Dextromethorphan

Antitussive NMDA receptor antagonist; sigma-1 receptor agonist; central cough suppressant

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 1-2 mg/kg q8-12h Duration: As needed for cough, typically 3-7 days
Notes: Use lowest effective dose. Not recommended for chronic cough without underlying diagnosis.
Cat PO 1-2 mg/kg q8-12h Duration: As needed for cough, typically 3-5 days
Notes: Use with caution; cats may be more sensitive to adverse effects. Not first-line for feline asthma.
Horse PO 0.5-1 mg/kg q12h Duration: As needed, but not recommended for routine use
Notes: Limited evidence; other antitussives preferred.

Clinical Indications & Species Uses

General Indications
  • Symptomatic relief of non-productive cough in small animals
Dog (Canine)
  • Non-productive cough associated with tracheitis, bronchitis, or kennel cough
  • Symptomatic relief of cough due to chronic bronchitis
  • Cough associated with collapsing trachea (as adjunctive therapy)
Cat (Feline)
  • Non-productive cough associated with bronchitis or asthma (though not first-line)
  • Symptomatic relief of cough due to upper respiratory infections
Horse (Equine)
  • May be used off-label for non-productive cough in some cases

Pharmacology & Mechanism of Action

Drug Class: Antitussive | Pharmacological Group: NMDA receptor antagonist; sigma-1 receptor agonist; central cough suppressant

Mechanism of Action: Dextromethorphan is a centrally acting antitussive that suppresses the cough reflex by elevating the threshold for coughing in the medullary cough center. Its primary mechanism is non-competitive antagonism of the N-methyl-D-aspartate (NMDA) receptor, which modulates glutamatergic transmission in the brainstem. It also acts as a sigma-1 receptor agonist, which may contribute to its antitussive and neuroprotective effects. At therapeutic doses, it does not depress respiration and has minimal sedative effects compared to opioid antitussives.

Pharmacodynamics: Dextromethorphan effectively reduces the frequency and intensity of non-productive cough. It has no analgesic or anesthetic properties at antitussive doses. In veterinary species, its efficacy as a cough suppressant is variable and often considered less effective than other antitussives like butorphanol or hydrocodone. It may also have weak bronchodilator and anti-inflammatory effects, but these are not clinically significant. At higher doses, it can produce dissociative effects due to NMDA antagonism, which are not desired in veterinary patients.

⚡ Pharmacokinetics Summary

Absorption: Rapidly absorbed from the gastrointestinal tract after oral administration. Peak plasma concentrations occur within 1-2 hours in most species. Food may delay absorption but does not significantly affect overall bioavailability.
Distribution: Widely distributed throughout the body, including the central nervous system. It crosses the blood-brain barrier. Volume of distribution is approximately 5-10 L/kg in humans; similar distribution is expected in animals. Protein binding is low to moderate.
Metabolism: Extensively metabolized in the liver via cytochrome P450 enzymes, primarily CYP2D6, to the active metabolite dextrorphan, which also has antitussive and NMDA antagonist activity. Other metabolic pathways include O-demethylation and N-demethylation. Significant first-pass metabolism reduces oral bioavailability.
Excretion: Excreted primarily in the urine as metabolites (dextrorphan and conjugates). A small fraction is excreted unchanged. Biliary excretion may also occur. Elimination half-life varies by species and is influenced by metabolic rate.
Half-Life: Dogs: approximately 2-4 hours; Cats: approximately 2-3 hours; Horses: approximately 1-2 hours; Humans: 1-4 hours (extensive metabolizers) to 8-24 hours (poor metabolizers).
Bioavailability: Oral bioavailability is approximately 11% in humans due to extensive first-pass metabolism; in dogs, it is approximately 20-30%. Bioavailability may be higher in cats.
Protein Binding: Approximately 45-60% bound to plasma proteins in humans; similar binding expected in animals.

