Toxicology – Antiprotozoal Medications

Core Concept

This older grouping focuses on three very different drugs:

• Chloroquine
• Hydroxychloroquine
• Metronidazole

They should not be treated as a single toxicologic class.

The most important distinction is:

• Chloroquine/hydroxychloroquine overdose → potentially rapid, life-threatening cardiotoxicity
• Metronidazole overdose → usually GI and neurologic toxicity, generally much less acutely cardiotoxic

Chloroquine and hydroxychloroquine poisoning can deteriorate extremely quickly, with hypotension, conduction abnormalities, ventricular dysrhythmias, seizures, and cardiac arrest.

1. Chloroquine

Chloroquine is an antimalarial and antiprotozoal medication that also has important cardiac electrophysiologic effects.

In overdose it behaves partly as a potent membrane-stabilizing/sodium-channel–blocking toxin.

It can also interfere with other cardiac ion channels.

The result can be:

Rapid conduction slowing + myocardial depression + vasodilation + electrolyte disturbance → cardiovascular collapse

Chloroquine Has a Narrow Safety Margin

Chloroquine is unusual because relatively modest multiples of therapeutic exposure can cause severe poisoning.

Historical fixed gram-based toxicity thresholds should not be used as the sole basis for risk assessment, particularly in children.

Any credible significant chloroquine overdose should be treated as a medical emergency.

Rapid Onset

Serious chloroquine toxicity generally develops early.

Possible progression:

GI symptoms → dizziness/agitation → hypotension → QRS/QT abnormalities → ventricular dysrhythmia → seizure/coma → cardiac arrest

A patient can deteriorate rapidly despite appearing relatively well initially.

Cardiovascular Toxicity

Major manifestations include:

• Hypotension
• Tachycardia
• QRS widening
• QT prolongation
• AV/intraventricular conduction abnormalities
• Ventricular ectopy
• Ventricular tachyarrhythmias
• Severe myocardial depression
• Cardiac arrest

Continuous ECG and hemodynamic monitoring are essential in significant poisoning.

Hypokalemia – Key Feature

A characteristic feature of severe chloroquine poisoning is hypokalemia.

This is largely caused by an intracellular shift of potassium rather than necessarily reflecting true whole-body potassium depletion.

Severe poisoning may therefore produce:

• Low serum potassium
• Muscle weakness
• Increased electrical instability

Potassium Replacement – Important Caution

Hypokalemia should be monitored carefully, but overly aggressive potassium replacement can be dangerous.

As chloroquine toxicity resolves:

Intracellularly shifted K⁺ can move back extracellularly → rebound hyperkalemia

Therefore potassium correction should be carefully titrated with frequent reassessment.

Neurologic Toxicity

Severe chloroquine poisoning may cause:

• Headache
• Agitation
• Confusion
• Seizures
• CNS depression
• Coma

Seizures can worsen:

• Hypoxia
• Acidosis
• Cardiovascular instability

Rapid control is therefore important.

Respiratory Toxicity

Respiratory compromise may result from:

• CNS depression
• Seizures
• Severe shock
• Cardiac arrest

Early airway control may be necessary in severe poisoning, especially when cardiovascular deterioration is progressing.

2. Hydroxychloroquine

Hydroxychloroquine is closely related to chloroquine.

Although widely used for autoimmune diseases rather than protozoal infection in contemporary practice, its overdose syndrome resembles chloroquine toxicity.

Important acute effects include:

• Hypotension
• Hypokalemia
• QRS widening
• QT prolongation
• Ventricular dysrhythmias
• Seizures
• CNS depression
• Cardiovascular collapse

A significant hydroxychloroquine overdose should therefore be approached as a potentially severe cardiotoxic poisoning.

Chloroquine vs Hydroxychloroquine

Both drugs can produce rapid cardiovascular toxicity.

Chloroquine has the strongest historical evidence base for classic severe poisoning management, while hydroxychloroquine overdose is generally managed using similar toxicologic principles because of their pharmacologic similarities.

Chronic Retinal Toxicity

Both chloroquine and hydroxychloroquine can cause retinal injury during chronic therapy.

Features may include:

• Reduced visual acuity
• Paracentral or central visual-field defects
• Difficulty reading
• Altered color vision
• Progressive retinopathy

This is primarily a cumulative therapeutic toxicity, not the expected manifestation of acute overdose.

