Terbinafine Hydrochloride — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	20-40 mg/kg	q24h	Duration: 4-6 weeks (dermatophytosis); may be longer for onychomycosis Notes: Administer with food to enhance absorption. For dermatophytosis, continue treatment for at least 2 weeks beyond clinical cure.
Cat	PO	20-40 mg/kg	q24h	Duration: 4-6 weeks (dermatophytosis); may be longer for onychomycosis Notes: Administer with food. Cats may require higher doses due to variable absorption. Monitor for hepatotoxicity.
Horse	PO	10-20 mg/kg	q24h	Duration: 4-8 weeks Notes: Oral absorption is variable; topical therapy may be preferred. Use with caution in horses with hepatic disease.
Cattle	PO	10-20 mg/kg	q24h	Duration: 4-6 weeks Notes: Not commonly used; topical therapy is preferred. Extra-label use requires veterinary oversight.
Small Ruminants (sheep, goats)	PO	10-20 mg/kg	q24h	Duration: 4-6 weeks Notes: Not commonly used; topical therapy is preferred. Extra-label use requires veterinary oversight.
Rabbit	PO	10-20 mg/kg	q24h	Duration: 4-6 weeks Notes: Limited data; use with caution. Topical therapy may be preferred.
Bird/Poultry	Not recommended	N/A	N/A	Duration: N/A Notes: Not indicated; systemic fungal infections in birds require other antifungals.
Exotic/Other (small mammals, reptiles)	PO	10-20 mg/kg	q24h	Duration: 4-6 weeks Notes: Off-label use; limited data. Use with caution.

Clinical Indications & Species Uses

General Indications

Treatment of dermatophytosis (ringworm) in various species
Treatment of onychomycosis

Dog (Canine)

Dermatophytosis (ringworm) caused by Microsporum canis, Trichophyton spp., and Microsporum gypseum
Onychomycosis (fungal nail infections)
Cutaneous candidiasis (off-label)

Cat (Feline)

Dermatophytosis (ringworm) caused by Microsporum canis
Onychomycosis
Cutaneous candidiasis (off-label)

Horse (Equine)

Dermatophytosis (ringworm) caused by Trichophyton equinum and Microsporum gypseum
Cutaneous candidiasis (off-label)

Cattle (Bovine)

Dermatophytosis (ringworm) caused by Trichophyton verrucosum

Small Ruminants (Sheep / Goat)

Dermatophytosis (ringworm) caused by Trichophyton verrucosum

Rabbit & Small Mammals

Dermatophytosis (ringworm) caused by Trichophyton mentagrophytes

Exotic & Other Species

Dermatophytosis in small mammals (e.g., guinea pigs, rats) - off-label
Fungal dermatitis in reptiles - off-label

Pharmacology & Mechanism of Action

Drug Class: Allylamine antifungal | Pharmacological Group: Antifungal agent

Mechanism of Action: Terbinafine hydrochloride is an allylamine antifungal that inhibits the enzyme squalene epoxidase, which is essential for the biosynthesis of ergosterol, a key component of the fungal cell membrane. This inhibition leads to an accumulation of squalene, which is toxic to the fungal cell, and a deficiency of ergosterol, resulting in disruption of the cell membrane and fungal cell death. Terbinafine is primarily fungicidal against dermatophytes (e.g., Trichophyton, Microsporum, Epidermophyton) and fungistatic against some yeasts such as Candida albicans.

Pharmacodynamics: Terbinafine exhibits concentration-dependent antifungal activity. It is highly lipophilic and keratophilic, allowing it to concentrate in skin, hair, and nails, where it persists for weeks to months after discontinuation. Its fungicidal action is rapid against susceptible dermatophytes. In veterinary medicine, it is used for dermatophytosis and other superficial fungal infections. It has poor activity against Malassezia pachydermatis and is not effective against systemic mycoses.

⚡ Pharmacokinetics Summary

Absorption: Terbinafine is well absorbed after oral administration in most species, but bioavailability is reduced by first-pass metabolism. In dogs, oral bioavailability is approximately 40-50% when given with food. In cats, absorption is variable; oral bioavailability is around 40-70%. Topical absorption is minimal.

Distribution: Terbinafine is extensively distributed to tissues, particularly to skin, hair, nails, and adipose tissue. It crosses the blood-brain barrier poorly. It is highly protein-bound (greater than 99% in humans; similar in animals). It is secreted in sebum and accumulates in hair follicles and stratum corneum.

Metabolism: Terbinafine is extensively metabolized in the liver via multiple cytochrome P450 enzymes (e.g., CYP2D6, CYP3A4) to inactive metabolites. It undergoes significant first-pass metabolism, reducing systemic bioavailability.

