Amantadine Hydrochloride — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	3-5 mg/kg	q24h	Duration: Chronic; may take 2-4 weeks for full effect Notes: Use with caution in patients with renal impairment; adjust dose or interval. Often used in combination with NSAIDs or gabapentin.
Cat	PO	3-5 mg/kg	q24h	Duration: Chronic; may take 2-4 weeks for full effect Notes: Cats may require lower doses due to longer half-life. Monitor for adverse effects. Use with caution in renal impairment.
Horse	PO	3-5 mg/kg	q12h	Duration: Chronic; may take 2-4 weeks for full effect Notes: Due to shorter half-life, twice daily dosing may be needed. Use with caution in renal impairment.
Cattle	PO	3-5 mg/kg	q24h	Duration: Chronic; off-label use Notes: Not approved for food animals; observe withdrawal times. Use with caution in renal impairment.
Small Ruminants (sheep, goats)	PO	3-5 mg/kg	q24h	Duration: Chronic; off-label use Notes: Limited data; use with caution. Observe withdrawal times.
Rabbit	PO	3-5 mg/kg	q24h	Duration: Chronic; off-label use Notes: Limited data; use with caution.
Birds/Poultry	PO	Not established	Not established	Duration: Not established Notes: Not recommended due to lack of data.

Clinical Indications & Species Uses

General Indications

Adjunctive therapy for chronic pain, especially neuropathic pain
Antiviral against influenza A (rarely used in veterinary practice)

Dog (Canine)

Adjunctive therapy for chronic pain associated with osteoarthritis
Management of neuropathic pain (e.g., intervertebral disc disease, degenerative myelopathy)
Treatment of influenza A virus infections (rarely used)
Adjunct for cancer pain

Cat (Feline)

Adjunctive therapy for chronic pain (e.g., osteoarthritis, neuropathic pain)
Management of pain associated with feline chronic gingivostomatitis (off-label)
Treatment of influenza A virus infections (rarely used)

Horse (Equine)

Adjunctive therapy for chronic laminitis pain
Management of neuropathic pain conditions
Treatment of equine influenza A virus infections (rarely used)

Cattle (Bovine)

Treatment of influenza A virus infections (not typical)

Pharmacology & Mechanism of Action

Drug Class: Antiviral; NMDA receptor antagonist | Pharmacological Group: Adamantane derivatives

Mechanism of Action: Amantadine hydrochloride is an adamantane derivative with multiple mechanisms of action. In veterinary medicine, its primary use is as an N-methyl-D-aspartate (NMDA) receptor antagonist. It noncompetitively binds to the NMDA receptor and blocks the excessive influx of calcium and sodium ions that occurs with glutamate-mediated excitotoxicity, thereby reducing central sensitization and neuropathic pain. Additionally, it has antiviral activity against influenza A viruses by inhibiting the viral M2 proton channel, preventing viral uncoating. It also enhances dopamine release and inhibits dopamine reuptake, contributing to its antiparkinsonian effects in humans, though this is less relevant in veterinary species.

Pharmacodynamics: Amantadine modulates pain transmission by antagonizing NMDA receptors, which are involved in the development of central sensitization and chronic pain states. It is often used as an adjunct to nonsteroidal anti-inflammatory drugs (NSAIDs) or opioids for the management of chronic pain, particularly in osteoarthritis and neuropathic pain conditions. Its analgesic effects are not immediate; clinical improvement may take several days to weeks. In addition, it exhibits antiviral activity against influenza A, but this is rarely exploited in veterinary practice. The drug also has mild stimulant effects on the central nervous system due to dopaminergic activity.

⚡ Pharmacokinetics Summary

Absorption: Amantadine is well absorbed after oral administration in most species. In dogs, oral bioavailability is approximately 100%. In horses, absorption is rapid but may be slightly lower. Food does not significantly affect absorption.

Distribution: Amantadine is widely distributed throughout the body, with a volume of distribution of approximately 4-5 L/kg in dogs. It crosses the blood-brain barrier and achieves concentrations in the cerebrospinal fluid that are about 50-60% of plasma levels. It also crosses the placenta and is excreted in milk.

Metabolism: Amantadine undergoes minimal hepatic metabolism. The majority of the drug is excreted unchanged in the urine. Minor metabolites include acetylated derivatives, but these are not pharmacologically significant.

Excretion: Amantadine is primarily excreted unchanged by the kidneys via glomerular filtration and active tubular secretion. Renal clearance is high, and the elimination half-life is prolonged in patients with renal impairment. In dogs, the elimination half-life is approximately 5-7 hours, while in cats it is longer, around 9-12 hours. In horses, the half-life is approximately 2-3 hours.

Half-Life: Dog: 5-7 hours; Cat: 9-12 hours; Horse: 2-3 hours; Cattle: approximately 4-6 hours (extrapolated).

Bioavailability: Oral bioavailability is high, approaching 100% in dogs and cats. In horses, it is approximately 80-90%.

