when applied extensively to:

• Large burns
• Open wounds
• Damaged mucosa
• Large body-surface areas

Repeated exposure can also produce contact sensitization, particularly with neomycin.

5. Hypersensitivity

Aminoglycosides may cause allergic reactions.

Manifestations can include:

• Rash
• Contact dermatitis
• Urticaria
• Rare severe immediate hypersensitivity

Neomycin is particularly well recognized as a cause of allergic contact dermatitis.

Therapeutic Drug Monitoring

The older fixed “toxic peak and trough” thresholds should not be treated as universal modern cutoffs.

Aminoglycoside monitoring depends on:

• Specific drug
• Dosing strategy
• Infection
• Renal function
• Duration of therapy
• Local pharmacokinetic protocol

Extended-interval dosing has changed how concentrations are interpreted.

Peak Concentration

Historically, peak concentrations were monitored to assess both efficacy and toxicity.

Modern practice recognizes that aminoglycosides exhibit concentration-dependent bacterial killing, so an appropriately high peak may actually be therapeutically desirable.

Therefore:

A high peak is not automatically synonymous with toxicity.

Interpretation depends on the agent and dosing strategy.

Trough Concentration

Persistent drug accumulation before subsequent doses is more concerning for toxicity.

Elevated trough or delayed clearance may indicate:

• Reduced renal elimination
• Excessive cumulative exposure
• Need for dose/interval adjustment

However, specific targets differ between drugs and treatment protocols.

Extended-Interval Dosing

Many patients now receive larger individual doses at longer intervals rather than traditional multiple-daily dosing.

This approach takes advantage of:

• Concentration-dependent killing
• Post-antibiotic effect
• Periods of very low drug concentration between doses

Consequently, old peak/trough values cannot simply be applied to every modern aminoglycoside regimen.

Diagnosis

Suspect aminoglycoside toxicity in a patient receiving one of these drugs who develops:

• Rising creatinine
• New hearing impairment
• Tinnitus
• Disequilibrium
• Vestibular dysfunction
• Unexpected weakness
• Respiratory compromise

Review:

• Drug
• Dose
• Dosing interval
• Duration
• Renal function
• Concentration data
• Other nephrotoxic/ototoxic drugs

Renal Evaluation

Important assessment includes:

• Serum creatinine
• BUN
• Electrolytes
• Urine output
• Serial renal function

Creatinine clearance/eGFR assists dosing assessment but must be interpreted cautiously during rapidly changing AKI because serum creatinine may lag behind true renal function.

Hearing Assessment

When ototoxicity is suspected, formal assessment may include:

• Audiometry
• High-frequency hearing evaluation
• Vestibular testing when indicated

Patients receiving prolonged high-risk therapy may benefit from baseline and follow-up hearing assessment.

Serum Aminoglycoside Concentrations

Drug concentrations can be clinically useful after:

• Significant parenteral dosing error
• Unexpected accumulation
• Renal impairment
• Prolonged therapy

Unlike many toxicologic drug concentrations, aminoglycoside levels can directly assist pharmacokinetic management.

Interpretation should account for the exact timing of blood sampling relative to the dose.

Initial Management

The general approach is:

Stop further exposure → assess renal function → obtain appropriately timed drug concentrations when useful → assess hearing/vestibular function → provide supportive care

Most single exposures in patients with normal renal function do not require aggressive treatment.

GI Decontamination

Because aminoglycosides are poorly absorbed orally, aggressive GI decontamination is generally unnecessary after an isolated oral exposure.

Do not induce vomiting.

Routine gastric lavage is obsolete.

Activated charcoal is generally of little practical value for most isolated aminoglycoside ingestions and should not be used routinely.

Hydration

Maintain appropriate intravascular volume and renal perfusion.

However, forced fluid administration does not “flush out” aminoglycosides and can cause volume overload.

Fluid therapy should therefore be guided by:

• Volume status
• Renal function
• Urine output
• Hemodynamics

Hypotension

Treat clinically significant hypotension with:

• Appropriate isotonic crystalloid when indicated
• Treatment of the underlying cause
• Vasopressor support if shock persists

Norepinephrine is generally favored for persistent vasodilatory shock.

Trendelenburg positioning and routine dopamine-first therapy are outdated.

Hemodialysis

Aminoglycosides have:

• Relatively low molecular weight
• Low protein binding
• Relatively small volume of distribution

Therefore, they are potentially dialyzable.

However, dialysis is usually unnecessary after a single overdose when renal function is normal because endogenous renal elimination is efficient.

When Dialysis May Become Relevant

Renal replacement therapy may be considered when there is:

• Severe renal failure with markedly impaired elimination
• Significant drug accumulation
• Serious toxicity with prolonged high concentrations
• Conventional renal indications such as severe electrolyte, acid–base, or volume abnormalities

The decision should be individualized with toxicology/nephrology input.

No Specific Antidote

There is no established specific antidote that reverses:

• Aminoglycoside nephrotoxicity
• Cochlear injury
• Vestibular injury

Management centers on preventing further exposure and providing organ support.

Differential Diagnosis – Acute Kidney Injury

Other causes of AKI include:

• Sepsis
• Shock
• Dehydration
• Rhabdomyolysis
• Urinary obstruction
• Other nephrotoxic medications
• Toxic alcohols
• Heavy metals

In critically ill patients, aminoglycosides may be only one of several simultaneous renal insults.

Differential Diagnosis – Hearing/Vestibular Symptoms

Consider:

• Other ototoxic medications
• Ear disease
• Vestibular neuritis
• Ménière disease
• Neurologic disorders
• Infection
• Age-related hearing loss

Other drugs with ototoxic potential can amplify risk.

Drug Interactions – Modern Perspective

The older interaction list should not be interpreted as a collection of absolute contraindications.

The clinically important principles are:

• Other nephrotoxins increase renal risk
• Other ototoxins may increase auditory/vestibular risk
• Neuromuscular blockers can have enhanced effects

Aminoglycosides and certain β-lactam antibiotics can also undergo chemical inactivation if physically mixed under inappropriate conditions, but this does not mean that clinically indicated combination therapy is universally prohibited.

Pregnancy

The historical FDA pregnancy letter categories are obsolete.

Systemic aminoglycoside use during pregnancy requires assessment of:

• Maternal infection severity
• Specific aminoglycoside
• Alternative antibiotics
• Gestational circumstances
• Potential fetal ototoxicity

Serious maternal infection may make aminoglycoside therapy appropriate when benefits outweigh potential fetal risks.

Safeguarding

Rigid historical age cutoffs for assuming neglect, abuse, or intentional poisoning are inappropriate.

Pediatric exposures should instead be evaluated according to:

• Developmental capability
• Medication accessibility
• Exposure circumstances
• Consistency of history
• Recurrent unexplained events
• Broader safeguarding concerns

Monitoring

During significant exposure or therapeutic toxicity, monitor:

• Serum creatinine
• Renal function trend
• Urine output
• Electrolytes
• Appropriately timed aminoglycoside concentrations
• Hearing when indicated
• Vestibular function when symptomatic
• Respiratory status if weakness develops

Renal toxicity may not become apparent immediately, so follow-up should reflect the exposure pattern rather than an arbitrary short observation period.

Disposition

An isolated accidental oral exposure in an asymptomatic patient with normal renal function generally has low systemic toxicity.

Further evaluation or admission may be needed for:

• Significant parenteral dosing error
• Renal impairment
• Rising creatinine
• Persistent excessive drug concentrations
• Hearing loss
• Significant vestibular dysfunction
• Neuromuscular weakness
• Respiratory compromise
• Other serious complications

Prognosis

Renal toxicity

Often improves after discontinuation, although severe cases can require temporary dialysis.

Ototoxicity

May be permanent.

Neuromuscular blockade

Usually resolves as drug concentrations fall if adequate respiratory support is provided.

The most important strategy is prevention through appropriate dosing and renal monitoring.

Important Modernization of the Older Source

• Aminoglycoside toxicity is generally a cumulative exposure problem, not a classic single-dose overdose syndrome.
• The major toxicities remain nephrotoxicity and ototoxicity.
• Nephrotoxicity primarily reflects proximal tubular injury and is often reversible.
• Ototoxicity may involve cochlear or vestibular systems and can be permanent.
• MT-RNR1 mitochondrial variants can greatly increase susceptibility to aminoglycoside hearing loss.
• Aminoglycosides can rarely produce clinically important neuromuscular blockade.
• Physostigmine should not be considered a standard antidote for aminoglycoside neuromuscular toxicity.
• Fixed historical peak/trough “toxic levels” should not be applied indiscriminately to modern extended-interval dosing.
• Appropriately timed serum concentrations remain useful for therapeutic monitoring and significant dosing errors.
• Oral aminoglycosides are poorly absorbed, making most isolated oral overdoses low risk.
• Routine gastric lavage is obsolete, and activated charcoal generally has little role.
• Hemodialysis can remove aminoglycosides but is generally reserved for severe accumulation with impaired renal clearance or other dialysis indications.
• The historical statement that gentamicin/tobramycin should not be combined therapeutically with certain β-lactams is overly broad; physical incompatibility and pharmacokinetic issues should be distinguished from clinically useful combination therapy.
• Trendelenburg and routine dopamine-first shock treatment are outdated.
• Pregnancy should no longer be described using the old FDA letter categories.

Key Points

• Aminoglycosides → kidney + inner-ear toxicity.
• Nephrotoxicity = proximal tubular injury and AKI.
• Ototoxicity = cochlear hearing loss and/or vestibular dysfunction.
• Kidney injury is often reversible; hearing or vestibular damage may not be.
• Toxicity is usually associated with repeated exposure, accumulation, or renal impairment.
• A single accidental oral ingestion usually has low systemic toxicity because GI absorption is poor.
• Renal impairment markedly prolongs elimination.
• Serum concentrations are useful when interpreted according to the specific dosing strategy.
• Rare severe toxicity can cause neuromuscular weakness and respiratory failure.
• There is no specific antidote.
• Management centers on stopping exposure, monitoring renal and auditory function, supportive care, and selected dialysis when clearance is severely impaired.

Toxicology – First-Generation OTC Antihistamines

Core Concept

Most traditional over-the-counter antihistamines are first-generation H1 receptor antagonists. Important examples include:

• Diphenhydramine
• Doxylamine
• Chlorpheniramine
• Brompheniramine
• Dimenhydrinate
• Meclizine
• Cyclizine
• Clemastine
• Cyproheptadine
• Triprolidine

Some older drugs listed in historical references are now rarely used, prescription-only in many regions, or discontinued.

These agents cross the blood–brain barrier much more readily than second-generation antihistamines.

Their overdose syndrome is primarily:

CNS toxicity + antimuscarinic toxicity

Large overdoses of certain agents, particularly diphenhydramine, can additionally produce:

Cardiac sodium-channel blockade → QRS widening → ventricular dysrhythmia

Severe poisoning may cause seizures, hyperthermia, rhabdomyolysis, coma, hypotension, and cardiac arrest.

⸻

Mechanism

Therapeutically, these drugs competitively block H1 histamine receptors.

In overdose, many also antagonize muscarinic acetylcholine receptors.

This produces the classic antimuscarinic syndrome:

• Agitation/delirium
• Mydriasis
• Dry mucous membranes
• Dry, flushed skin
• Tachycardia
• Hyperthermia
• Reduced bowel motility
• Urinary retention

Some first-generation antihistamines also affect cardiac ion channels at high concentrations.

⸻

CNS Effects

Because these agents readily enter the CNS, both depression and excitation can occur.

Possible manifestations include:

• Drowsiness
• Confusion
• Dysarthria
• Ataxia
• Agitation
• Hallucinations
• Delirium
• Seizures
• Coma

The clinical pattern varies with agent, dose, age, and coingestants.

⸻

Antimuscarinic Delirium

Severe poisoning may produce a characteristic delirium with:

• Severe agitation
• Visual hallucinations
• Incoherent speech
• Disorientation
• Picking at imaginary objects
• Paranoia
• Repeated attempts to climb out of bed

Peripheral antimuscarinic findings often accompany the delirium.

⸻

Classic Antimuscarinic Findings

A useful memory pattern is:

• Dilated pupils
• Dry mouth
• Dry skin
• Flushing
• Tachycardia
• Hyperthermia
• Reduced bowel sounds
• Urinary retention
• Delirium

However, not every finding must be present.

⸻

Sedation vs Excitation

First-generation antihistamines can produce either:

CNS depression

• Somnolence
• Ataxia
• Coma
• Respiratory compromise in severe poisoning

CNS excitation

• Restlessness
• Agitation
• Hallucinations
• Tremor
• Seizures

Children may sometimes demonstrate prominent excitation, but this pattern is not exclusive to children.

⸻

Diphenhydramine – Particularly Important

Diphenhydramine deserves special attention because a large overdose can produce more than a simple antimuscarinic syndrome.

It can cause:

• Severe delirium
• Seizures
• Fast sodium-channel blockade
• QRS widening
• Ventricular dysrhythmias
• QT abnormalities
• Hypotension
• Coma

Thus, severe diphenhydramine poisoning can resemble TCA poisoning.

⸻

Doxylamine

Doxylamine is another important first-generation antihistamine found in some sleep and cold preparations.

Large exposures may cause:

• Antimuscarinic delirium
• Seizures
• Coma
• Rhabdomyolysis

Rhabdomyolysis may occasionally be substantial even without prolonged seizures.