Excretion: Metabolites are excreted primarily in the urine (about 70-80%) and feces (about 20-30%). Less than 1% of the parent drug is excreted unchanged. In animals, elimination half-life varies by species.

Half-Life: Dogs: approximately 8-12 hours; Cats: approximately 8-12 hours; Horses: approximately 6-10 hours; Cattle: approximately 4-8 hours (estimated).

Bioavailability: Oral bioavailability: Dogs ~40-50%, Cats ~40-70%, Horses ~30-50% (estimated).

Protein Binding: Greater than 99% protein-bound in plasma.

Available Formulations & Strengths

Oral Tablet 250 mg (PO)

Oral Granules 125 mg, 250 mg sachets (PO)

Topical Cream 1% (Topical)

Topical Gel 1% (Topical)

Topical Spray 1% (Topical)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to terbinafine or any component of the formulation
Severe hepatic impairment
Severe renal impairment (creatinine clearance < 50 mL/min)
Use in pregnant or lactating animals unless benefits outweigh risks (limited safety data)

Warnings & Clinical Precautions:

Use with caution in animals with hepatic or renal disease; monitor liver enzymes and renal function periodically.
May cause gastrointestinal upset; administer with food to reduce nausea.
In cats, monitor for signs of hepatotoxicity (anorexia, vomiting, jaundice).
Not approved for use in food animals in many countries; extra-label use requires veterinary oversight and extended withdrawal times.
Topical formulations may cause skin irritation at application site.
Use in very young animals with caution due to limited safety data.
Terbinafine may be irritating to eyes; avoid contact with mucous membranes.

Adverse Effects & Reactions

Common:

Gastrointestinal upset (vomiting, diarrhea, anorexia)
Lethargy
Increased liver enzymes (transient)

Serious / Severe:

Hepatotoxicity (especially in cats)
Severe skin reactions (e.g., erythema multiforme, Stevens-Johnson syndrome)
Blood dyscrasias (neutropenia, thrombocytopenia)
Renal impairment

Rare:

Neurological signs (ataxia, seizures)
Allergic reactions (urticaria, angioedema)
Photosensitivity

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Cimetidine	Decreases metabolism of terbinafine, increasing its plasma concentration and risk of toxicity.	Moderate
Rifampin	Increases metabolism of terbinafine, reducing its efficacy.	Moderate
CYP2D6 substrates (e.g., tricyclic antidepressants, beta-blockers)	Terbinafine inhibits CYP2D6, increasing levels of these drugs and potential toxicity.	Moderate
Warfarin	Terbinafine may enhance the anticoagulant effect of warfarin; monitor prothrombin time.	Moderate
Cyclosporine	Terbinafine may increase cyclosporine levels; monitor for toxicity.	Mild

Overdose & Toxicity Management

Signs of Toxicity:

Gastrointestinal signs (vomiting, diarrhea)
Lethargy
Ataxia
Seizures (in severe cases)
Hepatotoxicity (with chronic overdose)

Emergency Treatment Protocol: Treatment is symptomatic and supportive. Induce emesis if recent ingestion and animal is conscious. Administer activated charcoal to reduce absorption. Provide intravenous fluids for dehydration and electrolyte imbalances. Monitor liver enzymes and renal function. Seizures may be treated with diazepam or barbiturates. There is no specific antidote.

Food Animal Withdrawal Times

🥩 Meat: 28 days🥛 Milk: 7 days

Withdrawal times are not established for terbinafine in food animals. Extra-label use requires extended withdrawal periods; consult regulatory guidelines. For cattle and small ruminants, a meat withdrawal of at least 28 days and milk withdrawal of at least 7 days is recommended, but these are estimates and may vary by country.

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.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use only. However, it is commonly used in veterinary medicine as an extra-label drug.

Extra-Label (Off-Label) Use: In the US, terbinafine is not FDA-approved for veterinary use; it is used extra-label under the Animal Medicinal Drug Use Clarification Act (AMDUCA). Extra-label use in food animals requires a valid veterinary-client-patient relationship, and withdrawal times must be extended. In some countries, terbinafine may be approved for use in certain species.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Terbinafine is a valuable antifungal for treating dermatophytosis in small animals, especially cats and dogs. It is often preferred over griseofulvin due to its fungicidal action and shorter duration of therapy. However, its use in cats requires careful monitoring for hepatotoxicity, as cats are more susceptible to adverse effects. In horses and food animals, topical therapy is often preferred due to cost and variable oral absorption. Terbinafine is not effective against systemic mycoses and should not be used for those conditions. When using terbinafine, it is important to combine with environmental decontamination and topical therapy to prevent reinfection. Always confirm diagnosis via fungal culture or PCR before initiating treatment. In food animals, adhere to withdrawal times and regulatory requirements.

References & Literature

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