Protein Binding: Amantadine has low protein binding, approximately 60-70% in humans, but in veterinary species it is generally less than 50%.

Available Formulations & Strengths

Oral Capsule 100 mg (PO)

Oral Tablet 100 mg (PO)

Oral Syrup 50 mg/5 mL (PO)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to amantadine or any component of the formulation
Severe renal impairment (unless dose adjusted and monitored)
Patients with a history of seizures or epilepsy (may lower seizure threshold)
Patients with congestive heart failure or peripheral edema (may exacerbate)
Pregnant or lactating animals (safety not established; use only if benefits outweigh risks)

Warnings & Clinical Precautions:

Use with caution in patients with renal insufficiency; reduce dose or extend dosing interval.
Use with caution in geriatric animals, as they may be more sensitive to adverse effects.
May cause CNS stimulation; monitor for agitation, anxiety, or seizures.
In horses, may cause excitement or ataxia at high doses.
Do not use as a sole analgesic; it is an adjunctive therapy.
May interact with anticholinergic drugs, increasing their effects.
In food animals, observe withdrawal times; extra-label use requires veterinary oversight.
Safety in pregnant or lactating animals has not been established; use only when clearly needed.

Adverse Effects & Reactions

Common:

Gastrointestinal upset (vomiting, diarrhea, anorexia)
CNS stimulation (restlessness, agitation, insomnia)
Ataxia or incoordination
Dry mouth (due to anticholinergic effects)

Serious / Severe:

Seizures (especially in patients with a history of seizures or at high doses)
Cardiac arrhythmias (rare)
Congestive heart failure exacerbation (due to fluid retention)
Renal toxicity (in patients with pre-existing renal disease)

Rare:

Hallucinations or behavioral changes
Livedo reticularis (skin discoloration)
Blood dyscrasias (leukopenia, neutropenia)
Hepatotoxicity (rare)

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Anticholinergic drugs (e.g., atropine, diphenhydramine)	Additive anticholinergic effects (dry mouth, urinary retention, constipation)	Moderate
CNS stimulants (e.g., amphetamines, methylphenidate)	Increased CNS stimulation and risk of seizures	High
Trimethoprim-sulfamethoxazole	May increase amantadine plasma concentrations by decreasing renal clearance	Moderate
Diuretics (e.g., furosemide, hydrochlorothiazide)	May reduce renal clearance of amantadine, increasing toxicity risk	Moderate
Quinidine or quinine	May reduce renal clearance of amantadine	Moderate
NSAIDs (e.g., carprofen, meloxicam)	No direct interaction, but additive renal effects; monitor renal function	Mild

Overdose & Toxicity Management

Signs of Toxicity:

Severe CNS stimulation (agitation, confusion, hallucinations)
Seizures
Cardiac arrhythmias (ventricular tachycardia, QT prolongation)
Hypertension or hypotension
Respiratory distress
Hyperthermia
Coma (in severe cases)

Emergency Treatment Protocol: Treatment is primarily supportive. Induce emesis if recent ingestion and the animal is conscious. Administer activated charcoal to reduce absorption. Provide IV fluids to enhance renal excretion. Monitor cardiac function and treat arrhythmias with appropriate antiarrhythmics. Control seizures with diazepam or barbiturates. In severe cases, hemodialysis may be considered (amantadine is dialyzable). Physostigmine has been used to reverse central anticholinergic effects, but its use is controversial and should be reserved for severe cases.

Food Animal Withdrawal Times

🥩 Meat: 7 days🥛 Milk: 4 days

Withdrawal times are not officially established for amantadine in food animals. The values provided are conservative estimates based on pharmacokinetic data and should be used as a guide. Extra-label use in food animals requires veterinary oversight and extended withdrawal periods may be necessary. Consult FARAD (Food Animal Residue Avoidance Databank) for current recommendations.

Storage, Handling & Regulatory Information

Storage Temperature: Store at room temperature (20-25°C or 68-77°F).

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

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use (influenza A and Parkinson's disease).

Extra-Label (Off-Label) Use: Amantadine is not FDA-approved for veterinary use in most species. Its use in animals is considered extra-label (off-label) and is permitted under the Animal Medicinal Drug Use Clarification Act (AMDUCA) in the United States, provided that a valid veterinarian-client-patient relationship exists, and appropriate withdrawal times are observed for food animals. In the European Union, similar regulations apply under the cascade system.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Amantadine is primarily used in veterinary medicine as an adjunctive analgesic for chronic pain, especially neuropathic pain and osteoarthritis. It is not effective as a sole analgesic and should be used in combination with NSAIDs, opioids, or other pain modulators. Clinical response may take 2-4 weeks, so it is important to set owner expectations. Due to its renal excretion, dose adjustments are necessary in patients with renal impairment. It is contraindicated in patients with a history of seizures. In food animals, extra-label use requires careful consideration of withdrawal times. Overall, amantadine is a valuable tool in managing chronic pain when used appropriately.

References & Literature

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