⸻

Dimenhydrinate

Dimenhydrinate is used for motion sickness.

It is pharmacologically related to diphenhydramine and can cause:

• Sedation
• Antimuscarinic delirium
• Hallucinations
• Seizures
• Cardiovascular toxicity after severe exposure

Some misuse occurs because of its psychoactive effects.

⸻

Combination Products – Major Pitfall

Many OTC “cold,” “night,” “allergy,” and “sleep” products contain more than one active ingredient.

Possible coformulated substances include:

• Acetaminophen
• Dextromethorphan
• Pseudoephedrine
• Phenylephrine
• Other antihistamines

The accompanying ingredient can be more dangerous than the antihistamine itself.

Always identify the exact product and every active ingredient.

⸻

Cardiovascular Effects

Mild/moderate toxicity commonly causes:

• Sinus tachycardia
• Mild hypertension

Severe poisoning may cause:

• QRS widening
• QT prolongation
• Ventricular ectopy
• Ventricular tachycardia
• Hypotension
• Cardiovascular collapse

The exact electrophysiologic effects differ among individual antihistamines.

⸻

Sodium-Channel Blockade

Large diphenhydramine exposures can inhibit fast myocardial sodium channels.

This produces:

Na⁺-channel blockade → slowed ventricular conduction → QRS widening → ventricular dysrhythmia

The ECG may resemble that of a TCA overdose.

⸻

ECG Findings

In significant poisoning, evaluate:

• Heart rate/rhythm
• PR interval
• QRS duration
• QT/QTc
• Ventricular ectopy
• Terminal QRS morphology

A prominent terminal R wave in aVR may occur with sodium-channel blockade but is not specific for a particular toxin.

⸻

Sodium Bicarbonate

When clinically important sodium-channel blockade produces:

• QRS widening
• Ventricular conduction disturbance
• Ventricular dysrhythmia

Sodium bicarbonate is an important treatment.

It works through sodium loading and alkalinization.

Treatment should be guided by:

• ECG response
• Blood pressure
• Perfusion
• Acid–base status
• Electrolytes

Excessive alkalinization can cause complications and should be avoided.

⸻

QT Prolongation

Some antihistamines can also interfere with cardiac potassium channels.

This can produce:

Delayed repolarization → QT prolongation → increased risk of polymorphic ventricular dysrhythmia

When important QT prolongation is present:

• Correct potassium
• Correct magnesium
• Correct other relevant electrolyte abnormalities
• Avoid additional QT-prolonging drugs

Torsades is managed with standard toxicologic resuscitation, including IV magnesium and electrical treatment when unstable.

⸻

Seizures

Seizures are an important feature of severe first-generation antihistamine poisoning.

They can cause:

• Hypoxemia
• Lactic acidosis
• Hyperthermia
• Rhabdomyolysis
• Hyperkalemia
• Aspiration

Seizures and acidemia can also worsen cardiovascular toxicity.

⸻

Seizure Treatment

Benzodiazepines are first-line.

Persistent toxicologic seizures may require:

• Additional benzodiazepines
• Phenobarbital
• Appropriate anesthetic treatment for refractory status epilepticus

Phenytoin is generally not preferred for toxicant-induced seizures, particularly when sodium-channel cardiotoxicity is also present.

⸻

Hyperthermia

Hyperthermia can result from:

• Impaired sweating
• Severe agitation
• Seizures
• Excessive muscular activity

Severe hyperthermia can cause:

• Rhabdomyolysis
• Acute kidney injury
• Hepatic injury
• Coagulopathy
• CNS injury

⸻

Hyperthermia Management

Priorities include:

• Control agitation
• Control seizures
• Remove excessive clothing
• Active external cooling
• Appropriate IV fluids
• Monitor core temperature

Antipyretics are ineffective because this is toxicologic hyperthermia, not a hypothalamic fever response.

⸻

Rhabdomyolysis

Rhabdomyolysis may follow:

• Seizures
• Severe agitation
• Hyperthermia
• Prolonged immobilization
• Doxylamine toxicity itself

Evaluate significant cases with:

• CK
• Potassium
• Creatinine
• Urinalysis
• Urine output

⸻

Urinary Retention

Antimuscarinic blockade can cause substantial bladder retention.

A distended bladder can worsen:

• Agitation
• Delirium
• Tachycardia

Clinically significant retention may require bladder decompression.

⸻

GI Effects

Antimuscarinic activity reduces gastrointestinal motility.

Possible findings include:

• Reduced bowel sounds
• Constipation
• Abdominal distension
• Ileus in severe cases

Slowed gastric emptying may contribute to prolonged absorption after large ingestion.

⸻

Respiratory Complications

Severe CNS depression or seizures may lead to:

• Loss of airway reflexes
• Hypoventilation
• Aspiration
• Hypoxemia

Airway management takes priority over decontamination.

⸻

Diagnosis

The diagnosis is usually clinical.

Important history includes:

• Exact product
• All active ingredients
• Amount
• Timing
• Formulation
• Intent
• Coingestants

Do not assume every OTC “allergy” or “sleep” medication contains only an antihistamine.

⸻

Laboratory Evaluation

A minimally symptomatic patient after a clearly identified small exposure may need little laboratory testing.

Significant poisoning may warrant:

• Glucose
• Electrolytes
• Bicarbonate
• Creatinine
• CK
• Blood gas/lactate in severe cases

Acetaminophen testing is often appropriate after intentional ingestion because combination products and occult coingestion are common.

⸻

Urine Drug Screening

Routine urine toxicology screening is generally not useful for diagnosing antihistamine poisoning.

Immunoassays have:

• False positives
• False negatives
• Cross-reactivity
• Limited ability to establish causation

A positive urine result demonstrates possible exposure, not necessarily the cause of the syndrome.

⸻

Neurodiagnostic Testing

Head CT, lumbar puncture, cultures, or other neurologic investigations are not routinely required when the toxidrome and exposure are clear.

Consider them when:

• Diagnosis is uncertain
• Focal neurologic findings occur
• Trauma is possible
• CNS infection is suspected
• Seizures or altered consciousness have an atypical course

⸻

Initial Management

Priorities are:

Airway/breathing → agitation/seizure control → core temperature → ECG → circulation → complications

A calm, low-stimulation environment can reduce worsening agitation.

⸻

Agitation

Benzodiazepines are appropriate for significant agitation, particularly when:

• The exposure is uncertain
• Seizures are a concern
• A mixed overdose is possible
• Physostigmine is inappropriate

Physical restraint alone can worsen:

• Muscular activity
• Hyperthermia
• Acidosis
• Rhabdomyolysis

If restraint is temporarily required for safety, adequate chemical sedation and frequent reassessment are important.

⸻

Physostigmine – Modern Role

The older description of physostigmine primarily as a diagnostic test is outdated.

Physostigmine can be a therapeutic treatment for carefully selected patients with severe, predominantly pure antimuscarinic delirium.

It may rapidly reverse:

• Delirium
• Hallucinations
• Severe agitation
• Confusion

Its use requires careful patient selection and monitoring.

⸻

When Physostigmine May Be Considered

The patient should have a convincing antimuscarinic syndrome with significant central delirium and no strong evidence of another dangerous toxic mechanism.

Before considering it, evaluate:

• ECG
• QRS duration
• Rhythm
• Coingestants
• Seizure risk

Toxicology/poison-center guidance is appropriate.

⸻

When Physostigmine Should Be Avoided

Avoid or use extreme caution with:

• QRS widening
• Suspected TCA poisoning
• Significant sodium-channel blockade
• Important conduction disease
• Bradycardia
• High-risk proconvulsant ingestion
• Uncertain mixed overdose

These are much more clinically important contraindications than some of the broad historical lists involving conditions such as diabetes.

⸻

Physostigmine Adverse Effects

Excessive cholinergic activity may cause:

• Bradycardia
• Salivation
• Sweating
• Bronchial secretions
• Nausea/vomiting
• Diarrhea
• Hypotension
• Seizures

It should therefore be used in a monitored environment with resuscitation capability.

Symptoms may recur because physostigmine can have a shorter duration than the causative antihistamine.

⸻

Hypotension

Potential causes include:

• Severe sodium-channel toxicity
• Dysrhythmia
• Dehydration
• Acidemia
• Coingestants

Management includes:

• Appropriate isotonic crystalloid
• Correction of cardiotoxicity
• Treatment of dysrhythmia
• Vasopressor support when required

For persistent vasodilatory shock, norepinephrine is generally favored over an automatic dopamine-first strategy.

Trendelenburg positioning is obsolete.

⸻

GI Decontamination

Do not induce vomiting.

Ipecac has no modern role.

Routine gastric lavage is obsolete.

A single dose of activated charcoal may occasionally be considered after a substantial recent ingestion when:

• The drug is adsorbable
• The airway is reliably protected
• Aspiration risk is acceptable

Delayed gastric emptying from antimuscarinic effects does not automatically justify late lavage or repeated charcoal.

⸻

Extracorporeal Elimination

Hemodialysis and hemoperfusion are generally ineffective for first-generation antihistamine poisoning because these drugs typically have pharmacokinetic characteristics unfavorable for extracorporeal removal.

Treatment remains primarily supportive.

⸻

Differential Diagnosis

Antimuscarinic-like toxicity can occur with:

• Atropine/scopolamine
• TCAs
• Antipsychotics
• Antiparkinsonian drugs
• Antispasmodics
• Datura species
• Other antimuscarinic plants or medications

Agitated patients with tachycardia and mydriasis may instead have:

• Amphetamine toxicity
• Cocaine toxicity
• Serotonin toxicity

A useful distinction is:

Antimuscarinic → usually dry

Sympathomimetic → usually sweaty

But no single sign is completely reliable.

⸻

Observation

The historical universal 6–12-hour rule should not be applied mechanically.

Observation depends on:

• Exact antihistamine
• Formulation
• Amount
• Symptoms
• ECG
• Mental-status trajectory
• Seizures
• Coingestants
• Combination-product ingredients

Large antimuscarinic ingestions can have prolonged effects because GI motility is reduced.

⸻

Admission

Monitored admission is appropriate for:

• Significant delirium
• Persistent altered mental status
• Seizure
• Hyperthermia
• QRS widening
• Important QT prolongation
• Ventricular dysrhythmia
• Persistent hypotension
• Rhabdomyolysis
• Respiratory compromise

Severe cardiotoxicity, recurrent seizures, extreme hyperthermia, or coma warrants ICU care.

⸻

Pregnancy

Historical FDA pregnancy letter categories are obsolete.

Management of significant poisoning during pregnancy prioritizes maternal:

• Airway
• Oxygenation
• Temperature
• Seizure control
• Hemodynamics
• Cardiac rhythm

Treatment decisions should be based on the specific drug and clinical situation rather than the old A/B/C/D/X categories.

⸻

Safeguarding

Rigid historical age thresholds for assuming neglect, abuse, or intentional poisoning are inappropriate.

Pediatric poisoning should instead be assessed according to:

• Developmental capability
• Medication access
• Exposure circumstances
• Consistency of the history
• Recurrent unexplained events
• Broader safeguarding concerns

Intentional self-poisoning requires appropriate safety assessment after medical stabilization.

⸻

Prognosis

Most mild and moderate antihistamine poisonings recover completely with supportive care.

Severe complications include:

• Status epilepticus
• Extreme hyperthermia
• Aspiration
• Rhabdomyolysis
• Acute kidney injury
• QRS widening
• Ventricular dysrhythmia
• Shock
• Hypoxic brain injury

Large diphenhydramine or doxylamine exposures deserve particular caution.

⸻

Important Modernization of the Older Source

• The clinically important OTC agents are predominantly first-generation H1 antihistamines.
• Toxicity is not purely antimuscarinic; some agents, especially diphenhydramine, also block myocardial sodium channels.
• Severe diphenhydramine poisoning can resemble TCA poisoning.
• Doxylamine is particularly associated with severe CNS toxicity and rhabdomyolysis after substantial exposure.
• Combination products are a major source of additional toxicity; always identify every active ingredient.
• Benzodiazepines are first-line for agitation and seizures.
• Phenytoin is generally not preferred for toxicologic seizures.
• Sodium bicarbonate is important for clinically significant antihistamine-associated sodium-channel blockade/QRS widening.
• Physostigmine is a therapeutic option for carefully selected pure antimuscarinic delirium, not merely a diagnostic test.
• Physostigmine should generally be avoided when QRS widening, TCA exposure, significant sodium-channel blockade, or a dangerous mixed ingestion is suspected.
• Routine urine drug screening has limited diagnostic value.
• Antipyretics do not treat toxicologic hyperthermia.
• Ipecac and routine gastric lavage are obsolete.
• Antimuscarinic delayed gastric emptying does not justify routine late lavage or repeated charcoal.
• Trendelenburg and routine dopamine-first treatment of shock are outdated.
• Fixed observation periods should be replaced by agent-, symptom-, ECG-, and trajectory-based assessment.