Available Formulations & Strengths

Oral Tablet 15 mg, 30 mg (PO)
Oral Capsule 30 mg (PO)
Oral Liquid (Syrup) 7.5 mg/5 mL, 15 mg/5 mL, 30 mg/5 mL (PO)
Oral Liquid (Drops) 7.5 mg/mL (PO)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to dextromethorphan
  • Concurrent use with monoamine oxidase inhibitors (MAOIs) or within 14 days of MAOI therapy
  • Use in patients with productive cough (cough with significant mucus production) as suppression may impair clearance
  • Use in patients with respiratory depression or compromised respiratory function
  • Use in neonates or very young animals due to immature metabolic pathways
Warnings & Clinical Precautions:
  • Use with caution in patients with hepatic impairment, as metabolism may be reduced.
  • Use with caution in patients with renal impairment, as excretion may be delayed.
  • May cause sedation; caution when used with other CNS depressants.
  • In cats, monitor for signs of serotonin syndrome if used with other serotonergic drugs.
  • Not recommended for use in food animals due to lack of withdrawal data and potential for residues.
  • Do not use in animals with a history of seizures, as high doses may lower seizure threshold.
  • Avoid use in animals with glaucoma or urinary retention due to potential anticholinergic effects (though weak).

Adverse Effects & Reactions

Common:

  • Sedation
  • Drowsiness
  • Gastrointestinal upset (vomiting, diarrhea)
  • Ataxia (at high doses)

Serious / Severe:

  • Respiratory depression (rare at therapeutic doses, more likely with overdose)
  • Serotonin syndrome (when combined with serotonergic drugs)
  • Excitation or dysphoria (especially in cats)
  • Seizures (at high doses)

Rare:

  • Hypersensitivity reactions (rash, urticaria)
  • Hepatotoxicity (with chronic high doses)
  • Urinary retention

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Monoamine oxidase inhibitors (MAOIs) e.g., selegiline Risk of serotonin syndrome, hyperthermia, and death. Avoid concurrent use. High
Selective serotonin reuptake inhibitors (SSRIs) e.g., fluoxetine Increased risk of serotonin syndrome due to additive serotonergic effects. Moderate
Tramadol Additive serotonergic effects and potential for seizures. Moderate
Amiodarone, quinidine Inhibit CYP2D6, leading to increased dextromethorphan levels and toxicity. Moderate
CNS depressants (barbiturates, benzodiazepines, opioids) Additive sedation and respiratory depression. Moderate

Overdose & Toxicity Management

Signs of Toxicity:

  • Excitation
  • Ataxia
  • Dysphoria
  • Hallucinations (in animals, may manifest as agitation or vocalization)
  • Tachycardia
  • Hypertension
  • Seizures
  • Respiratory depression
  • Coma

Emergency Treatment Protocol: Treatment is primarily supportive. Induce vomiting if recent ingestion and animal is conscious. Administer activated charcoal to reduce absorption. Provide intravenous fluids, monitor vital signs, and manage seizures with benzodiazepines (e.g., diazepam). In severe cases, consider naloxone (though limited efficacy) and supportive care. No specific antidote exists.

Food Animal Withdrawal Times

Not approved for use in food animals. No withdrawal times established. Do not use in animals intended for food production.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature (20-25°C, excursions permitted to 15-30°C).

Handling & Special Conditions: Protect from moisture. Keep container tightly closed. Do not freeze oral liquids.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use as an OTC antitussive.

Extra-Label (Off-Label) Use: In the US, dextromethorphan is not FDA-approved for veterinary use. Extra-label use in animals is permitted under AMDUCA provided a valid veterinarian-client-patient relationship exists and appropriate withdrawal times are observed for food animals (though not recommended).

Clinical Pearls & Practice Notes

💡 Clinical Insights: Dextromethorphan is a centrally acting antitussive that may be used in small animals for symptomatic relief of non-productive cough. However, its efficacy in veterinary medicine is questionable, and other antitussives such as butorphanol or hydrocodone are often preferred for more severe cough. It is important to diagnose the underlying cause of cough before using antitussive therapy, as suppression of a productive cough can be harmful. Dextromethorphan should be used with caution in cats, as they may exhibit paradoxical excitation. It is not recommended for use in food animals due to lack of residue data. Always use the lowest effective dose for the shortest duration necessary.

References & Literature

  • 📚 Plumb's Veterinary Drug Handbook
  • 📚 Papich Veterinary Pharmacology and Therapeutics
  • 📚 Compendium of Veterinary Products (CVP)