Terbinafine for Horses: Uses, Dosing & Side Effects

This information is for educational purposes only. Never give your pet any medication without your veterinarian's guidance. Dosing, frequency, and safety depend on your pet's specific health profile.

Terbinafine is an allylamine antifungal medication used extra-label in horses. It works by blocking fungal sterol production early in the cell membrane pathway, which leads to fungal cell death. In veterinary references, the listed equine oral dose range is 10-30 mg/kg by mouth every 24 hours, but published sources also note that horses have lower oral bioavailability than dogs and cats because of substantial first-pass metabolism. That means your vet may use it selectively and often as part of a broader treatment plan rather than as a routine first choice.

In horses, terbinafine is not one of the most commonly used antifungals. Clinical efficacy in equine patients has not been well established, so your vet may recommend it only when the suspected fungus, infection site, prior treatment response, and practical factors all line up. It is most often discussed for difficult fungal disease where other antifungals, topical therapy, surgery, or combination treatment may also be needed.

Because equine fungal infections can involve the skin, cornea, guttural pouch, respiratory tract, or deeper tissues, the right plan depends on where the infection is and how severe it is. Your vet may recommend fungal culture, cytology, biopsy, or ophthalmic testing before deciding whether terbinafine makes sense for your horse.

In horses, terbinafine is used extra-label for selected fungal infections. Veterinary pharmacology references describe activity against dermatophytes, some Aspergillus species, and other fungi, and note that terbinafine may be used to enhance the effect of other antifungal drugs in certain fungal disorders. In real-world equine practice, that usually means it is considered for challenging or persistent infections, not as a one-size-fits-all medication.

Your vet may discuss terbinafine for problems such as fungal skin disease or as one part of treatment for equine keratomycosis when a horse has a fungal corneal ulcer. For eye disease, oral terbinafine is usually not the only therapy. Horses with fungal keratitis often need frequent topical antifungals, pain control, anti-collagenase therapy, and sometimes surgery or referral.

Terbinafine may also come up when a horse has not responded well to another antifungal, when culture results suggest susceptibility, or when a combination approach is being considered. Since fungal infections can look similar to bacterial, parasitic, inflammatory, or immune-mediated disease, your vet will usually want a diagnosis before starting treatment.

Published veterinary references list terbinafine at 10-30 mg/kg by mouth every 24 hours in horses. That is a broad range, and the exact dose your vet chooses depends on the infection site, suspected organism, severity, treatment response, and whether terbinafine is being used alone or with another antifungal. Because horses appear to have lower relative bioavailability after oral dosing, your vet may weigh expected benefit carefully before prescribing it.

Treatment length can vary widely. Superficial infections may need weeks of therapy, while deeper or ocular fungal disease may require a longer course plus topical medications, repeat exams, and lab monitoring. Do not stop early because the skin or eye looks better. Fungal infections often improve slowly, and stopping too soon can lead to relapse.

Terbinafine is commonly given by mouth, often using human tablets or a compounded preparation when needed. If your horse is hard to medicate, ask your vet whether the formulation, tablet size, or dosing schedule can be adjusted. Follow your vet's instructions closely, and let them know if your horse misses doses, spits out medication, goes off feed, or develops diarrhea.

Because terbinafine is metabolized by the liver, your vet may recommend baseline and follow-up bloodwork in horses needing longer treatment courses or in horses with a history of liver disease, poor appetite, weight loss, or other concurrent medications.

Most reported adverse effects of oral terbinafine in veterinary references involve the gastrointestinal tract and skin. In practice, that means your horse's care team may ask you to watch for decreased appetite, loose manure, diarrhea, nausea-like behavior, or general digestive upset. Some horses may also develop itching, hives, or other skin reactions.

A more serious but less common concern is liver irritation or hepatobiliary dysfunction. Call your vet promptly if your horse becomes dull, stops eating, loses weight, develops diarrhea that persists, or shows signs that could fit liver trouble such as jaundice, photosensitivity, or worsening lethargy. These signs are not specific to terbinafine, but they matter during treatment.

If terbinafine is being used for an eye infection, remember that worsening squinting, cloudiness, discharge, or eye pain is an emergency even if the horse is already on medication. Oral antifungals alone are usually not enough for a rapidly progressive fungal corneal ulcer.

Do not change the dose on your own if side effects appear. Instead, contact your vet so they can decide whether to continue, pause, lower the dose, switch medications, or add monitoring.

Terbinafine is metabolized through hepatic cytochrome P450 pathways, so drug interactions are possible. Veterinary references specifically note that cimetidine, a CYP450 inhibitor, can increase terbinafine blood levels and decrease clearance. That can raise the chance of side effects, especially during longer treatment courses.

Terbinafine is also sometimes used with other antifungals, including drugs such as amphotericin B or azoles, when your vet wants broader coverage or possible synergy. Combination therapy can be appropriate in some cases, but it also increases the need for careful monitoring because each medication brings its own risks and practical challenges.

Tell your vet about every medication and supplement your horse receives, including ulcer medications, anti-inflammatories, compounded products, and anything used for eye disease. Horses with liver disease or kidney disease may need extra caution, dose adjustments, or a different antifungal plan.

If your horse competes, ask your vet about medication rules and withdrawal guidance. Even when a drug is medically appropriate, competition regulations may affect timing and documentation.