Key Points

• First-generation antihistamine overdose = antimuscarinic + CNS toxicity.
• Typical findings: tachycardia, mydriasis, dry skin/mouth, hyperthermia, urinary retention, agitation, hallucinations, and delirium.
• Severe poisoning can cause seizures, coma, QRS widening, ventricular dysrhythmias, hypotension, and rhabdomyolysis.
• Diphenhydramine can cause clinically important sodium-channel blockade.
• Benzodiazepines are first-line for agitation and seizures.
• Sodium bicarbonate is used for significant QRS widening/sodium-channel cardiotoxicity.
• Physostigmine is reserved for carefully selected predominantly pure antimuscarinic delirium.
• Always check for acetaminophen, dextromethorphan, decongestants, or other ingredients in combination products.
• Management is primarily supportive, ECG-directed, and complication-focused.

Toxicology – Second-Generation (“Nonsedating”) Antihistamines

Core Concept

The older term “nonsedating antihistamines” generally refers to second-generation H1 antihistamines.

Modern examples include:

• Cetirizine
• Levocetirizine
• Loratadine
• Desloratadine
• Fexofenadine
• Acrivastine in some regions

These agents penetrate the CNS less readily than first-generation antihistamines and therefore usually produce less sedation and fewer antimuscarinic effects.

Most isolated overdoses are relatively mild.

The major cardiac toxicity emphasized in older literature—marked QT prolongation and torsades—was primarily associated with astemizole and terfenadine, drugs that have been withdrawn or are no longer routinely marketed in many countries.

Historical Agents – Astemizole and Terfenadine

Astemizole and terfenadine are important historically because they demonstrated that an apparently “nonsedating” antihistamine could still cause severe cardiac toxicity.

Both could block cardiac repolarizing potassium currents and produce:

QT prolongation → early afterdepolarizations → torsades de pointes → ventricular fibrillation/cardiac arrest

Risk became especially important when their metabolism was inhibited by interacting medications.

Modern second-generation antihistamines generally have a much more favorable cardiac safety profile.

Mechanism

Second-generation antihistamines primarily antagonize peripheral H1 receptors.

H1 blockade reduces allergic manifestations such as:

• Pruritus
• Sneezing
• Rhinorrhea
• Urticaria

Their limited CNS penetration accounts for less sedation compared with agents such as diphenhydramine.

However, “nonsedating” is not absolute.

Cetirizine

Cetirizine is relatively more likely than some other second-generation agents to cause:

• Drowsiness
• Fatigue
• Dizziness

Overdose usually produces mild CNS effects rather than severe cardiotoxicity.

Loratadine

Loratadine and its active metabolite desloratadine generally have limited CNS penetration.

Overdose may produce:

• Somnolence
• Headache
• Tachycardia

Serious isolated cardiotoxicity is uncommon.

Fexofenadine

Fexofenadine is the active metabolite related to the older drug terfenadine.

Unlike terfenadine, it has substantially less potential for clinically important cardiac potassium-channel blockade.

This explains why:

Terfenadine → important historical torsades risk

whereas:

Fexofenadine → much lower cardiac risk

Most isolated fexofenadine overdoses are mild.

Acrivastine

Acrivastine can produce:

• Drowsiness
• Dizziness
• Headache
• GI symptoms

Some preparations contain pseudoephedrine.

This distinction is crucial because toxicity from a combination product may reflect the sympathomimetic component rather than the antihistamine.

Combination Products

Always determine whether the preparation contains another active ingredient.

Examples include antihistamine products combined with:

• Pseudoephedrine
• Other decongestants
• Analgesics in some formulations

Pseudoephedrine can produce:

• Agitation
• Tremor
• Tachycardia
• Hypertension
• Insomnia

Therefore, a markedly hyperadrenergic presentation after an “antihistamine” ingestion should prompt inspection of the actual formulation.

Toxic Dose

There is no single useful toxic-dose threshold covering all second-generation antihistamines.

For currently used agents, isolated accidental overdose generally has a relatively wide safety margin.

Risk assessment should consider:

• Exact agent
• Amount
• Patient age/size
• Symptoms
• Coingestants
• Combination-product ingredients
• Renal/hepatic impairment where relevant

Historical astemizole and terfenadine toxicity should not be extrapolated directly to modern agents.

Clinical Features

Most uncomplicated overdoses produce only:

• Drowsiness
• Dizziness
• Headache
• Nausea
• Dry mouth
• Mild tachycardia

Large exposures may occasionally produce:

• Agitation
• Tremor
• Ataxia
• Significant somnolence

Severe neurologic or cardiovascular toxicity should prompt consideration of another agent or coingestant.

Sedation

The label “nonsedating” is misleading if interpreted literally.

Second-generation antihistamines cause less CNS H1 blockade, but some sedation remains possible.

Cetirizine is particularly associated with somnolence compared with fexofenadine.

Antimuscarinic Effects

Modern second-generation antihistamines generally have much weaker antimuscarinic effects than first-generation agents.

Prominent findings such as:

• Severe delirium
• Marked mydriasis
• Very dry skin
• Urinary retention
• Ileus
• Major QRS widening

should raise concern for:

• Diphenhydramine
• Doxylamine
• Another first-generation antihistamine
• TCA
• Another antimuscarinic/sodium-channel blocker
• Mixed ingestion

Cardiovascular Effects

With current second-generation agents, cardiovascular effects are usually limited.

Possible findings include:

• Mild tachycardia
• Occasionally palpitations

Major ventricular dysrhythmia is uncommon in isolated overdose of modern agents.

Astemizole Cardiotoxicity

Historical astemizole toxicity could produce:

• QT prolongation
• Syncope
• Torsades de pointes
• Ventricular dysrhythmia
• Hypotension
• Cardiovascular collapse

Its prolonged pharmacokinetics also allowed delayed toxicity.

These historical features explain the prolonged monitoring recommendations in older texts but should not automatically be applied to cetirizine, loratadine, or fexofenadine.

Drug Interactions – Historical Lesson

Astemizole and terfenadine depended heavily on hepatic metabolism.

Inhibition of their metabolism could increase parent-drug concentrations and markedly increase QT-related toxicity.

Historically important interacting drugs included some:

• Azole antifungals
• Macrolide antibiotics
• CYP inhibitors

This interaction was one of the major reasons these drugs disappeared from routine clinical use.

QT Prolongation – Modern Approach

If clinically important QT prolongation occurs:

• Review the exact antihistamine
• Look for coingestants
• Check potassium
• Check magnesium
• Consider calcium when appropriate
• Review other QT-prolonging medications
• Consider congenital/acquired long-QT conditions

Marked QT prolongation should not automatically be attributed to a modern second-generation antihistamine.

Torsades de Pointes

If torsades develops:

• Stop QT-prolonging drugs
• Correct hypokalemia
• Correct hypomagnesemia
• Give IV magnesium
• Treat unstable ventricular dysrhythmia electrically

Recurrent pause-dependent torsades associated with bradycardia may require heart-rate acceleration, including selected use of overdrive pacing.

Avoid adding medications that further prolong the QT interval.

Neurologic Effects

Large exposures may cause:

• Somnolence
• Dizziness
• Headache
• Tremor
• Agitation
• Ataxia

Seizures are unusual with modern second-generation antihistamines.

If seizures occur, evaluate for:

• Large/mixed ingestion
• First-generation antihistamine
• Sympathomimetic combination product
• Other proconvulsant
• Metabolic or neurologic cause

Seizure Treatment

Benzodiazepines are first-line therapy for toxicant-induced seizures.

Persistent seizures may require:

• Additional benzodiazepines
• Phenobarbital
• Appropriate anesthetic status-epilepticus treatment

The older recommendation to routinely progress to phenytoin is outdated; phenytoin is generally not preferred for toxicologic seizures.

Hepatic Effects

Rare liver injury has been reported with antihistamines, but it is not a defining manifestation of acute second-generation antihistamine overdose.

Liver testing should therefore be driven by:

• Symptoms
• Prolonged exposure
• Coingestants
• Other clinical concerns

rather than performed routinely after every uncomplicated exposure.

Diagnosis

Diagnosis is usually clinical.

Determine:

• Exact product
• Immediate vs combination formulation
• Amount
• Time
• Symptoms
• Coingestants
• Relevant medical conditions

Identifying the exact product is particularly important because a “non-drowsy allergy tablet” may contain a decongestant.

ECG

An ECG is reasonable when there is:

• Syncope
• Palpitations
• Significant tachycardia
• Intentional or substantial overdose
• Suspected QT-active drug
• Electrolyte abnormality
• Concerning coingestant
• Cardiovascular symptoms

Routine prolonged ECG monitoring is generally unnecessary after every small asymptomatic exposure to a modern second-generation antihistamine.

Laboratory Evaluation

Small uncomplicated exposures may require no laboratory testing.

When clinically indicated, consider:

• Glucose
• Electrolytes
• Potassium
• Magnesium
• Creatinine

Additional testing depends on:

• Intentional overdose
• Coingestants
• ECG abnormalities
• Altered mental status

Drug concentrations are not routinely useful.

Occult Coingestion

Intentional overdose should prompt evaluation for clinically important coingestants.

Acetaminophen testing is often relevant because early toxicity can be asymptomatic.

Other testing should be directed by history and clinical findings rather than performed mechanically.

Initial Management

Management is predominantly supportive:

Airway/breathing → mental status → circulation → identify exact formulation → ECG when indicated → treat complications

Most isolated modern second-generation antihistamine overdoses do not require aggressive intervention.

GI Decontamination

Do not induce vomiting.

Routine gastric lavage is obsolete.

Activated charcoal may occasionally be considered after a substantial recent ingestion when:

• The substance is adsorbable
• The airway is safe
• Aspiration risk is low
• Expected benefit justifies treatment

Routine decontamination is unnecessary after many minor accidental exposures.

Hypotension

Significant hypotension is unusual after isolated modern second-generation antihistamine overdose.

If it occurs, evaluate for:

• Coingestants
• Dysrhythmia
• Dehydration
• Another medical cause

Treatment may include appropriate isotonic crystalloid and, for persistent vasodilatory shock, a vasopressor such as norepinephrine.

Trendelenburg positioning and routine dopamine-first therapy are outdated.

Extracorporeal Elimination

Hemodialysis and hemoperfusion have no routine role in second-generation antihistamine poisoning.

Supportive care is generally sufficient.

Monitoring

Monitoring should be proportional to the exposure.

Mild modern-agent exposure

Monitor:

• Mental status
• Vital signs
• Development of unexpected symptoms

Significant or symptomatic exposure

Consider:

• ECG
• Cardiac rhythm
• Electrolytes
• Neurologic status

QT prolongation

Continue ECG monitoring until clinically important repolarization abnormalities and associated risk factors have resolved.

Observation

The historical blanket recommendation of:

• 6 hours for all uncomplicated exposures
• 24 hours for hospitalized patients

is too rigid.

Observation should depend on:

• Exact agent
• Amount
• Symptoms
• ECG
• Combination ingredients
• Coingestants
• Clinical trajectory

Historical prolonged monitoring for astemizole should not be automatically transferred to currently used agents.

Admission

Hospital admission may be appropriate for:

• Significant altered mental status
• Seizure
• Syncope with concerning ECG findings
• Important QT prolongation
• Ventricular dysrhythmia
• Persistent hypotension
• Significant coingestion

ICU care is appropriate for torsades, cardiovascular collapse, or other severe toxicity.

Pregnancy and Breastfeeding

The historical FDA pregnancy letter categories are obsolete.

Cetirizine and loratadine have substantial clinical experience in pregnancy, but medication decisions should consider:

• Specific drug
• Dose
• Gestational stage
• Clinical indication

The old blanket statement that cetirizine or loratadine should generally be avoided during breastfeeding is also too broad.

Drug transfer into milk and infant effects vary by agent, and current lactation-specific guidance should be used.

Safeguarding

Rigid historical age thresholds for neglect, abuse, or intentional poisoning are inappropriate.

Pediatric exposures should instead be assessed according to:

• Developmental capability
• Medication accessibility
• Exposure circumstances
• Consistency of history
• Recurrent unexplained exposures
• Broader safeguarding concerns

Intentional self-poisoning requires appropriate safety assessment after medical stabilization.

Prognosis

Most isolated overdoses involving currently used second-generation antihistamines have a favorable outcome.

Severe toxicity should prompt particular attention to:

• Combination products
• Coingestants
• Electrolyte disturbances
• Unexpected first-generation antihistamine exposure
• Historical QT-active agents

Important Modernization of the Older Source

• “Nonsedating” is better described as second-generation H1 antihistamines; sedation can still occur.
• Astemizole is largely historical and its severe QT toxicity should not define the entire modern class.
• Terfenadine is likewise largely historical because of serious interaction-mediated cardiotoxicity.
• Modern agents such as cetirizine, loratadine, and fexofenadine generally have a much wider cardiovascular safety margin.
• Fexofenadine lacks the major torsadogenic liability of its historical precursor terfenadine.
• Cetirizine can still cause clinically noticeable drowsiness.
• Combination products containing pseudoephedrine can produce a sympathomimetic syndrome.
• Severe antimuscarinic delirium or QRS widening is atypical for modern second-generation agents and should prompt a search for another toxicant.
• Routine prolonged cardiac monitoring is unnecessary after every minor modern-agent exposure.
• Phenytoin is generally not preferred for toxicologic seizures.
• Ipecac and routine gastric lavage are obsolete.
• Trendelenburg and dopamine-first shock treatment are outdated.
• Observation should be agent-, symptom-, ECG-, and coingestant-specific.
• There is no specific antidote.