Hydroxychloroquine Retinopathy

Risk relates to factors such as:

• Daily exposure relative to body size
• Duration of therapy
• Cumulative exposure
• Renal dysfunction
• Concurrent retinal risk factors

Retinal injury may continue to progress after the drug has been stopped because of prolonged tissue retention.

Ophthalmologic Monitoring

Long-term hydroxychloroquine treatment requires appropriate retinal screening using modern ophthalmologic methods.

These may include:

• Automated visual-field testing
• Spectral-domain OCT
• Other specialized retinal tests when indicated

Routine acute overdose management does not depend on an immediate retinal examination unless visual symptoms or another indication exists.

Other Chronic Chloroquine/Hydroxychloroquine Toxicity

Long-term therapy can rarely cause:

• Skeletal myopathy
• Neuropathy
• Cardiomyopathy
• Conduction disease
• Hearing abnormalities
• Skin pigmentation changes

These should be distinguished from the rapid cardiovascular syndrome of acute overdose.

3. Metronidazole

Metronidazole is a nitroimidazole antimicrobial used against:

• Anaerobic bacteria
• Trichomonas
• Giardia
• Entamoeba and other susceptible organisms

Acute overdose is generally much less dangerous than chloroquine or hydroxychloroquine poisoning.

Acute Metronidazole Toxicity

Common effects include:

• Nausea
• Vomiting
• Abdominal discomfort
• Metallic taste
• Headache
• Dizziness
• Drowsiness
• Ataxia

Most isolated acute exposures are managed supportively.

Metronidazole Neurotoxicity

Prolonged or excessive exposure can produce clinically important neurologic toxicity.

Manifestations include:

• Ataxia
• Dysarthria
• Confusion
• Encephalopathy
• Peripheral neuropathy
• Seizures

Metronidazole-Induced Encephalopathy

Characteristic findings may include:

• Gait instability
• Dysarthria
• Altered mental status
• Cerebellar dysfunction

MRI can show characteristic abnormalities, including lesions involving the dentate nuclei and other CNS structures.

Symptoms frequently improve after discontinuation, although recovery can take time.

Peripheral Neuropathy

Prolonged metronidazole exposure can cause:

• Numbness
• Tingling
• Burning sensations
• Distal sensory impairment

Risk is more closely related to cumulative exposure than to a single modest ingestion.

Metronidazole and Alcohol

Older literature commonly states that metronidazole reliably causes a classic disulfiram-like reaction with ethanol.

Modern evidence is less convincing.

Although avoidance of alcohol during treatment is still commonly recommended in product guidance, the mechanism and consistency of a true disulfiram-like interaction are uncertain.

Therefore, flushing or vomiting after alcohol plus metronidazole should not automatically be assumed to prove an acetaldehyde-mediated disulfiram reaction.

Metronidazole Drug Interactions

Important interactions include:

Warfarin

Metronidazole can increase anticoagulant effect, potentially raising INR and bleeding risk.

Lithium

Lithium concentrations may increase in some patients, particularly when renal function changes.

Enzyme-inducing antiseizure medications

Some can increase metronidazole metabolism and reduce exposure.

A severe or atypical presentation should therefore include a complete medication review.

Dark Urine

Metronidazole can occasionally cause dark or reddish-brown urine due to metabolites.

This finding alone does not necessarily indicate hematuria or renal failure.

Hematologic Effects

Rare effects include:

• Leukopenia
• Neutropenia
• Thrombocytopenia

These are more relevant to therapeutic or prolonged exposure than uncomplicated acute overdose.

Hepatic Considerations

Metronidazole is metabolized hepatically.

Significant hepatic dysfunction can reduce clearance and increase systemic exposure.

Rare clinically important hepatotoxicity can also occur.

Diagnosis

The first priority is determining the exact drug.

For chloroquine/hydroxychloroquine

Assess immediately:

• Amount and timing
• Symptoms
• Blood pressure
• ECG
• Potassium
• Glucose
• Other electrolytes
• Acid–base status when severely ill
• Coingestants

For metronidazole

Assess:

• Acute vs cumulative exposure
• Neurologic findings
• Hepatic function when relevant
• Interacting medications
• Coingestants

ECG in Chloroquine/Hydroxychloroquine Poisoning

Obtain an ECG early and monitor serially.

Assess:

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

Progressive conduction abnormalities may precede cardiovascular collapse.

Laboratory Evaluation

For significant chloroquine/hydroxychloroquine exposure, consider:

• Potassium
• Magnesium
• Calcium
• Sodium
• Glucose
• Renal function
• Blood gas/lactate in severe poisoning

Frequent potassium reassessment may be required because concentrations can change rapidly during recovery.

