Amphotericin B 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.

Amphotericin B is a prescription antifungal medication in the polyene class. In equine medicine, your vet may use it for severe or hard-to-treat fungal infections when a fungus is invading tissue rather than sitting on the surface. It is a potent drug, but it also has a narrow safety margin, so horses receiving it usually need close monitoring.

Most equine use is extra-label, which means the drug is being used under veterinary supervision in a way that is not specifically listed on the product label for horses. That is common in veterinary medicine. Amphotericin B may be given intravenously, used in a compounded ophthalmic preparation for fungal keratitis, placed intrauterine in some mares with fungal endometritis, or used in specialized local-delivery techniques such as regional limb perfusion.

This medication is best known for its activity against serious fungal organisms, but it is not a routine first-line drug for every horse with a suspected fungal problem. Your vet will usually weigh the likely fungus, the location of infection, kidney risk, hydration status, and whether safer oral antifungals could work before recommending amphotericin B.

In horses, amphotericin B is mainly reserved for serious fungal disease. Examples include some cases of systemic mycosis, difficult fungal respiratory disease, invasive candidiasis, and selected infections caused by organisms such as Blastomyces, Histoplasma, or other opportunistic fungi. Merck also notes its use for systemic mycotic infections and describes regional limb perfusion use in horses for pythiosis.

Your vet may also use amphotericin B in fungal eye disease, especially fungal keratitis, as a compounded topical medication when culture results or clinical suspicion support it. In broodmares, Merck lists 100 mg per mare intrauterine for fungal or yeast endometritis, typically given over several consecutive days during estrus.

Because fungal disease in horses can look like bacterial infection, inflammation, or trauma, amphotericin B is usually not started casually. Your vet may recommend fungal culture, cytology, biopsy, endoscopy, ultrasound, or ophthalmic testing first. That extra work can help match the treatment plan to the actual organism and may reduce unnecessary drug exposure.

Amphotericin B dosing in horses depends heavily on the formulation, route, and disease being treated. Merck lists a general equine intravenous dose of 0.3 mg/kg IV every 24 hours for 3 days in one dosing table, while another Merck reference lists 0.15-0.5 mg/kg every 48 hours until a cumulative dose of 4-12 mg/kg is reached. Those differences matter because protocols vary by case, fungal organism, kidney tolerance, and whether your vet is using conventional deoxycholate or a lipid-based product.

For mares with fungal endometritis, Merck describes 100 mg per mare intrauterine, often continued for several consecutive days, preferably during estrus. For ophthalmic use, amphotericin B is commonly compounded into a topical eye medication rather than used from the injectable vial directly by pet parents at home. Concentration and frequency vary widely based on the corneal lesion and culture results.

This is not a medication to dose without veterinary direction. Your vet may adjust the plan based on body weight, hydration, kidney values, urine output, electrolyte levels, and response to treatment. Many horses need baseline and repeat bloodwork during therapy. If kidney values rise or the horse shows signs of intolerance, your vet may pause treatment, lower the dose, increase fluid support, or switch to another antifungal option.

The biggest concern with amphotericin B is kidney injury. This drug is well known for causing nephrotoxicity, especially with conventional formulations or in horses that are dehydrated, already have kidney compromise, or are receiving other kidney-stressing medications. Your vet may monitor creatinine, BUN, electrolytes, hydration, and urine output during treatment.

Other reported adverse effects include loss of appetite, nausea, diarrhea, fever, hypersensitivity reactions, anemia, cardiac rhythm changes, liver dysfunction, and neurologic signs. Merck also notes that electrolyte and acid-base problems can occur, including hypokalemia and metabolic acidosis. In practical terms, pet parents may notice dullness, reduced appetite, increased drinking, reduced manure, weakness, or a horse that seems "off" after treatment.

If your horse is receiving amphotericin B and develops marked lethargy, poor appetite, colic signs, decreased urination, swelling, weakness, or worsening eye or respiratory signs, contact your vet promptly. These changes do not always mean the drug is the cause, but they do mean your horse needs reassessment.

Amphotericin B has several important interactions because it can stress the kidneys and alter electrolytes. Merck advises avoiding or using extra caution with aminoglycosides such as gentamicin or amikacin because the combination can increase the risk of nephrotoxicity. That matters in horses, since aminoglycosides are commonly used for serious bacterial infections.

Merck also lists caution with digitalis drugs because amphotericin B can increase toxicity risk, and with mineralocorticoids or thiazide diuretics because they can worsen hypokalemia. Additional caution is advised with curarizing agents, antineoplastic drugs, and cyclosporine.

Even when a combination is medically appropriate, your vet may change the monitoring plan rather than avoid the pairing completely. Be sure your vet knows about every medication, supplement, electrolyte product, and compounded eye treatment your horse is receiving so the full risk picture is clear.