Key Points

• Second-generation antihistamines usually cause mild toxicity in isolated overdose.
• Common effects are drowsiness, dizziness, headache, nausea, and mild tachycardia.
• Cetirizine is relatively more sedating; fexofenadine has very little CNS effect.
• Astemizole and terfenadine are historical exceptions with major QT/torsades toxicity.
• Marked QT prolongation with a modern agent should prompt evaluation for electrolytes, interactions, and coingestants.
• IV magnesium + electrolyte correction are central if torsades occurs.
• Benzodiazepines are first-line for the unusual toxicologic seizure.
• Always check whether the product also contains pseudoephedrine or another active drug.
• Management is primarily supportive.

Toxicology – Second-Generation (“Nonsedating”) Antihistamines

Core Concept

The older term “nonsedating antihistamines” generally refers to second-generation H1 antihistamines.

Modern examples include:

• Cetirizine
• Levocetirizine
• Loratadine
• Desloratadine
• Fexofenadine
• Acrivastine in some regions

These agents penetrate the CNS less readily than first-generation antihistamines and therefore usually produce less sedation and fewer antimuscarinic effects.

Most isolated overdoses are relatively mild.

The major cardiac toxicity emphasized in older literature—marked QT prolongation and torsades—was primarily associated with astemizole and terfenadine, drugs that have been withdrawn or are no longer routinely marketed in many countries.

Historical Agents – Astemizole and Terfenadine

Astemizole and terfenadine are important historically because they demonstrated that an apparently “nonsedating” antihistamine could still cause severe cardiac toxicity.

Both could block cardiac repolarizing potassium currents and produce:

QT prolongation → early afterdepolarizations → torsades de pointes → ventricular fibrillation/cardiac arrest

Risk became especially important when their metabolism was inhibited by interacting medications.

Modern second-generation antihistamines generally have a much more favorable cardiac safety profile.

Mechanism

Second-generation antihistamines primarily antagonize peripheral H1 receptors.

H1 blockade reduces allergic manifestations such as:

• Pruritus
• Sneezing
• Rhinorrhea
• Urticaria

Their limited CNS penetration accounts for less sedation compared with agents such as diphenhydramine.

However, “nonsedating” is not absolute.

Cetirizine

Cetirizine is relatively more likely than some other second-generation agents to cause:

• Drowsiness
• Fatigue
• Dizziness

Overdose usually produces mild CNS effects rather than severe cardiotoxicity.

Loratadine

Loratadine and its active metabolite desloratadine generally have limited CNS penetration.

Overdose may produce:

• Somnolence
• Headache
• Tachycardia

Serious isolated cardiotoxicity is uncommon.

Fexofenadine

Fexofenadine is the active metabolite related to the older drug terfenadine.

Unlike terfenadine, it has substantially less potential for clinically important cardiac potassium-channel blockade.

This explains why:

Terfenadine → important historical torsades risk

whereas:

Fexofenadine → much lower cardiac risk

Most isolated fexofenadine overdoses are mild.

Acrivastine

Acrivastine can produce:

• Drowsiness
• Dizziness
• Headache
• GI symptoms

Some preparations contain pseudoephedrine.

This distinction is crucial because toxicity from a combination product may reflect the sympathomimetic component rather than the antihistamine.

Combination Products

Always determine whether the preparation contains another active ingredient.

Examples include antihistamine products combined with:

• Pseudoephedrine
• Other decongestants
• Analgesics in some formulations

Pseudoephedrine can produce:

• Agitation
• Tremor
• Tachycardia
• Hypertension
• Insomnia

Therefore, a markedly hyperadrenergic presentation after an “antihistamine” ingestion should prompt inspection of the actual formulation.

Toxic Dose

There is no single useful toxic-dose threshold covering all second-generation antihistamines.

For currently used agents, isolated accidental overdose generally has a relatively wide safety margin.

Risk assessment should consider:

• Exact agent
• Amount
• Patient age/size
• Symptoms
• Coingestants
• Combination-product ingredients
• Renal/hepatic impairment where relevant

Historical astemizole and terfenadine toxicity should not be extrapolated directly to modern agents.

Clinical Features

Most uncomplicated overdoses produce only:

• Drowsiness
• Dizziness
• Headache
• Nausea
• Dry mouth
• Mild tachycardia

Large exposures may occasionally produce:

• Agitation
• Tremor
• Ataxia
• Significant somnolence

Severe neurologic or cardiovascular toxicity should prompt consideration of another agent or coingestant.

Sedation

The label “nonsedating” is misleading if interpreted literally.

Second-generation antihistamines cause less CNS H1 blockade, but some sedation remains possible.

Cetirizine is particularly associated with somnolence compared with fexofenadine.

Antimuscarinic Effects

Modern second-generation antihistamines generally have much weaker antimuscarinic effects than first-generation agents.

Prominent findings such as:

• Severe delirium
• Marked mydriasis
• Very dry skin
• Urinary retention
• Ileus
• Major QRS widening

should raise concern for:

• Diphenhydramine
• Doxylamine
• Another first-generation antihistamine
• TCA
• Another antimuscarinic/sodium-channel blocker
• Mixed ingestion

Cardiovascular Effects

With current second-generation agents, cardiovascular effects are usually limited.

Possible findings include:

• Mild tachycardia
• Occasionally palpitations

Major ventricular dysrhythmia is uncommon in isolated overdose of modern agents.

Astemizole Cardiotoxicity

Historical astemizole toxicity could produce:

• QT prolongation
• Syncope
• Torsades de pointes
• Ventricular dysrhythmia
• Hypotension
• Cardiovascular collapse

Its prolonged pharmacokinetics also allowed delayed toxicity.

These historical features explain the prolonged monitoring recommendations in older texts but should not automatically be applied to cetirizine, loratadine, or fexofenadine.

Drug Interactions – Historical Lesson

Astemizole and terfenadine depended heavily on hepatic metabolism.

Inhibition of their metabolism could increase parent-drug concentrations and markedly increase QT-related toxicity.

Historically important interacting drugs included some:

• Azole antifungals
• Macrolide antibiotics
• CYP inhibitors

This interaction was one of the major reasons these drugs disappeared from routine clinical use.

QT Prolongation – Modern Approach

If clinically important QT prolongation occurs:

• Review the exact antihistamine
• Look for coingestants
• Check potassium
• Check magnesium
• Consider calcium when appropriate
• Review other QT-prolonging medications
• Consider congenital/acquired long-QT conditions

Marked QT prolongation should not automatically be attributed to a modern second-generation antihistamine.

Torsades de Pointes

If torsades develops:

• Stop QT-prolonging drugs
• Correct hypokalemia
• Correct hypomagnesemia
• Give IV magnesium
• Treat unstable ventricular dysrhythmia electrically

Recurrent pause-dependent torsades associated with bradycardia may require heart-rate acceleration, including selected use of overdrive pacing.

Avoid adding medications that further prolong the QT interval.

Neurologic Effects

Large exposures may cause:

• Somnolence
• Dizziness
• Headache
• Tremor
• Agitation
• Ataxia

Seizures are unusual with modern second-generation antihistamines.

If seizures occur, evaluate for:

• Large/mixed ingestion
• First-generation antihistamine
• Sympathomimetic combination product
• Other proconvulsant
• Metabolic or neurologic cause

Seizure Treatment

Benzodiazepines are first-line therapy for toxicant-induced seizures.

Persistent seizures may require:

• Additional benzodiazepines
• Phenobarbital
• Appropriate anesthetic status-epilepticus treatment

The older recommendation to routinely progress to phenytoin is outdated; phenytoin is generally not preferred for toxicologic seizures.

Hepatic Effects

Rare liver injury has been reported with antihistamines, but it is not a defining manifestation of acute second-generation antihistamine overdose.

Liver testing should therefore be driven by:

• Symptoms
• Prolonged exposure
• Coingestants
• Other clinical concerns

rather than performed routinely after every uncomplicated exposure.

Diagnosis

Diagnosis is usually clinical.

Determine:

• Exact product
• Immediate vs combination formulation
• Amount
• Time
• Symptoms
• Coingestants
• Relevant medical conditions

Identifying the exact product is particularly important because a “non-drowsy allergy tablet” may contain a decongestant.

ECG

An ECG is reasonable when there is:

• Syncope
• Palpitations
• Significant tachycardia
• Intentional or substantial overdose
• Suspected QT-active drug
• Electrolyte abnormality
• Concerning coingestant
• Cardiovascular symptoms

Routine prolonged ECG monitoring is generally unnecessary after every small asymptomatic exposure to a modern second-generation antihistamine.

Laboratory Evaluation

Small uncomplicated exposures may require no laboratory testing.

When clinically indicated, consider:

• Glucose
• Electrolytes
• Potassium
• Magnesium
• Creatinine

Additional testing depends on:

• Intentional overdose
• Coingestants
• ECG abnormalities
• Altered mental status

Drug concentrations are not routinely useful.

Occult Coingestion

Intentional overdose should prompt evaluation for clinically important coingestants.

Acetaminophen testing is often relevant because early toxicity can be asymptomatic.

Other testing should be directed by history and clinical findings rather than performed mechanically.

Initial Management

Management is predominantly supportive:

Airway/breathing → mental status → circulation → identify exact formulation → ECG when indicated → treat complications

Most isolated modern second-generation antihistamine overdoses do not require aggressive intervention.

GI Decontamination

Do not induce vomiting.

Routine gastric lavage is obsolete.

Activated charcoal may occasionally be considered after a substantial recent ingestion when:

• The substance is adsorbable
• The airway is safe
• Aspiration risk is low
• Expected benefit justifies treatment

Routine decontamination is unnecessary after many minor accidental exposures.

Hypotension

Significant hypotension is unusual after isolated modern second-generation antihistamine overdose.

If it occurs, evaluate for:

• Coingestants
• Dysrhythmia
• Dehydration
• Another medical cause

Treatment may include appropriate isotonic crystalloid and, for persistent vasodilatory shock, a vasopressor such as norepinephrine.

Trendelenburg positioning and routine dopamine-first therapy are outdated.

Extracorporeal Elimination

Hemodialysis and hemoperfusion have no routine role in second-generation antihistamine poisoning.

Supportive care is generally sufficient.

Monitoring

Monitoring should be proportional to the exposure.

Mild modern-agent exposure

Monitor:

• Mental status
• Vital signs
• Development of unexpected symptoms

Significant or symptomatic exposure

Consider:

• ECG
• Cardiac rhythm
• Electrolytes
• Neurologic status

QT prolongation

Continue ECG monitoring until clinically important repolarization abnormalities and associated risk factors have resolved.

Observation

The historical blanket recommendation of:

• 6 hours for all uncomplicated exposures
• 24 hours for hospitalized patients

is too rigid.

Observation should depend on:

• Exact agent
• Amount
• Symptoms
• ECG
• Combination ingredients
• Coingestants
• Clinical trajectory

Historical prolonged monitoring for astemizole should not be automatically transferred to currently used agents.

Admission

Hospital admission may be appropriate for:

• Significant altered mental status
• Seizure
• Syncope with concerning ECG findings
• Important QT prolongation
• Ventricular dysrhythmia
• Persistent hypotension
• Significant coingestion

ICU care is appropriate for torsades, cardiovascular collapse, or other severe toxicity.

Pregnancy and Breastfeeding

The historical FDA pregnancy letter categories are obsolete.

Cetirizine and loratadine have substantial clinical experience in pregnancy, but medication decisions should consider:

• Specific drug
• Dose
• Gestational stage
• Clinical indication

The old blanket statement that cetirizine or loratadine should generally be avoided during breastfeeding is also too broad.

Drug transfer into milk and infant effects vary by agent, and current lactation-specific guidance should be used.

Safeguarding

Rigid historical age thresholds for neglect, abuse, or intentional poisoning are inappropriate.

Pediatric exposures should instead be assessed according to:

• Developmental capability
• Medication accessibility
• Exposure circumstances
• Consistency of history
• Recurrent unexplained exposures
• Broader safeguarding concerns

Intentional self-poisoning requires appropriate safety assessment after medical stabilization.

Prognosis

Most isolated overdoses involving currently used second-generation antihistamines have a favorable outcome.

Severe toxicity should prompt particular attention to:

• Combination products
• Coingestants
• Electrolyte disturbances
• Unexpected first-generation antihistamine exposure
• Historical QT-active agents

Important Modernization of the Older Source

• “Nonsedating” is better described as second-generation H1 antihistamines; sedation can still occur.
• Astemizole is largely historical and its severe QT toxicity should not define the entire modern class.
• Terfenadine is likewise largely historical because of serious interaction-mediated cardiotoxicity.
• Modern agents such as cetirizine, loratadine, and fexofenadine generally have a much wider cardiovascular safety margin.
• Fexofenadine lacks the major torsadogenic liability of its historical precursor terfenadine.
• Cetirizine can still cause clinically noticeable drowsiness.
• Combination products containing pseudoephedrine can produce a sympathomimetic syndrome.
• Severe antimuscarinic delirium or QRS widening is atypical for modern second-generation agents and should prompt a search for another toxicant.
• Routine prolonged cardiac monitoring is unnecessary after every minor modern-agent exposure.
• Phenytoin is generally not preferred for toxicologic seizures.
• Ipecac and routine gastric lavage are obsolete.
• Trendelenburg and dopamine-first shock treatment are outdated.
• Observation should be agent-, symptom-, ECG-, and coingestant-specific.
• There is no specific antidote.

