Toxicology – Butorphanol

Core concept

Butorphanol is a synthetic opioid analgesic with mixed opioid-receptor activity.

The major overdose syndrome is the usual opioid toxidrome:

CNS depression + respiratory depression ± miosis

Severe poisoning may progress to:

Hypoventilation → hypoxia → coma → respiratory arrest

The most important treatment priorities are:

Airway/ventilation + naloxone when clinically significant opioid respiratory depression is present

Current product labeling specifically identifies respiratory depression as the major life-threatening manifestation of overdose.

Forms and Uses

Butorphanol remains available in human formulations including:

• Nasal spray
• IV/IM injection

Current nasal spray contains 10 mg/mL, with each metered spray delivering approximately 1 mg.

Current injectable preparations include concentrations such as:

• 1 mg/mL
• 2 mg/mL

For analgesia, current labeling lists a usual adult dose of:

• 1 mg IV
• 2 mg IM

with dose individualization according to clinical response.

Butorphanol is a Schedule IV controlled substance in the United States.

Pathophysiology

The older description of butorphanol as simply a μ-antagonist/κ-agonist is incomplete.

Current FDA labeling describes butorphanol as:

• Partial agonist at the μ-opioid receptor
• Full agonist at the κ-opioid receptor

Therefore:

μ/κ opioid-receptor activity → analgesia + sedation + respiratory depression

κ-receptor activity also contributes to:

• Dysphoria
• Psychotomimetic effects
• Unpleasant perceptual symptoms

Butorphanol can produce substantial respiratory depression through direct suppression of brainstem respiratory centers.

Mixed Agonist-Antagonist Effect

Although butorphanol has partial μ-agonist activity, it can functionally oppose the effects of a full μ-opioid agonist.

Therefore, in a patient physically dependent on drugs such as:

• Morphine
• Heroin
• Oxycodone
• Fentanyl
• Methadone

butorphanol may:

Displace/reduce full μ-agonist activity → precipitate acute opioid withdrawal

Current labeling specifically warns that butorphanol may reduce the analgesic effect of full opioid agonists and precipitate withdrawal symptoms.

Toxic Dose

There is no well-defined single toxic dose.

Severity depends on:

• Amount
• Route
• Opioid tolerance
• Age
• Coingestants
• Renal function
• Hepatic function

Therapeutic doses can cause:

• Dizziness
• Sedation
• Dysphoria
• Nausea

Large doses may cause profound opioid toxicity.

Importantly, current labeling warns that accidental exposure to even one dose of nasal butorphanol can cause a fatal overdose in a child.

Thus, tolerance in a chronic user should never be interpreted as protection against serious toxicity.

Pharmacokinetics

Butorphanol is metabolized primarily to hydroxybutorphanol.

Most elimination occurs through the urine.

The usual elimination half-life is approximately several hours, but clearance is significantly prolonged in:

• Renal impairment
• Hepatic impairment
• Older adults

In severe renal impairment, the half-life may approximately double to around 10.5 hours.

In hepatic impairment, the half-life may approximately triple to around 16.8 hours.

Therefore:

Renal/hepatic dysfunction → prolonged toxicity and increased risk of recurrent sedation or respiratory depression

Risk Factors for Severe Toxicity

Greater concern is warranted in:

• Children
• Older adults
• Opioid-naïve patients
• Renal impairment
• Hepatic impairment
• Underlying pulmonary disease
• Sleep-disordered breathing
• Concurrent CNS depressants

Particularly dangerous coexposures include:

• Benzodiazepines
• Alcohol
• Other opioids
• Sedative-hypnotics
• Muscle relaxants
• Antipsychotics
• General anesthetics

The combination of butorphanol with benzodiazepines or other CNS depressants can produce:

Profound sedation + respiratory depression + coma + death.

Clinical Features

Classic Opioid Toxidrome

Think:

Depressed consciousness + hypoventilation + small pupils

However, the full triad is not always present.

Neurologic

Possible manifestations include:

• Drowsiness
• Dizziness
• Sedation
• Dysphoria
• Confusion
• Somnolence
• Stupor
• Coma

Severe overdose may produce:

• Skeletal muscle flaccidity
• Loss of airway reflexes

Current labeling also notes that toxic leukoencephalopathy has been reported after opioid overdose, sometimes presenting after apparent initial recovery.