Drug Concentrations

Routine serum chloroquine, hydroxychloroquine, or metronidazole concentrations are generally not sufficiently available or actionable to guide acute emergency management.

Treatment should be driven by:

• Clinical severity
• ECG
• Hemodynamics
• Electrolytes

Management of Chloroquine/Hydroxychloroquine Poisoning

Priorities are:

Airway → continuous ECG → circulation → potassium/electrolytes → seizures → aggressive treatment of shock and dysrhythmia

Early toxicology/poison-center consultation is strongly appropriate for any significant exposure.

Airway Management

Severe chloroquine poisoning may deteriorate abruptly.

Early controlled airway management should be considered when there is:

• Severe CNS depression
• Recurrent seizures
• Progressive shock
• Respiratory failure

Peri-intubation cardiovascular collapse is a major concern in severely poisoned patients, so resuscitation must proceed concurrently.

Epinephrine

Epinephrine has an important role in severe chloroquine poisoning with hypotension and myocardial depression.

It can improve:

• Blood pressure
• Cardiac output
• Contractility

Vasopressor therapy should be titrated to clinical response under intensive monitoring rather than according to rigid historical dose targets.

High-Dose Diazepam – Important Historical Therapy

Older literature describes high-dose diazepam as a central treatment for severe chloroquine poisoning.

Modern interpretation is more nuanced.

Benzodiazepines are clearly appropriate for:

• Seizures
• Agitation
• Sedation when required

Historical observational evidence suggested benefit from high-dose diazepam combined with epinephrine in severe chloroquine poisoning, but the evidence is limited and does not establish diazepam as a universal antidote.

Very high-dose diazepam regimens should therefore be undertaken only with specialist toxicology guidance and intensive airway/hemodynamic monitoring.

Sodium Bicarbonate

Because chloroquine can produce sodium-channel blockade and QRS widening, sodium bicarbonate may be considered in selected severe conduction toxicity.

However, this requires caution because alkalemia can further reduce serum potassium.

Treatment should therefore be guided by:

• ECG
• Potassium
• Acid–base status
• Hemodynamics
• Toxicology expertise

It is not a routine treatment for every chloroquine ingestion.

Ventricular Dysrhythmias

Priorities include:

• Optimize oxygenation
• Correct severe acidemia
• Carefully manage potassium
• Correct magnesium when indicated
• Treat shock
• Address sodium-channel blockade when appropriate

Antiarrhythmic selection requires caution because some agents can worsen conduction or QT abnormalities.

Seizures

Benzodiazepines are first-line.

Persistent seizures may require:

• Additional benzodiazepines
• Phenobarbital
• Appropriate anesthetic therapy

Correct:

• Hypoglycemia
• Hypoxia
• Electrolyte abnormalities
• Acid–base disturbance

GI Decontamination

Do not induce vomiting.

Ipecac is obsolete.

Routine gastric lavage is not standard management.

Because chloroquine/hydroxychloroquine can become life-threatening rapidly, airway and cardiovascular stabilization take priority over GI decontamination.

Activated charcoal may be considered after a clinically important recent ingestion when:

• The airway is protected
• Aspiration risk is acceptable
• Administration will not delay resuscitation

Metronidazole Management

Most acute metronidazole overdoses require:

• Supportive care
• Fluids if clinically dehydrated
• Antiemetic therapy when appropriate
• Neurologic observation
• Seizure treatment if required

For chronic neurotoxicity:

Stop metronidazole and provide supportive neurologic care.

There is no specific antidote.

Enhanced Elimination

Routine enhanced elimination is not recommended for chloroquine, hydroxychloroquine, or metronidazole poisoning.

Chloroquine and hydroxychloroquine have extensive tissue distribution, making conventional dialysis ineffective for meaningful toxin removal.

Extracorporeal life support such as VA-ECMO may be considered as circulatory rescue in selected refractory cardiogenic/cardiotoxic collapse, but this supports the patient while toxicity resolves—it does not meaningfully eliminate the drug.

Observation

A universal fixed observation period is inappropriate.

Chloroquine/hydroxychloroquine

Significant exposures require monitored medical evaluation because severe toxicity can develop rapidly.

Disposition depends on:

• Exposure magnitude
• Symptoms
• ECG
• Potassium
• Blood pressure
• Mental status
• Clinical trajectory

Metronidazole

Minor asymptomatic exposures generally require much less intensive monitoring.