Key Points

• Second-generation antihistamines usually cause mild toxicity in isolated overdose.
• Common effects are drowsiness, dizziness, headache, nausea, and mild tachycardia.
• Cetirizine is relatively more sedating; fexofenadine has very little CNS effect.
• Astemizole and terfenadine are historical exceptions with major QT/torsades toxicity.
• Marked QT prolongation with a modern agent should prompt evaluation for electrolytes, interactions, and coingestants.
• IV magnesium + electrolyte correction are central if torsades occurs.
• Benzodiazepines are first-line for the unusual toxicologic seizure.
• Always check whether the product also contains pseudoephedrine or another active drug.
• Management is primarily supportive.

Toxicology – Antifungal Medications

Core Concept

Antifungal medications are a diverse group, so there is no single antifungal toxidrome. Toxicity depends strongly on the drug class, route, duration of exposure, renal/hepatic function, and interacting medications.

Important groups include:

• Polyenes – amphotericin B, nystatin
• Azoles – fluconazole, itraconazole, voriconazole, posaconazole, isavuconazole, ketoconazole, and topical imidazoles
• Antimetabolite – flucytosine
• Echinocandins – caspofungin, micafungin, anidulafungin
• Allylamines – terbinafine

Major toxicologic concerns include:

• Amphotericin B → nephrotoxicity and electrolyte disturbances
• Flucytosine → bone-marrow suppression and GI toxicity
• Systemic azoles → hepatotoxicity, drug interactions, and agent-specific QT effects
• Most topical/vaginal antifungals → predominantly local or mild GI effects after accidental exposure

There is generally no specific antidote.

1. Amphotericin B

Amphotericin B is a polyene antifungal used for serious systemic fungal infections.

It binds fungal membrane ergosterol, forming membrane pores and disrupting cellular integrity.

Unfortunately, amphotericin can also interact with mammalian cell membranes and produce significant toxicity.

Amphotericin B – Major Toxicities

The principal adverse effects are:

• Nephrotoxicity
• Potassium wasting
• Magnesium wasting
• Infusion-related reactions
• Anemia
• Less commonly severe cardiovascular effects

Toxicity is more important after parenteral exposure than accidental oral ingestion because conventional amphotericin B is poorly absorbed from the GI tract.

Amphotericin B Nephrotoxicity

Amphotericin can cause:

• Renal vasoconstriction
• Reduced GFR
• Direct tubular injury

Clinical consequences include:

• Rising creatinine
• Azotemia
• Renal potassium wasting
• Renal magnesium wasting
• Renal tubular dysfunction
• Distal renal tubular acidosis in some patients

Renal injury is especially important with prolonged conventional amphotericin B therapy.

Liposomal Amphotericin

Modern lipid-associated formulations, particularly liposomal amphotericin B, generally cause less nephrotoxicity than conventional amphotericin B deoxycholate.

However, renal injury and electrolyte disturbances can still occur.

The formulation therefore matters when assessing toxicity.

Potassium and Magnesium

A major correction to the older source is that therapeutic amphotericin toxicity characteristically produces:

• Hypokalemia
• Hypomagnesemia

These abnormalities can themselves increase dysrhythmia risk.

Hyperkalemia is not the usual chronic electrolyte pattern, although acute severe cellular injury or massive administration errors can produce different abnormalities.

Amphotericin Infusion Reactions

Infusion-related reactions can include:

• Fever
• Chills/rigors
• Nausea
• Headache
• Hypotension
• Dyspnea

Rare severe infusion reactions may involve:

• Bronchospasm
• Severe hypotension
• Cardiovascular instability

Rapid or erroneous IV administration can be particularly dangerous.

“Red Man Syndrome” – Correction

The term red man syndrome is classically associated with rapid vancomycin infusion and is not the preferred description of amphotericin toxicity.

Amphotericin can certainly produce infusion-associated flushing and other reactions, but these should be described as amphotericin infusion reactions rather than equated with classic vancomycin infusion reaction.

Cardiovascular Toxicity

Severe amphotericin toxicity has been associated with:

• Hypotension
• Bradyarrhythmia
• Ventricular dysrhythmia
• Cardiac arrest

Potential contributors include:

• Electrolyte abnormalities
• Infusion-related reactions
• Severe renal dysfunction
• Administration errors

Continuous ECG monitoring is appropriate after a substantial IV overdose or when cardiovascular abnormalities develop.

Amphotericin Drug Interactions

Risk of renal injury increases when amphotericin is combined with other nephrotoxic exposures.

Examples can include:

• Aminoglycosides
• Calcineurin inhibitors
• Other nephrotoxic medications

Diuretics or other causes of potassium/magnesium loss may further increase electrolyte complications.

2. Flucytosine

Flucytosine is an antifungal antimetabolite.

Fungal cells convert it through metabolic pathways to compounds related to 5-fluorouracil, interfering with:

• DNA synthesis
• RNA function

Human cells do not efficiently perform the initial conversion, but excessive systemic exposure can still cause substantial toxicity.

Flucytosine – Major Toxicities

The main target organs are rapidly dividing tissues.

Important effects include:

• Bone-marrow suppression
• GI toxicity
• Hepatotoxicity

Possible hematologic abnormalities include:

• Leukopenia
• Neutropenia
• Thrombocytopenia
• Anemia
• Pancytopenia in severe cases

Flucytosine and Renal Function

Flucytosine is substantially eliminated by the kidneys.

Therefore:

Renal impairment → reduced clearance → accumulation → greater marrow and GI toxicity

Renal function is one of the most important determinants of toxicity during therapy.

Flucytosine GI Toxicity

Possible manifestations include:

• Nausea
• Vomiting
• Diarrhea
• Abdominal discomfort

Severe systemic exposure can produce significant mucosal injury, but the historical description that severe GI injury is universal is too strong.

Flucytosine Concentrations

Unlike the older blanket statement that antifungal concentrations are never clinically useful, therapeutic drug monitoring can be clinically useful for flucytosine, particularly during treatment when:

• Renal function is impaired
• High exposure is suspected
• Toxicity develops

This is primarily a therapeutic-monitoring issue rather than a reason to delay acute supportive care.

3. Azole Antifungals

Azoles inhibit fungal lanosterol 14-α-demethylase, disrupting ergosterol synthesis and fungal cell membranes.

Modern systemic azoles include:

• Fluconazole
• Itraconazole
• Voriconazole
• Posaconazole
• Isavuconazole

Ketoconazole remains historically important but systemic oral use has been greatly restricted in many settings because of toxicity.

Azole Toxicity

Accidental single ingestions are often relatively mild.

Possible acute effects include:

• Nausea
• Vomiting
• Abdominal discomfort
• Headache
• Dizziness

More clinically important problems during systemic therapy include:

• Hepatotoxicity
• Drug interactions
• QT effects with several agents
• Agent-specific endocrine or neurologic effects

Azole Hepatotoxicity

Systemic azoles can cause:

• Transaminase elevation
• Hepatitis
• Rare severe hepatic injury

Risk and frequency differ among individual drugs.

Significant symptoms such as jaundice, persistent vomiting, right-upper-quadrant discomfort, or unexplained systemic illness warrant liver assessment.

Ketoconazole

Oral ketoconazole is no longer treated as a routine systemic antifungal in many modern settings because it can cause serious:

• Hepatotoxicity
• Adrenal steroid synthesis inhibition
• Drug interactions

Endocrine effects can include:

• Adrenal insufficiency
• Reduced androgen synthesis
• Gynecomastia
• Sexual/reproductive effects during prolonged exposure

Topical ketoconazole has far less systemic exposure.

Voriconazole

Voriconazole can cause distinctive adverse effects including:

• Transient visual disturbances
• Hallucinations or other neuropsychiatric effects
• Hepatotoxicity
• QT prolongation
• Photosensitivity with prolonged use

Toxicity may become more likely when concentrations rise because of nonlinear pharmacokinetics and metabolic variability.

Therapeutic drug monitoring is clinically useful in selected patients.

Itraconazole

Itraconazole can cause:

• GI symptoms
• Hepatotoxicity
• Drug interactions
• Edema
• Negative inotropic effects

It can worsen or precipitate heart failure in susceptible patients.

This cardiac adverse effect is important during therapeutic use and is not simply an overdose phenomenon.

Fluconazole

Fluconazole is generally better tolerated than many older systemic azoles.

Possible toxicity includes:

• GI symptoms
• Hepatotoxicity
• Rash
• QT prolongation

Serious skin reactions such as Stevens–Johnson syndrome are rare but recognized.

Isavuconazole – Important Exception

Most clinically important systemic azoles can prolong the QT interval.

Isavuconazole is an important exception because it tends to shorten the QT interval.

This distinction can be useful when interpreting ECG findings.

QT Prolongation

For QT-prolonging antifungals, dysrhythmia risk increases with:

• Hypokalemia
• Hypomagnesemia
• Bradycardia
• Congenital long-QT syndrome
• Other QT-prolonging drugs

Management centers on:

• Stopping the offending agent
• Correcting electrolytes
• ECG monitoring when clinically indicated

Torsades is treated with standard measures including IV magnesium and electrical therapy when unstable.

Azole Drug Interactions

Drug interactions are among the most important hazards of systemic azole therapy.

Many azoles inhibit CYP enzymes to varying degrees.

Consequently, concentrations of other medications may rise.

Clinically important interactions can involve drugs such as:

• Certain anticoagulants
• Some statins
• Calcineurin inhibitors
• Some benzodiazepines
• Antiarrhythmics
• Antiseizure medications
• Some glucose-lowering drugs

The exact interaction profile differs substantially among individual azoles.

Important Correction – CYP Statement

The older source’s broad interaction description is oversimplified.

Azoles generally act as CYP inhibitors, not simply as nonspecific agents that raise a fixed list of drug concentrations.

The affected CYP pathways and interaction magnitude vary by antifungal.

Medication-specific interaction checking is therefore important.

4. Echinocandins

Modern echinocandins include:

• Caspofungin
• Micafungin
• Anidulafungin

They inhibit fungal β-(1,3)-D-glucan synthesis, impairing the fungal cell wall.

They were not represented adequately in older antifungal toxicology references.

Echinocandin Toxicity

These agents generally have a favorable toxicity profile.

Possible adverse effects include:

• Infusion reactions
• Histamine-mediated flushing
• Rash
• GI symptoms
• Transaminase elevation

Severe acute poisoning is uncommon.

Management is mainly supportive.

5. Nystatin

Nystatin is a polyene but is poorly absorbed from intact GI mucosa and skin.

Therefore, accidental oral exposure generally causes limited systemic toxicity.

Possible effects are mainly:

• Nausea
• Vomiting
• Diarrhea

Systemic toxicity is unusual with conventional topical/oral preparations.

6. Topical and Vaginal Antifungals

Agents include various formulations of:

• Clotrimazole
• Miconazole
• Terconazole
• Tioconazole
• Nystatin

Systemic absorption from ordinary topical use is generally limited.

Adverse effects are more commonly:

• Local burning
• Irritation
• Erythema
• Contact dermatitis

Accidental small oral exposures are usually much less concerning than systemic antifungal overdoses.

Diagnosis

There is no universal antifungal poisoning syndrome.

Assessment should identify:

• Exact drug
• Formulation
• Route
• Amount
• Timing
• Acute overdose vs chronic therapeutic toxicity
• Renal function
• Hepatic function
• Interacting medications

This distinction is particularly important because many serious antifungal toxicities develop during therapeutic treatment, not after a single overdose.

Laboratory Evaluation

Testing should be drug-specific.

Amphotericin B

Consider:

• Creatinine
• BUN
• Potassium
• Magnesium
• Bicarbonate
• Calcium
• ECG when significant toxicity is suspected
• CBC during prolonged treatment

Flucytosine

Consider:

• CBC with differential
• Platelets
• Creatinine
• Liver tests
• Electrolytes

Systemic azoles

Consider:

• Liver tests
• Electrolytes
• ECG for QT-risk situations
• Renal function where relevant

Therapeutic Drug Monitoring

The older statement that antifungal levels are universally useless is incorrect.

Drug concentrations can have clinical roles for selected systemic antifungals, particularly:

• Flucytosine
• Voriconazole
• Itraconazole
• Posaconazole in selected circumstances

This is especially relevant during prolonged therapy, treatment failure, organ dysfunction, suspected toxicity, or major drug interactions.

Acute stabilization should never be delayed while waiting for a concentration.

Initial Management

General priorities are:

Airway/breathing → circulation → identify exact antifungal → ECG/electrolytes when relevant → renal/hepatic/marrow assessment → supportive care

There is no universal antidote for antifungal poisoning.

GI Decontamination

Ipecac should not be used.

Induced vomiting is obsolete in poisoning management.

Routine gastric lavage is also inappropriate.

A single dose of activated charcoal may occasionally be considered after a substantial recent ingestion of an adsorbable systemic antifungal when:

• The airway is safe
• Aspiration risk is low
• Expected benefit justifies treatment

Many accidental antifungal ingestions do not require GI decontamination.

Hypotension

Treat the cause.

Management may include:

• Appropriate isotonic crystalloid
• Treatment of severe infusion reactions
• Correction of electrolyte abnormalities
• Vasopressor support for persistent shock

Norepinephrine is generally favored for persistent vasodilatory shock.

Routine Trendelenburg positioning and a dopamine-first strategy are outdated.