Pupils

Miosis is typical.

However:

Mydriasis may occur in severe hypoxia

so normal or dilated pupils do not exclude opioid poisoning.

Respiratory

The most important manifestation is:

Respiratory depression

Possible findings include:

• Bradypnea
• Shallow breathing
• Reduced tidal volume
• Hypoventilation
• Hypercapnia
• Hypoxemia
• Upper-airway obstruction
• Snoring respirations
• Apnea

Respiratory arrest is the principal mechanism by which opioid overdose progresses to cardiac arrest.

Cardiovascular

Possible effects include:

• Bradycardia
• Hypotension
• Peripheral vasodilation

Rarely:

• Hypertension
• Dysrhythmia secondary to severe hypoxia or other factors

Severe circulatory depression may require vasopressor support.

Pulmonary Edema

Noncardiogenic pulmonary edema may occur after severe opioid overdose.

Possible findings:

• Dyspnea
• Hypoxemia
• Crackles
• Frothy secretions
• Bilateral infiltrates

Treatment is supportive, including positive-pressure ventilation when needed.

Gastrointestinal

Possible manifestations include:

• Nausea
• Vomiting
• Reduced gastrointestinal motility
• Constipation

Nausea and vomiting can occur even at therapeutic doses.

Metabolic

Hypoglycemia has been reported with opioids, including current butorphanol labeling.

Therefore:

Check glucose in any patient with altered mental status.

Diagnosis

Diagnosis is primarily clinical:

Known/suspected butorphanol exposure + opioid toxidrome

Do not delay treatment while waiting for toxicology testing.

Essential Assessment

Evaluate:

• Airway
• Respiratory rate
• Depth of ventilation
• Oxygen saturation
• Mental status
• Pupils
• Heart rate
• Blood pressure
• Bedside glucose

Ventilation Assessment

Pulse oximetry alone may be misleading, particularly if supplemental oxygen has been given.

A patient can have:

• Normal SpO₂
• Severe hypoventilation
• Rising CO₂

Therefore, consider:

• Continuous capnography
• Blood gas

when significant respiratory depression is present.

Additional Tests

Depending on severity:

• ECG
• Electrolytes
• BUN/creatinine
• Liver tests
• Blood gas
• Chest radiograph if aspiration or pulmonary edema is suspected

In intentional overdose consider:

• Acetaminophen concentration
• Salicylate concentration
• Evaluation for coingestants

Urine Drug Screening

A routine “opiate” urine immunoassay primarily detects morphine-like compounds.

Many structurally different synthetic and semisynthetic opioids are not reliably detected by standard screening assays. ACMT emphasizes that management of acute opioid poisoning should be based on the clinical presentation rather than a screening test result.

Older literature specifically notes that butorphanol may not be detected by common opioid immunoassays.

Therefore:

A negative routine urine opioid screen does not exclude butorphanol poisoning.

Differential Diagnosis

Other causes of:

CNS depression + respiratory depression ± miosis

include:

• Fentanyl
• Heroin
• Morphine
• Methadone
• Oxycodone
• Buprenorphine
• Clonidine
• Other sedative-hypnotics

Also consider:

• Benzodiazepine poisoning
• Alcohol intoxication
• Hypoglycemia
• Hypercapnia
• Intracranial hemorrhage
• Stroke
• CNS infection

Treatment

1. Airway and Breathing Come First

The most important principle is:

Ventilate the patient.

AHA guidance for life-threatening opioid poisoning emphasizes:

• Opening the airway
• Rescue breathing or bag-mask ventilation
• Supplemental oxygen as appropriate

before or while opioid antagonist therapy is being given.

If ventilation remains inadequate:

• Bag-mask ventilate
• Consider supraglottic airway
• Intubate if necessary

Do not wait for naloxone to work if the patient is severely hypoventilating.

Naloxone

Role

Naloxone is the specific antidotal treatment for clinically significant butorphanol-induced respiratory or circulatory depression.

Current naloxone labeling specifically includes butorphanol among opioids whose respiratory depression may be reversed by naloxone.

Treatment Goal

The goal is:

Adequate spontaneous ventilation and airway protection

not necessarily complete awakening.

A somnolent patient who is:

• Breathing adequately
• Maintaining the airway
• Hemodynamically stable

may not require aggressive naloxone dosing. AHA notes that patients with opioid exposure who are breathing normally with intact airway reflexes may be carefully observed.