Prolonged exposure requires assessment for delayed neurologic toxicity.

Admission

Hospitalization is appropriate for chloroquine/hydroxychloroquine poisoning with:

• Significant or uncertain overdose
• Hypotension
• Hypokalemia
• QRS/QT abnormality
• Ventricular ectopy/dysrhythmia
• Seizure
• Altered mental status
• Respiratory compromise

ICU care is appropriate for clinically significant cardiotoxicity.

For metronidazole, admission may be appropriate for:

• Persistent encephalopathy
• Severe ataxia
• Seizures
• Significant dehydration
• Serious coingestion or interaction

Pregnancy and Breastfeeding

Historical FDA pregnancy letter categories are obsolete.

The statement that metronidazole should simply be avoided throughout pregnancy is also outdated.

Modern decisions consider:

• Drug
• Infection
• Gestational age
• Maternal benefit
• Fetal risk
• Available alternatives

Chloroquine and hydroxychloroquine have established therapeutic uses during pregnancy in appropriate clinical circumstances.

Breastfeeding recommendations should similarly be drug- and indication-specific.

Safeguarding

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

Evaluate pediatric exposures according to:

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

Prognosis

Chloroquine/Hydroxychloroquine

Outcome depends heavily on the severity of early cardiovascular toxicity.

Poor prognostic features include:

• Severe hypotension
• Marked conduction abnormalities
• Ventricular dysrhythmias
• Severe hypokalemia
• Seizures
• Cardiac arrest

Patients who survive the acute cardiotoxic phase may recover substantially, although hypoxic injury can cause persistent deficits.

Metronidazole

Most acute exposures have a favorable prognosis.

Chronic neurotoxicity often improves after discontinuation, although peripheral neuropathy may recover slowly or occasionally persist.

Important Modernization of the Older Source

• Chloroquine and hydroxychloroquine overdose should be separated toxicologically from metronidazole.
• Chloroquine/hydroxychloroquine poisoning is a rapidly developing cardiotoxic emergency.
• Sodium-channel blockade contributes to QRS widening and ventricular dysrhythmias.
• Hypokalemia is a characteristic marker of severe chloroquine toxicity, largely reflecting intracellular redistribution.
• Potassium replacement requires caution because rebound hyperkalemia can occur during recovery.
• Significant hydroxychloroquine overdose can produce a syndrome similar to chloroquine poisoning.
• Chronic retinal toxicity should not be confused with the acute overdose syndrome.
• Epinephrine remains an important vasopressor/inotropic therapy in severe chloroquine poisoning.
• High-dose diazepam is historically associated with severe chloroquine treatment, but evidence is limited; it should not be described as a universally proven antidote.
• Sodium bicarbonate may have a role for selected significant sodium-channel blockade but requires careful potassium and acid–base monitoring.
• Dialysis is ineffective for meaningful chloroquine/hydroxychloroquine elimination.
• VA-ECMO can be considered as rescue support for refractory cardiovascular collapse; it is not an elimination technique.
• Metronidazole overdose is usually much less severe.
• Prolonged metronidazole exposure can cause encephalopathy, cerebellar dysfunction, and peripheral neuropathy.
• The classic metronidazole–ethanol “disulfiram reaction” is less firmly established than older teaching suggested.
• Metronidazole can interact importantly with warfarin and lithium.
• Ipecac is obsolete and routine gastric lavage is not standard.
• Fixed 2-, 4-, or 8-hour discharge rules are too rigid.
• Historical FDA pregnancy categories and blanket metronidazole pregnancy avoidance are outdated.

Key Points

• Chloroquine/hydroxychloroquine → rapid, potentially fatal cardiotoxicity.
• QRS widening + hypotension + hypokalemia are particularly concerning in chloroquine poisoning.
• Severe cases can progress quickly to ventricular dysrhythmia, seizure, shock, and cardiac arrest.
• Epinephrine has an important role in severe chloroquine-associated cardiovascular collapse.
• Benzodiazepines treat seizures; historical high-dose diazepam therapy requires specialist guidance.
• Potassium must be corrected cautiously because rebound hyperkalemia can occur.
• Metronidazole → GI symptoms and, with substantial/cumulative exposure, neurotoxicity.
• Metronidazole-induced encephalopathy often features cerebellar dysfunction and may have characteristic MRI abnormalities.
• There is no universal specific antidote for these agents.
• Significant chloroquine or hydroxychloroquine exposure warrants rapid monitored evaluation and early toxicology/poison-center involvement.