Dysrhythmias

When dysrhythmias occur:

• Stop the causative drug
• Correct potassium
• Correct magnesium
• Correct significant calcium abnormalities
• Treat hypoxemia/acidemia
• Follow standard resuscitation principles

For torsades associated with QT prolongation:

• IV magnesium
• Correction of electrolytes
• Electrical treatment if unstable

Bone-Marrow Suppression

This is particularly important with flucytosine.

Monitor:

• Hemoglobin
• Leukocyte/neutrophil count
• Platelets

Clinically important marrow suppression may require:

• Discontinuation of the drug
• Supportive hematologic care
• Management of infection or bleeding complications

Recovery depends partly on exposure severity and renal clearance.

Renal Injury

Amphotericin-associated renal toxicity requires:

• Serial renal function
• Careful volume assessment
• Potassium monitoring
• Magnesium monitoring
• Review of other nephrotoxic drugs

Electrolyte abnormalities may persist even when creatinine changes are modest.

Hepatic Injury

Systemic azoles and, less commonly, other antifungals may produce clinically significant liver injury.

Evaluate significant cases with:

• AST/ALT
• Bilirubin
• Coagulation studies when severe

Severe hepatic dysfunction warrants specialist assessment.

Extracorporeal Treatment

There is no general role for dialysis simply because an antifungal overdose has occurred.

Potential usefulness depends on the specific agent and its:

• Protein binding
• Volume of distribution
• Molecular characteristics
• Renal clearance

Dialysis may still be required for conventional indications such as severe renal failure or dangerous electrolyte abnormalities.

Monitoring

Monitoring should match the causative drug.

Important parameters may include:

• Vital signs
• ECG
• Potassium
• Magnesium
• Renal function
• Liver function
• CBC
• Platelets

A patient receiving amphotericin requires a very different monitoring strategy from someone with a small accidental topical azole ingestion.

Observation and Disposition

A universal 6-hour observation period is not appropriate for every antifungal exposure.

Disposition depends on:

• Agent
• Route
• Formulation
• Amount
• Symptoms
• Organ function
• Laboratory abnormalities
• Drug interactions
• Intentional vs accidental exposure

Many small accidental topical or oral exposures can be managed conservatively, whereas significant amphotericin administration errors, marrow toxicity, hepatic injury, or cardiac abnormalities require monitored care.

Admission

Hospital management may be required for:

• Significant amphotericin overdose
• Dysrhythmia
• Important electrolyte abnormalities
• Acute kidney injury
• Persistent hypotension
• Severe vomiting/dehydration
• Significant marrow suppression
• Hepatitis
• Coagulopathy
• Severe drug interaction

ICU care is appropriate for severe cardiovascular instability or multiorgan toxicity.

Pregnancy

The old FDA A/B/C/D/X pregnancy categories are obsolete.

Antifungal selection during pregnancy depends strongly on:

• Specific agent
• Route
• Dose
• Duration
• Gestational stage
• Severity of fungal infection

The historical claim that all vaginal antifungals are automatically safe in pregnancy is too broad.

Topical azoles have extensive use in pregnancy, but treatment decisions should still be agent- and formulation-specific.

Similarly, breastfeeding recommendations cannot accurately be reduced to “avoid all azoles.”

Safeguarding

Rigid age thresholds for assuming neglect, abuse, or intentional poisoning are outdated.

Pediatric exposure should instead be assessed according to:

• Developmental capability
• Access to medication
• Exposure circumstances
• Consistency of history
• Recurrent unexplained events
• Broader safeguarding concerns

Prognosis

Most small accidental antifungal exposures have a favorable outcome.

Prognosis becomes more concerning with:

• Major amphotericin administration errors
• Severe renal injury
• Dangerous electrolyte abnormalities
• Dysrhythmias
• Severe flucytosine-associated marrow suppression
• Significant azole hepatotoxicity
• Serious drug interactions

Important Modernization of the Older Source

• Antifungals should not be divided simply into polyenes, flucytosine, and “imidazoles”; modern therapy includes triazoles, echinocandins, allylamines, and other agents.
• Amphotericin B primarily causes nephrotoxicity with potassium and magnesium wasting.
• Liposomal amphotericin generally causes less nephrotoxicity than conventional amphotericin B.
• Amphotericin infusion reactions should not simply be called “red man syndrome.”
• Flucytosine toxicity is strongly influenced by renal clearance and can cause severe marrow suppression.
• Flucytosine concentrations can be clinically useful; the blanket statement that antifungal levels have no value is incorrect.
• Selected azoles also use therapeutic drug monitoring.
• Systemic azoles can produce clinically important hepatotoxicity and CYP-mediated drug interactions.
• Several azoles can prolong QT, whereas isavuconazole characteristically shortens QT.
• Oral ketoconazole has been greatly restricted because of serious hepatic, endocrine, and interaction risks.
• Itraconazole can worsen heart failure because of negative inotropic effects.
• Modern echinocandins generally have relatively low acute toxicity.
• Nystatin has little systemic absorption with conventional oral/topical use.
• Ipecac and routine gastric lavage are obsolete.
• Trendelenburg and dopamine-first shock management are outdated.
• Observation and laboratory testing should be drug-specific, not based on a universal antifungal protocol.
• There is generally no specific antidote.

Key Points

• Antifungal toxicity varies dramatically by drug class.
• Amphotericin B → kidney injury + hypokalemia + hypomagnesemia + infusion reactions.
• Flucytosine → bone-marrow suppression + GI toxicity, especially with renal impairment.
• Systemic azoles → liver injury + major drug interactions + agent-specific cardiac effects.
• Topical agents and nystatin usually have limited systemic toxicity.
• Check ECG and electrolytes when QT-active drugs or amphotericin-related electrolyte disturbances are involved.
• Monitor CBC during significant flucytosine toxicity.
• Monitor renal function, potassium, and magnesium with amphotericin.
• Monitor liver function with clinically significant systemic azole toxicity.
• Management is predominantly supportive and agent-specific.

Toxicology – Class IB Antidysrhythmic Agents

Core Concept

Class IB antidysrhythmic drugs are fast sodium-channel blockers. Historically important examples include:

• Lidocaine
• Mexiletine
• Tocainide — now largely obsolete/discontinued in many regions

Phenytoin also has Class IB-like electrophysiologic properties but is better considered separately.

The most important modern toxicology distinction is that lidocaine toxicity is also local anesthetic systemic toxicity (LAST).

Major manifestations include:

• Early neurologic excitation
• Circumoral or tongue paresthesia
• Tinnitus
• Dizziness
• Seizures
• CNS depression/coma
• Hypotension
• Bradycardia
• Conduction disturbances
• Ventricular dysrhythmias
• Cardiovascular collapse

Severe local-anesthetic toxicity has an important modern rescue treatment:

Intravenous lipid emulsion (ILE)

Thus, the historical statement that there is “no specific antidote” requires qualification for severe lidocaine-associated LAST.

Mechanism

Class IB drugs block fast voltage-gated sodium channels.

Their effects are particularly prominent in:

• Depolarized tissue
• Ischemic myocardium
• Rapidly firing sodium channels

At therapeutic concentrations they can suppress abnormal ventricular automaticity.

With excessive exposure, sodium-channel blockade extends to normal cardiac and nervous tissue.

CNS Sodium-Channel Toxicity

Local anesthetics inhibit neuronal sodium channels.

As concentrations rise, inhibitory CNS pathways may be affected first, producing an initial excitatory phase.

Early symptoms may include:

• Circumoral numbness
• Tongue paresthesia
• Metallic or abnormal taste
• Tinnitus
• Dizziness
• Lightheadedness
• Visual disturbance
• Restlessness
• Tremor
• Confusion

This can progress to:

Agitation → seizure → CNS depression → coma → respiratory arrest

Cardiac Sodium-Channel Toxicity

At higher concentrations:

Na⁺-channel blockade → impaired conduction + reduced myocardial excitability/contractility

Possible consequences include:

• PR prolongation
• QRS widening
• AV block
• Bradycardia
• Ventricular dysrhythmias
• Hypotension
• Cardiogenic shock
• Asystole

Lidocaine

Lidocaine is used as:

• Local anesthetic
• Regional anesthetic
• Topical anesthetic
• Selected ventricular antiarrhythmic therapy

Toxicity may follow:

• Excessive injection
• Accidental intravascular administration
• Rapid systemic absorption
• Excessive topical exposure
• Impaired metabolism
• Medication error
• Rare oral exposure

Acute intravascular exposure can produce toxicity within minutes.

Local Anesthetic Systemic Toxicity – LAST

The modern syndrome of serious systemic toxicity from lidocaine or another local anesthetic is termed:

Local Anesthetic Systemic Toxicity (LAST)

LAST can produce both:

Neurologic toxicity

• Paresthesia
• Tinnitus
• Agitation
• Seizure
• Coma

Cardiovascular toxicity

• Hypotension
• Bradycardia
• Conduction block
• Ventricular dysrhythmia
• Cardiovascular collapse

The traditional progression from neurologic symptoms to cardiovascular toxicity is useful but not universal.

Some patients can present primarily with cardiovascular collapse.

Why Toxicity May Occur at Therapeutic Use

Risk depends on more than the total administered dose.

Important factors include:

• Injection site vascularity
• Accidental intravascular administration
• Rate of administration
• Patient size
• Age
• Cardiac output
• Hepatic function
• Protein binding
• Acid–base status
• Concomitant drugs

Therefore, a single rigid “toxic dose” cannot reliably predict LAST.

Lidocaine Metabolism

Lidocaine is predominantly metabolized in the liver.

Reduced clearance can occur with:

• Severe hepatic dysfunction
• Reduced hepatic blood flow
• Heart failure
• Shock

Repeated dosing or prolonged infusion can therefore cause accumulation.

Acidemia

Acidemia is particularly undesirable in severe local-anesthetic toxicity.

It may increase the active drug fraction and worsen:

• CNS toxicity
• Sodium-channel blockade
• Myocardial depression

Therefore, adequate oxygenation and ventilation are important during resuscitation.

Mexiletine

Mexiletine is an orally active Class IB sodium-channel blocker structurally and pharmacologically related to lidocaine.

Toxicity may produce:

• Nausea/vomiting
• Tremor
• Dizziness
• Ataxia
• Confusion
• Seizures
• Bradycardia
• Hypotension
• Conduction abnormalities
• Ventricular dysrhythmias

Neurologic toxicity may be prominent.

Tocainide

Tocainide is an older orally active lidocaine-like drug.

It has largely disappeared from contemporary clinical practice because of safety concerns and availability of alternatives.

Older toxicology descriptions remain historically relevant but are less important for current practice.

Neurologic Warning Signs

One of the important characteristics of Class IB toxicity is that neurologic symptoms may precede cardiovascular deterioration.

Early findings can include:

• Restlessness
• Dizziness
• Paresthesias
• Tinnitus
• Tremor
• Confusion
• Visual disturbances

These symptoms should not automatically be attributed to anxiety or hyperventilation when systemic local-anesthetic exposure is possible.

Seizures

Seizures are a major manifestation of severe lidocaine/LAST toxicity.

Seizures worsen toxicity by producing:

• Hypoxemia
• Hypercarbia
• Lactic acidosis
• Increased metabolic demand

These changes can intensify cardiovascular toxicity.

Therefore:

Rapid seizure control + oxygenation + ventilation are critical.

Seizure Treatment

Benzodiazepines are generally first-line therapy.

Persistent seizures may require escalation with appropriate anticonvulsant/anesthetic therapy.

Large doses of medications that significantly depress myocardial function should be used cautiously in a patient already developing cardiovascular collapse.

CNS Depression

As toxicity progresses, initial CNS excitation may transition to:

• Drowsiness
• Respiratory depression
• Coma
• Apnea

Airway and ventilation support may therefore become necessary rapidly.

Cardiovascular Findings

Severe toxicity may produce:

• Hypotension
• Bradycardia
• AV block
• QRS widening
• Ventricular dysrhythmias
• Cardiac arrest

Unlike Class IA agents, therapeutic Class IB drugs do not characteristically produce major QT prolongation.

Marked QT prolongation should therefore prompt consideration of:

• Coingestants
• Electrolyte abnormalities
• Another drug mechanism

ECG

Obtain an ECG in significant systemic toxicity.

Assess:

• Rate
• Rhythm
• PR interval
• QRS duration
• QT/QTc
• AV conduction
• Ventricular ectopy

However:

A normal ECG does not exclude early lidocaine toxicity.

Neurologic symptoms may occur before obvious ECG abnormalities.

Methemoglobinemia – Important Clarification

Methemoglobinemia is classically associated with certain local anesthetics, especially:

• Benzocaine
• Prilocaine

Lidocaine can cause methemoglobinemia, but it is substantially less characteristic than with benzocaine or prilocaine.

Consider it when there is:

• Cyanosis disproportionate to respiratory findings
• Low pulse-oximetry readings that do not normalize as expected with oxygen
• Chocolate-brown blood
• An exposure capable of oxidizing hemoglobin

Confirm with co-oximetry.

Laboratory Evaluation

Significant poisoning may warrant:

• Glucose
• Electrolytes
• Potassium
• Magnesium
• Calcium
• Bicarbonate
• Creatinine

Severe poisoning may additionally require:

• Blood gas
• Lactate
• Serial acid–base assessment

Co-oximetry is appropriate if methemoglobinemia is suspected.