Adult Naloxone Dose

FDA-labeled treatment for known or suspected opioid overdose is:

0.4–2 mg IV

repeated every:

2–3 minutes

if adequate respiratory improvement does not occur.

If there is no response after a cumulative 10 mg, another diagnosis or substantial non-opioid contribution should be considered.

Opioid-Dependent Patients

In a patient with known or suspected opioid dependence, rapid complete reversal can precipitate severe withdrawal.

A cautious approach is to use small IV doses, for example:

0.04–0.4 mg IV, titrated every few minutes to adequate ventilation.

The principle is:

Titrate to breathing, not wakefulness.

Butorphanol May Require More Naloxone

An important butorphanol-specific point:

Larger naloxone doses may sometimes be required because butorphanol is a mixed opioid agonist/antagonist with strong receptor affinity.

Current butorphanol labeling explicitly warns of this possibility.

Recurrent Toxicity

Naloxone may wear off before butorphanol.

Therefore:

Response to naloxone does not mean the patient is safe for immediate discharge.

Patients must be monitored for:

• Recurrent sedation
• Recurrent respiratory depression
• Need for repeat naloxone

Current labeling specifically states that patients should be monitored until spontaneous respiration is reliably re-established because opioid reversal may be shorter than butorphanol’s duration of action.

Naloxone Infusion

If repeated naloxone boluses are required to maintain adequate ventilation, a continuous naloxone infusion may be appropriate.

The infusion is titrated to:

• Respiratory rate
• Ventilation
• Mental status sufficient for airway protection

rather than to complete alertness.

Precipitated Withdrawal

Naloxone may precipitate:

• Agitation
• Vomiting
• Diaphoresis
• Tachycardia
• Hypertension
• Severe pain
• Acute withdrawal

Butorphanol itself may also precipitate withdrawal in patients dependent on full μ-opioid agonists.

This is why both:

• Naloxone
• Butorphanol exposure

require particular care in opioid-dependent patients.

Gastrointestinal Decontamination

Do Not Induce Vomiting

Do not induce emesis.

Opioid poisoning can rapidly produce:

• Sedation
• Loss of airway reflexes
• Aspiration

The historical recommendation for emesis should not be used.

Activated Charcoal

Routine activated charcoal is not necessary for most butorphanol exposures.

It might be considered after a substantial, very recent oral ingestion if:

• The airway is fully protected
• Aspiration risk is low
• A clinically meaningful oral dose was actually swallowed

However, airway stabilization and ventilation take priority.

Gastric Lavage

Routine gastric lavage is not recommended.

Hypotension

Treat clinically significant hypotension with:

• IV crystalloid when appropriate
• Vasopressors if shock persists

Current labeling specifically supports vasopressors when required for circulatory shock.

Pulmonary Edema

Management includes:

• Oxygen
• Positive-pressure ventilation when necessary
• Appropriate ventilatory support

Treatment should focus on oxygenation and ventilation rather than routine diuresis unless another indication exists.

Cardiac Arrest

For opioid-associated cardiac arrest:

High-quality CPR and ventilation take priority.

Naloxone may be given if opioid poisoning is suspected, but it should not delay standard resuscitation.

Enhanced Elimination

There is no established role for:

• Hemodialysis
• Hemoperfusion
• Urinary alkalinization

in routine butorphanol overdose.

Management is primarily:

• Ventilatory support
• Naloxone
• Supportive care

Monitoring

Symptomatic patients require:

• Continuous pulse oximetry
• Respiratory-rate monitoring
• Frequent mental-status assessment
• Cardiac monitoring in significant poisoning

Consider:

• Continuous capnography

for significant respiratory depression.

Also monitor:

• Glucose
• Blood pressure
• Renal function when prolonged toxicity is possible

Observation and Disposition

The historical fixed 4-hour observation period should not be applied automatically to every patient.

Observation should be individualized according to:

• Dose
• Route
• Clinical severity
• Naloxone requirement
• Coingestants
• Renal function
• Hepatic function

Patients who require naloxone should be observed until:

• Respiratory function is reliably normal
• Mental status is stable
• Vital signs are stable
• Risk of recurrent opioid toxicity is sufficiently low

AHA specifically recommends continued healthcare observation after naloxone when recurrent toxicity remains possible.

