Dimenhydrinate

Antiemetic, Antihistamine (H1 receptor antagonist) Ethanolamine derivative

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 4-8 mg/kg q8h Duration: As needed for motion sickness; for vestibular disease, 3-5 days or as directed
Notes: Administer 30-60 minutes before travel. For vestibular disease, may be used for several days.
Cat PO 4-8 mg/kg q8h Duration: As needed for motion sickness; for vestibular disease, 3-5 days or as directed
Notes: Administer 30-60 minutes before travel. Use with caution in cats due to potential for excitement.
Horse PO 0.5-1 mg/kg q8-12h Duration: As needed for transport
Notes: Off-label use; not commonly used in equine practice.
Rabbit PO 2-4 mg/kg q8h Duration: As needed
Notes: Off-label; use with caution due to limited safety data.

Clinical Indications & Species Uses

General Indications
  • Prevention of motion sickness
  • Symptomatic treatment of nausea and vomiting of vestibular origin
Dog (Canine)
  • Prevention and treatment of motion sickness
  • Vestibular syndrome
  • Nausea and vomiting associated with chemotherapy (adjunctive)
  • Nausea associated with inner ear infections
Cat (Feline)
  • Prevention and treatment of motion sickness
  • Vestibular syndrome
  • Nausea associated with inner ear infections
Horse (Equine)
  • Motion sickness during transport
  • Vestibular disease (off-label)
Rabbit & Small Mammals
  • Motion sickness (off-label)
  • Vestibular disease (off-label)
Exotic & Other Species
  • Motion sickness in small mammals (e.g., ferrets, guinea pigs) - off-label

Pharmacology & Mechanism of Action

Drug Class: Antiemetic, Antihistamine (H1 receptor antagonist) | Pharmacological Group: Ethanolamine derivative

Mechanism of Action: Dimenhydrinate is a combination of diphenhydramine and 8-chlorotheophylline. Its antiemetic effect is primarily due to the diphenhydramine component, which antagonizes histamine H1 receptors in the vestibular apparatus and the chemoreceptor trigger zone (CTZ) of the brain. This reduces stimulation of the vomiting center and decreases vestibular excitation. The 8-chlorotheophylline component provides a mild stimulant effect that may counteract the sedative properties of diphenhydramine, though the overall effect is still sedative in many animals.

Pharmacodynamics: Dimenhydrinate suppresses nausea and vomiting by inhibiting histamine-mediated signals in the vestibular system and CTZ. It also has anticholinergic, antiemetic, and mild local anesthetic effects. In dogs and cats, it reduces motion sickness and vestibular-associated nausea. Its sedative effects are variable and may be more pronounced in some individuals. The drug does not have significant anti-inflammatory or immunosuppressive activity.

⚡ Pharmacokinetics Summary

Absorption: Dimenhydrinate is well absorbed from the gastrointestinal tract after oral administration. Onset of action is typically within 30-60 minutes. Peak plasma concentrations occur approximately 1-2 hours after oral dosing.
Distribution: The drug is widely distributed throughout the body, including the central nervous system. It crosses the blood-brain barrier and placenta. Volume of distribution is not well characterized in veterinary species but is likely moderate to large.
Metabolism: Dimenhydrinate is rapidly metabolized in the liver, primarily to diphenhydramine and 8-chlorotheophylline. Diphenhydramine undergoes further hepatic metabolism via N-demethylation and oxidation.
Excretion: Metabolites are excreted primarily in the urine. A small portion is excreted unchanged. Biliary excretion may also occur. The elimination half-life of diphenhydramine in dogs is approximately 4-8 hours, but the half-life of dimenhydrinate itself is not well documented.
Half-Life: Dogs: 4-8 hours (for diphenhydramine component); Cats: approximately 4-6 hours; Horses: not well documented.
Bioavailability: Oral bioavailability is high, estimated at 70-90% in dogs, though first-pass metabolism may reduce systemic availability.
Protein Binding: Diphenhydramine is approximately 80-85% protein bound in plasma.

Available Formulations & Strengths

Oral Tablet 25 mg, 50 mg (PO)
Oral Liquid 12.5 mg/5 mL, 15 mg/5 mL (PO)
Injectable Solution 50 mg/mL (IM, IV)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to dimenhydrinate or other antihistamines
  • Patients with glaucoma (due to anticholinergic effects)
  • Patients with prostatic hyperplasia or urinary retention
  • Patients with severe cardiovascular disease
  • Neonates and debilitated animals (use with caution)
Warnings & Clinical Precautions:
  • Use with caution in animals with hepatic or renal impairment
  • May cause sedation; avoid concurrent use with other CNS depressants
  • In cats, may cause paradoxical excitement
  • Use with caution in animals with seizure disorders (may lower seizure threshold)
  • Not recommended for use in food animals due to lack of withdrawal data
  • Injectable formulation should be administered slowly IV to avoid hypotension

Adverse Effects & Reactions

Common:

  • Sedation
  • Drowsiness
  • Dry mouth
  • Constipation
  • Urinary retention

Serious / Severe:

  • Hypotension
  • Tachycardia
  • Seizures (especially in overdose)
  • Paradoxical excitement (especially in cats)

Rare:

  • Blood dyscrasias
  • Allergic reactions
  • Hepatotoxicity

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
CNS depressants (barbiturates, opioids, anesthetics) Additive sedation and respiratory depression High
Anticholinergic drugs (atropine, glycopyrrolate) Additive anticholinergic effects (dry mouth, urinary retention, tachycardia) Moderate
MAO inhibitors Prolonged anticholinergic effects Moderate
Ototoxic drugs (aminoglycosides) May mask signs of ototoxicity Mild

Overdose & Toxicity Management

Signs of Toxicity:

  • Severe sedation
  • Hypotension
  • Tachycardia
  • Seizures
  • Respiratory depression
  • Coma

Emergency Treatment Protocol: Treatment is symptomatic and supportive. Induce emesis if recent ingestion and animal is conscious. Administer activated charcoal to reduce absorption. Provide IV fluids for hypotension. Use diazepam or barbiturates to control seizures. Monitor respiratory and cardiac function. Physostigmine may be used to reverse severe anticholinergic effects, but use with caution.

Food Animal Withdrawal Times

Not approved for use in food animals; no withdrawal times established. Do not use in animals intended for food.

Storage, Handling & Regulatory Information

Storage Temperature: Store at room temperature (20-25°C)

Handling & Special Conditions: Protect from moisture. Keep container tightly closed.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Approved for human use; not FDA-approved for veterinary use, but commonly used off-label in veterinary medicine.

Extra-Label (Off-Label) Use: In the US, extra-label use in food animals is prohibited if no withdrawal times are established. In companion animals, extra-label use is permitted under AMDUCA with veterinary oversight.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Dimenhydrinate is a useful antiemetic for motion sickness and vestibular disease in dogs and cats. It is generally well tolerated, but sedation is a common side effect. It is less effective for vomiting due to chemotherapy or metabolic causes. For motion sickness, administer 30-60 minutes before travel. In cats, monitor for paradoxical excitement. Not recommended for food animals due to lack of withdrawal data. Always use the lowest effective dose and monitor for adverse effects.

References & Literature

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