Serum Drug Concentrations

Lidocaine or mexiletine concentrations can occasionally support a diagnosis but should not delay treatment.

Clinical toxicity correlates imperfectly with an isolated serum concentration.

Management should be based primarily on:

• Neurologic findings
• ECG
• Hemodynamics
• Respiratory status
• Exposure history

Routine serial drug levels are generally not required to determine recovery in most poisoning cases.

Initial Management

Priorities are:

Stop exposure → airway/oxygenation → control seizures → ECG/circulation → treat cardiovascular toxicity

For suspected LAST, preparation for lipid-emulsion therapy should occur early when severe neurologic or cardiovascular toxicity develops.

Stop Local Anesthetic Administration

If toxicity develops during local or regional anesthesia:

Immediately stop administration of the local anesthetic.

Preventing further systemic exposure is the first intervention.

Oxygenation and Ventilation

Avoid:

• Hypoxemia
• Hypercarbia
• Severe acidemia

These physiologic abnormalities can worsen local-anesthetic cardiotoxicity.

Early airway support may be necessary with:

• Recurrent seizures
• Coma
• Apnea
• Severe cardiovascular collapse

Intravenous Lipid Emulsion

This is the major modern addition missing from the older source.

IV lipid emulsion is an established rescue therapy for severe local anesthetic systemic toxicity, particularly when cardiovascular instability is present.

Its proposed mechanisms include:

• Sequestration/redistribution of lipophilic local anesthetic
• Improved myocardial substrate availability
• Direct beneficial effects on myocardial function

It is especially relevant to severe toxicity from highly lipophilic local anesthetics, but it is also used for severe lidocaine-associated LAST.

Exact administration should follow a current LAST resuscitation protocol.

Cardiac Arrest in LAST

Resuscitation differs somewhat from ordinary cardiac arrest because the myocardium is profoundly affected by the local anesthetic.

Priorities include:

• High-quality CPR
• Adequate oxygenation and ventilation
• Seizure control
• Correction of acidemia
• Early lipid emulsion for severe LAST
• Appropriate defibrillation when indicated

Resuscitation may need to be prolonged because severe local-anesthetic cardiotoxicity can be reversible as the drug redistributes and is metabolized.

Vasopressors

Vasopressor therapy may be required for severe hypotension.

In LAST, standard resuscitation drugs may need modification because excessive catecholamine exposure can worsen arrhythmogenicity and interfere with successful resuscitation.

Current LAST-specific resuscitation guidance should therefore be followed rather than simply applying an old dopamine-first algorithm.

Trendelenburg – Obsolete

Routine Trendelenburg positioning is not recommended as treatment for hypotension.

It does not provide reliable sustained improvement in perfusion and may worsen respiratory mechanics.

Sodium Bicarbonate

Sodium bicarbonate may be considered when significant sodium-channel-blocker physiology is present, particularly with:

• QRS widening
• Severe conduction slowing
• Acidemia

However, in severe LAST it does not replace lipid emulsion and comprehensive resuscitation.

Bradycardia and AV Block

Management centers on:

• Correcting hypoxemia/acidemia
• Treating systemic local-anesthetic toxicity
• Hemodynamic support

Temporary pacing may be considered for refractory severe bradycardia or high-grade block.

However, electrical pacing may be ineffective when myocardial sodium-channel blockade and contractile depression are profound.

Ventricular Dysrhythmias

Management includes:

• Correction of hypoxemia
• Correction of acidemia
• Treatment of electrolyte abnormalities
• Defibrillation/cardioversion when clinically indicated
• LAST-specific resuscitation

Avoid reflexively adding additional sodium-channel-blocking antiarrhythmics because they may worsen the underlying toxicity.

Antiarrhythmics in LAST

The older recommendation to treat lidocaine-induced dysrhythmia with additional Class I drugs should be approached cautiously.

Agents that further impair myocardial conduction can aggravate toxicity.

Treatment should instead focus on reversing the toxic physiology and following contemporary LAST resuscitation guidance.

Torsades de Pointes

Torsades is not the defining dysrhythmia of typical Class IB poisoning.

If polymorphic VT with prolonged QT occurs:

• Correct potassium
• Correct magnesium
• Remove QT-prolonging agents
• Give IV magnesium
• Electrically treat unstable dysrhythmia

Consider another drug or electrolyte abnormality if major QT prolongation dominates the presentation.

GI Decontamination

GI decontamination applies primarily to oral drugs such as mexiletine.

Do not induce vomiting.

Routine gastric lavage is obsolete.

Activated charcoal may occasionally be considered after a substantial recent oral ingestion when:

• The substance is adsorbable
• The airway is safe
• Aspiration risk is acceptable

It has no role in an already absorbed parenteral lidocaine exposure.

Extracorporeal Removal

Routine:

• Hemodialysis
• Hemoperfusion
• Hemofiltration
• Forced diuresis
• Urinary manipulation

do not represent standard toxin-removal strategies for lidocaine poisoning.

In catastrophic refractory cardiovascular collapse, the relevant extracorporeal intervention is circulatory support, not conventional dialysis.

VA-ECMO

Severe LAST is potentially reversible.

When cardiovascular collapse remains refractory despite appropriate resuscitation and lipid therapy, VA-ECMO may be considered at an experienced center.

This modernizes the older concept of emergency cardiopulmonary bypass.

ECMO supports circulation while the local anesthetic redistributes and is metabolized.

Differential Diagnosis

The combination of neurologic toxicity, seizures, and cardiovascular abnormalities can also occur with:

• Other Class I antiarrhythmics
• Tricyclic antidepressants
• Diphenhydramine
• Cocaine
• Chloroquine
• β-blockers
• Calcium-channel blockers
• Other local anesthetics

Nontoxicologic causes include:

• Hypoglycemia
• Electrolyte abnormalities
• Intracranial pathology
• CNS infection
• Primary seizure disorders

Monitoring

Significant toxicity requires monitoring of:

• Mental status
• Respiratory function
• Oxygenation
• Heart rate
• Blood pressure
• Continuous ECG
• Acid–base status when severe

Monitor severe cases for recurrent:

• Seizures
• Dysrhythmias
• Hypotension
• Respiratory failure

Observation

The historical universal 6-hour observation rule should not be applied mechanically.

Observation depends on:

• Drug
• Route
• Formulation
• Amount
• Timing
• Neurologic findings
• ECG
• Hemodynamics
• Coingestants

Parenteral intravascular lidocaine toxicity generally develops rapidly, whereas oral Class IB agents may have a different time course.

Admission

Monitored admission is appropriate for:

• Seizures
• Significant altered mental status
• Respiratory depression
• Hypotension
• Bradycardia
• AV block
• QRS widening
• Ventricular dysrhythmia
• Any clinically significant LAST

Severe cardiovascular or neurologic toxicity warrants intensive care.

Pregnancy

The historical FDA pregnancy letter categories are obsolete.

Management of severe poisoning during pregnancy prioritizes maternal:

• Airway
• Oxygenation
• Ventilation
• Seizure control
• Hemodynamics

Maternal stabilization is also fundamental to fetal oxygenation and perfusion.

Safeguarding

Rigid historical age cutoffs for neglect, abuse, or intentional poisoning are outdated.

Pediatric exposures should instead be assessed according to:

• Developmental capability
• Access
• Circumstances
• Consistency of history
• Recurrent unexplained exposure
• Broader safeguarding concerns

Prognosis

Early neurologic toxicity is often completely reversible when recognized and treated promptly.

Poor outcomes are associated with:

• Prolonged seizures
• Severe hypoxemia
• Acidemia
• Refractory ventricular dysrhythmia
• Prolonged cardiovascular collapse
• Hypoxic brain injury

Even severe LAST can be reversible with prolonged, aggressive resuscitation.

Important Modernization of the Older Source

• Lidocaine toxicity should be recognized within the modern syndrome of local anesthetic systemic toxicity (LAST).
• Neurologic manifestations often precede cardiovascular toxicity, but this sequence is not universal.
• A normal early ECG does not exclude LAST.
• Severe toxicity can cause seizures, conduction block, ventricular dysrhythmias, and cardiovascular collapse.
• IV lipid emulsion is a major modern rescue treatment for severe LAST and is absent from the historical source.
• Avoid hypoxemia, hypercarbia, and acidemia because they worsen toxicity.
• Benzodiazepines are first-line for seizures.
• Sodium bicarbonate may have a role in significant conduction toxicity but does not replace LAST-specific resuscitation.
• Additional sodium-channel-blocking antiarrhythmics can worsen toxicity.
• Major QT prolongation/torsades is less characteristic of Class IB poisoning than Class IA poisoning.
• Methemoglobinemia is much more strongly associated with benzocaine/prilocaine than with lidocaine.
• Routine gastric lavage is obsolete.
• Dialysis is not a useful primary toxin-removal strategy.
• Trendelenburg and dopamine-first hypotension management are outdated.
• “Cardiac bypass” is better conceptualized today as VA-ECMO for selected refractory, potentially reversible cardiovascular collapse.
• Fixed observation periods should be replaced by agent-, route-, formulation-, and symptom-based monitoring.

Key Points

• Class IB drugs block fast Na⁺ channels.
• Major examples are lidocaine and mexiletine; tocainide is largely historical.
• Lidocaine systemic toxicity = LAST.
• Early LAST may cause circumoral paresthesia, tinnitus, dizziness, agitation, and tremor.
• Severe toxicity progresses to seizures, coma, respiratory depression, bradycardia, conduction block, ventricular dysrhythmias, and cardiovascular collapse.
• Control seizures rapidly and prevent hypoxemia, hypercarbia, and acidemia.
• IV lipid emulsion is an important rescue therapy for severe LAST.
• ECG abnormalities may occur late; a normal ECG does not exclude early neurologic toxicity.
• Avoid adding drugs that worsen sodium-channel blockade.
• Severe refractory cardiovascular collapse may require VA-ECMO.

Toxicology – Class IA Antidysrhythmics: Quinidine and Disopyramide

Core Concept

Quinidine and disopyramide are Class IA sodium-channel-blocking antidysrhythmics. Their use has declined substantially because safer or more effective alternatives are available for many arrhythmias.

In overdose, the major dangers are:

• Fast sodium-channel blockade → QRS widening
• Potassium-channel blockade → QT prolongation
• Ventricular dysrhythmias
• Torsades de pointes
• Hypotension
• Bradycardia/AV block
• Seizures and CNS depression

Quinidine additionally produces cinchonism, while disopyramide has particularly prominent antimuscarinic and negative-inotropic effects.

There is no specific antidote.

⸻

Mechanism – Class IA

Class IA agents block fast myocardial sodium channels.

This slows phase-0 depolarization:

Na⁺-channel blockade → slowed conduction → QRS widening

They also inhibit repolarizing potassium currents:

K⁺-channel blockade → prolonged repolarization → QT prolongation

Thus, Class IA poisoning can produce the dangerous combination of:

Wide QRS + prolonged QT

⸻

Electrophysiologic Effects

Toxicity may cause:

• Slowed atrial conduction
• Slowed AV/intraventricular conduction
• QRS widening
• QT prolongation
• Bradyarrhythmias
• Ventricular tachycardia
• Ventricular fibrillation
• Torsades de pointes

These abnormalities may deteriorate rapidly.

⸻

Quinidine

Quinidine is closely related to quinine.

In addition to sodium- and potassium-channel effects, it can produce:

• α-adrenergic blockade
• Hypotension
• GI symptoms
• Cinchonism

Its role in contemporary rhythm management is much narrower than suggested by older toxicology texts.

⸻

Disopyramide

Disopyramide has important:

• Sodium-channel-blocking activity
• Potassium-channel effects
• Antimuscarinic activity
• Negative inotropic effects

Consequently, toxicity can include:

• Dry mouth
• Mydriasis
• Urinary retention
• Ileus
• Tachycardia
• Delirium
• Hypotension
• Heart failure/cardiogenic shock
• Conduction abnormalities

⸻

Toxic Dose

There is no reliable universal toxic-dose threshold.

Toxicity can occur relatively close to therapeutic concentrations, especially when clearance is impaired or interacting medications are present.

Risk depends on:

• Agent
• Formulation
• Amount
• Renal/hepatic function
• Baseline conduction disease
• Electrolytes
• Drug interactions
• Coingestants

Clinical status and serial ECG findings are more important than the reported dose alone.

⸻

Sustained-Release Preparations

Modified-release formulations can produce:

• Delayed absorption
• Delayed peak toxicity
• Prolonged cardiotoxicity

Antimuscarinic slowing of GI motility may further delay absorption.

An initially normal ECG therefore does not necessarily exclude later deterioration after a substantial sustained-release exposure.

⸻

Cinchonism

Quinidine can produce a syndrome known as cinchonism.

Possible manifestations include:

• Tinnitus
• Hearing disturbance
• Headache
• Dizziness
• Nausea/vomiting
• Visual disturbance
• Confusion

More severe toxicity may cause marked visual impairment and serious neurologic or cardiovascular abnormalities.

Sudden tinnitus or visual symptoms in a patient taking quinidine should raise concern for toxicity.

⸻

Neurologic Toxicity

Severe poisoning may produce:

• Confusion
• Somnolence
• Seizures
• Coma
• Respiratory depression

Seizures can worsen cardiotoxicity through:

Seizure → lactic acidosis → acidemia → increased sodium-channel toxicity

Rapid seizure control is therefore important.