Admission

Hospital admission is appropriate for:

• Persistent respiratory depression
• Repeated naloxone requirements
• Naloxone infusion
• Coma
• Aspiration
• Pulmonary edema
• Hemodynamic instability
• Significant coingestants
• Prolonged toxicity from renal/hepatic dysfunction

Severe toxicity requiring ventilatory support warrants ICU care.

Pregnancy

The historical FDA pregnancy categories such as Category C are obsolete.

Current FDA labeling instead provides narrative pregnancy-risk information.

Important points include:

• Butorphanol crosses the placenta
• Prolonged maternal opioid use can cause neonatal opioid withdrawal syndrome
• Maternal overdose should still be treated promptly, with maternal airway and ventilation as the priority

Current labeling warns that prolonged use during pregnancy may cause potentially serious neonatal withdrawal.

Breastfeeding

Butorphanol can pass into breast milk.

Current labeling advises monitoring exposed infants for:

• Excessive sleepiness
• Respiratory difficulty
• Limpness

because these may indicate opioid toxicity.

Prognosis

With prompt recognition and adequate ventilation:

Complete recovery is expected in most isolated butorphanol overdoses.

Poor outcomes usually result from:

• Delayed recognition
• Prolonged hypoxia
• Aspiration
• Severe coingestion
• Respiratory arrest

Potential post-hypoxic neurologic injury depends more on the duration of hypoxia than on a direct neurotoxic effect of butorphanol.

Important Pitfalls

1. Treating the patient’s level of consciousness instead of ventilation

Naloxone should be titrated primarily to:

Adequate ventilation and airway protection

rather than complete wakefulness.

2. Waiting for naloxone instead of ventilating

A severely hypoventilating patient needs immediate:

• Airway opening
• Bag-mask ventilation

Naloxone is an adjunct to, not a replacement for, respiratory support.

3. Using a negative urine opioid screen to exclude poisoning

Routine opioid immunoassays may not detect butorphanol.

Clinical findings are more important than the screen.

4. Forgetting precipitated withdrawal

Butorphanol may provoke withdrawal in a patient dependent on a full μ-opioid agonist.

Naloxone can do the same.

5. Assuming respiratory depression has a complete ceiling

Butorphanol may show some ceiling characteristics at higher therapeutic doses, but life-threatening and fatal respiratory depression remains possible, particularly with overdose or CNS depressant coingestion.

6. Missing recurrent toxicity after naloxone

Naloxone may wear off before butorphanol.

Continue respiratory observation after an initial response.

7. Missing renal or hepatic impairment

Butorphanol elimination may be substantially prolonged in:

• Severe renal disease
• Hepatic impairment

so monitoring may need to be extended.

8. Combining butorphanol with benzodiazepines or alcohol

The combination markedly increases risk of:

• Profound sedation
• Respiratory depression
• Coma
• Death

High-Yield Toxicology Pearls

Butorphanol overdose = opioid toxidrome

Think:

Somnolence/coma + respiratory depression ± miosis

Key points:

• Butorphanol is a synthetic Schedule IV opioid
• Current receptor description:
• Partial μ agonist
• Full κ agonist
• κ activity may cause dysphoria/psychotomimetic effects
• Main life-threatening toxicity: respiratory depression
• Severe overdose may cause:
• Coma
• Bradycardia
• Hypotension
• Pulmonary edema
• Respiratory arrest
• A single accidental nasal dose can be dangerous to a small child
• Coingestion with benzodiazepines, alcohol, or other CNS depressants markedly increases toxicity
• Butorphanol may precipitate opioid withdrawal in patients dependent on full μ agonists
• Main treatment: airway + ventilation
• Antidote: naloxone
• FDA naloxone overdose dosing: 0.4–2 mg IV, repeat every 2–3 min as needed
• In opioid-dependent patients, use smaller titrated naloxone doses
• Goal of naloxone: restore adequate breathing, not necessarily complete alertness
• Butorphanol may require larger cumulative naloxone doses
• Monitor for recurrent respiratory depression
• A negative routine urine opioid screen does not exclude butorphanol
• Routine activated charcoal and gastric lavage are not required
• Renal and hepatic impairment can substantially prolong elimination
• Most patients recover completely if hypoxia is prevented