⸻

Cardiovascular Toxicity

Major manifestations include:

• QRS widening
• QT prolongation
• AV conduction delay
• Bradycardia
• Hypotension
• Ventricular tachycardia
• Ventricular fibrillation
• Torsades de pointes

Severe poisoning can progress to shock or cardiac arrest.

⸻

Why Hypotension Occurs

Hypotension may result from several simultaneous mechanisms:

• Myocardial depression
• Sodium-channel blockade
• Dysrhythmia
• α-adrenergic blockade, particularly with quinidine
• Negative inotropy, particularly with disopyramide
• Acidemia

Therefore, treatment must address both circulation and the underlying electrical toxicity.

⸻

ECG – Essential Test

Obtain an ECG promptly in suspected significant poisoning.

Assess:

• Heart rate
• Rhythm
• PR interval
• QRS duration
• QT/QTc
• AV conduction
• Ventricular ectopy

Symptomatic or significant exposures require continuous cardiac monitoring.

⸻

QRS Widening

QRS widening reflects myocardial sodium-channel blockade.

Increasing QRS duration suggests increasing conduction toxicity and greater risk of ventricular dysrhythmia.

The ECG and clinical trajectory are more useful than a serum drug concentration for acute management.

⸻

QT Prolongation

Class IA agents delay ventricular repolarization.

This produces:

QT prolongation → early afterdepolarizations → polymorphic VT → torsades de pointes

Risk increases with:

• Hypokalemia
• Hypomagnesemia
• Bradycardia
• Other QT-prolonging drugs
• Congenital long-QT susceptibility

⸻

Quinidine and Diarrhea

Quinidine commonly causes gastrointestinal adverse effects, including diarrhea.

Significant diarrhea can cause:

K⁺ loss → hypokalemia → increased QT-related dysrhythmia risk

Thus electrolyte abnormalities can convert otherwise modest drug exposure into a more dangerous electrophysiologic situation.

⸻

Syncope

Syncope in a patient receiving a QT-prolonging Class IA agent should prompt urgent consideration of a transient ventricular dysrhythmia, including torsades.

However, it should not be assumed automatically; other causes of syncope still require evaluation.

⸻

Laboratory Evaluation

Important tests in significant poisoning include:

• Potassium
• Magnesium
• Calcium
• Bicarbonate
• Creatinine
• Glucose

Severe cases may additionally require:

• Blood gas
• Lactate
• Renal and hepatic assessment

Renal function is particularly relevant to drugs with substantial renal elimination.

⸻

Serum Drug Concentrations

Quinidine or disopyramide concentrations may occasionally provide supportive information, but they generally do not determine emergency treatment.

Management should be guided by:

• ECG
• Blood pressure
• Mental status
• Seizures
• Perfusion
• Electrolytes

Do not delay treatment while awaiting a drug level.

⸻

Initial Management

Priorities are:

Airway/breathing → ECG → circulation → correct electrolytes → control seizures → treat conduction abnormalities/torsades

Continuous monitoring is important because deterioration can be abrupt.

⸻

Sodium Bicarbonate

Sodium bicarbonate is an important treatment when significant sodium-channel blockade is present.

Clinical indications include:

• Significant QRS widening
• Ventricular conduction toxicity
• Ventricular dysrhythmia associated with sodium-channel blockade
• Cardiovascular instability in the appropriate toxicologic context

Its effects come from:

• Sodium loading
• Alkalinization
• Reduced drug interaction with myocardial sodium channels

Treatment is guided by ECG response, hemodynamics, and acid–base/electrolyte status rather than blindly targeting a fixed dose.

⸻

Avoid Acidemia

Acidemia can intensify sodium-channel-blocker cardiotoxicity.

Therefore:

• Treat seizures promptly
• Maintain adequate ventilation
• Correct hypoperfusion
• Address severe metabolic abnormalities

Excessive alkalinization should also be avoided because it can cause clinically important electrolyte disturbances.

⸻

Torsades de Pointes

Management priorities include:

• Stop QT-prolonging drugs
• Correct hypokalemia
• Correct hypomagnesemia
• Correct other important electrolyte abnormalities
• IV magnesium
• Electrical defibrillation/cardioversion if unstable

For recurrent pause-dependent torsades associated with bradycardia, increasing the heart rate with overdrive pacing may be appropriate.

Isoproterenol has selected roles in acquired bradycardia-dependent torsades but is not appropriate for every prolonged-QT situation.

⸻

Drugs to Avoid in Torsades

Avoid adding medications that further prolong repolarization.

In particular, additional Class IA drugs are inappropriate.

Other QT-prolonging antiarrhythmics may also worsen acquired long-QT-associated torsades.

The older recommendation lists should therefore be interpreted according to the mechanism rather than memorized as isolated drug names.

⸻

Bradycardia and AV Block

Severe sodium-channel toxicity can produce:

• Bradycardia
• AV block
• Intraventricular conduction delay

Standard bradycardia measures may be attempted as clinically appropriate, but severe toxicologic conduction blockade may respond poorly.

Temporary pacing may be considered when clinically significant bradycardia or high-grade block persists despite correction of the toxicologic abnormalities.

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Important Pacing Limitation

Electrical capture does not guarantee effective mechanical cardiac output in profound sodium-channel-blocker poisoning.

Therefore, pacing should not distract from:

• Sodium bicarbonate therapy
• Correction of acidemia
• Electrolyte correction
• Hemodynamic support

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Hypotension

Management includes:

• Appropriate isotonic crystalloid when volume responsive
• Treatment of the dysrhythmia
• Sodium bicarbonate when sodium-channel cardiotoxicity is present
• Correction of acidemia

Persistent shock may require vasopressor therapy.

Norepinephrine is generally more appropriate than the historical routine dopamine-first approach for persistent vasodilatory hypotension.

Disopyramide-associated myocardial depression may complicate the hemodynamic picture.

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Seizures

Benzodiazepines are first-line therapy.

Persistent seizures may require:

• Additional benzodiazepines
• Phenobarbital
• Appropriate anesthetic therapy for refractory status epilepticus

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Phenytoin – Modern Correction

The historical recommendation to use phenytoin/fosphenytoin for refractory ventricular dysrhythmias is not part of routine modern management of Class IA poisoning.

Phenytoin itself interacts with sodium channels and can produce cardiovascular toxicity during IV administration.

It is also generally not preferred for toxicant-induced seizures.

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Lidocaine

Lidocaine may have a specialist-guided role in refractory ventricular dysrhythmias caused by sodium-channel blockers after appropriate sodium bicarbonate therapy.

It should not replace bicarbonate as the fundamental treatment of significant Class IA sodium-channel toxicity.

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Other Class I Antiarrhythmics

Avoid using additional Class IA agents because they can worsen:

• Sodium-channel blockade
• QRS widening
• QT prolongation
• Hypotension

Class IC sodium-channel blockers can likewise worsen conduction toxicity.

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GI Decontamination

Do not induce vomiting.

Seizures, dysrhythmias, or altered consciousness may develop abruptly.

Routine gastric lavage is obsolete.

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Activated Charcoal

A single dose of activated charcoal may be considered after a substantial recent oral exposure when:

• The drug is adsorbable
• The airway is safe
• Aspiration risk is acceptable

It should never delay cardiovascular stabilization.

Routine repeat-dose charcoal solely because GI motility is slowed is not established.

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Whole-Bowel Irrigation

Whole-bowel irrigation is not routine.

It may occasionally be considered after a substantial sustained-release ingestion when:

• The patient is sufficiently stable
• Bowel function is adequate
• The airway is protected
• There is no obstruction, ileus, or perforation

Because antimuscarinic effects can impair GI motility, WBI may sometimes be impractical or inappropriate.

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Extracorporeal Removal

Quinidine is generally poorly suited to routine extracorporeal removal because of its pharmacokinetic characteristics.

Historical suggestions that disopyramide should routinely undergo hemodialysis should also be interpreted cautiously.

Extracorporeal treatment is not the primary strategy for Class IA cardiotoxicity and should not delay:

• Sodium bicarbonate
• Dysrhythmia treatment
• Vasopressor support
• Electrolyte correction

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Refractory Cardiovascular Collapse

Massive sodium-channel-blocker poisoning may rarely cause shock or dysrhythmia refractory to conventional therapy.

Selected severe cases may require advanced rescue measures such as VA-ECMO at an experienced center.

The older reference to “cardiac bypass” is better understood in modern practice as temporary extracorporeal circulatory support for potentially reversible poisoning.

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Chronic Quinidine Adverse Effects

Therapeutic quinidine can rarely cause immune-mediated complications such as:

• Thrombocytopenia
• Hemolytic anemia
• Drug-induced lupus-like reactions

These are distinct from the acute electrophysiologic toxicity of overdose.

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Drug Interactions

Risk increases when Class IA agents are combined with:

• Other QT-prolonging medications
• Other sodium-channel blockers
• Drugs altering their metabolism
• Drugs causing hypokalemia or hypomagnesemia

A medication review is therefore particularly important when toxicity occurs at apparently therapeutic dosing.

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Differential Diagnosis

The combination of:

CNS toxicity + seizure + QRS widening

can also occur with:

• Tricyclic antidepressants
• Diphenhydramine and some other antihistamines
• Cocaine
• Flecainide/propafenone
• Chloroquine
• Other sodium-channel-blocking xenobiotics

QT prolongation and torsades have their own broad drug and electrolyte differential.

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Monitoring

Significant poisoning requires:

• Continuous ECG
• Blood pressure monitoring
• Respiratory monitoring
• Serial electrolytes
• Renal function
• Serial assessment of QRS and QT

Severe cases also require monitoring for:

• Seizures
• Acidemia
• Shock
• End-organ hypoperfusion

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Observation

A fixed 6-hour rule should not be applied universally.

Observation depends on:

• Agent
• Immediate- vs sustained-release formulation
• Amount
• ECG findings
• Symptoms
• Electrolytes
• Coingestants
• Clinical trajectory

Sustained-release products can produce delayed and prolonged toxicity.

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Admission

Monitored inpatient care is appropriate for:

• QRS widening
• Significant QT prolongation
• Dysrhythmia
• Syncope concerning for dysrhythmia
• Hypotension
• Bradycardia/high-grade AV block
• Seizures
• Altered mental status
• Significant cinchonism
• Large sustained-release exposure

Severe cardiovascular instability warrants ICU care.

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Pregnancy

Historical FDA pregnancy letter categories are obsolete.

Clinically significant poisoning during pregnancy should be managed according to maternal physiology, with priority given to:

• Airway
• Oxygenation
• Cardiac rhythm
• Blood pressure
• Seizure control

Maternal dysrhythmia and shock also threaten fetal perfusion.

⸻

Safeguarding

Rigid historical age cutoffs for neglect, abuse, or intentional poisoning are outdated.

Pediatric exposure should instead be assessed according to developmental capability, access, circumstances, history consistency, recurrent events, and other safeguarding concerns.

⸻

Prognosis

Mild poisoning may resolve with monitoring and supportive treatment.

Severe poisoning carries risk of:

• Ventricular dysrhythmia
• Torsades
• Cardiogenic/vasodilatory shock
• Seizures
• Respiratory failure
• Cardiac arrest
• Hypoxic brain injury

Outcome is strongly influenced by early recognition and correction of electrophysiologic toxicity.

⸻

Important Modernization of the Older Source

• Quinidine and disopyramide are now used much less commonly than historically.
• Class IA = sodium-channel blockade + potassium-channel blockade.
• Therefore, poisoning can produce both QRS widening and QT prolongation.
• Quinidine can cause cinchonism.
• Disopyramide has prominent antimuscarinic and negative-inotropic effects.
• Sodium bicarbonate is central when clinically important sodium-channel blockade is present.
• Correct K⁺ and Mg²⁺ aggressively when QT-related dysrhythmia risk is present.
• IV magnesium is central to torsades management.
• Overdrive pacing may help selected recurrent bradycardia-dependent torsades.
• Phenytoin/fosphenytoin is not routine modern therapy for Class IA cardiotoxicity or toxicologic seizures.
• Additional Class IA/IC sodium-channel blockers should generally be avoided.
• Norepinephrine is generally favored over routine dopamine-first treatment for persistent vasodilatory shock.
• Routine gastric lavage is obsolete.
• Repeated charcoal is not automatically indicated because of slowed GI motility.
• WBI has only a selective role in substantial sustained-release exposure.
• Drug concentrations should not delay ECG-directed treatment.
• Severe refractory cardiovascular collapse may warrant advanced extracorporeal circulatory support.

Key Points

• Class IA toxicity = Na⁺ blockade + K⁺ blockade.
• Na⁺ blockade → QRS widening and conduction slowing.
• K⁺ blockade → QT prolongation and torsades risk.
• Quinidine additionally causes cinchonism.
• Disopyramide has strong antimuscarinic and myocardial-depressant effects.
• Obtain an early ECG and continuously monitor significant poisoning.
• Sodium bicarbonate is first-line for important sodium-channel cardiotoxicity.
• Magnesium and electrolyte correction are central for torsades.
• Benzodiazepines are first-line for seizures.
• Avoid additional Class IA/IC agents that worsen conduction.
• Sustained-release formulations may cause delayed toxicity.
• There is no specific antidote.