Gentamicin 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.

Gentamicin is an aminoglycoside antibiotic used in horses to treat certain bacterial infections, especially infections caused by aerobic Gram-negative bacteria. It is usually given by intravenous injection in adult horses and is a prescription medication that should only be used under your vet's direction.

This drug is concentration-dependent, which means higher peak blood levels tend to improve bacterial killing. That is one reason many equine protocols use once-daily dosing rather than smaller doses spread through the day. In practice, your vet may pair gentamicin with another antibiotic such as a penicillin when broader coverage is needed.

Gentamicin can be very useful, but it also has an important downside: it can be hard on the kidneys. Horses that are dehydrated, septic, already have kidney disease, or are receiving other kidney-stressing drugs need especially careful planning and monitoring. For that reason, gentamicin is not a medication pet parents should ever dose on their own.

In adult horses, gentamicin is most commonly used for suspected or confirmed bacterial infections where Gram-negative organisms are a concern. Regulatory and reference sources support its use for lower respiratory tract infections caused by susceptible Gram-negative bacteria, and equine clinicians also use it in broader hospital settings when culture results, severity of illness, or infection location support that choice.

Examples may include bacterial pneumonia, some uterine infections in mares, and selected joint, soft tissue, or systemic infections when your vet believes gentamicin is appropriate. In many hospitalized horses, it is used alongside a beta-lactam antibiotic to widen coverage while culture and sensitivity results are pending.

Gentamicin is not effective against viruses, and it is not the right fit for every bacterial infection. The best use is targeted use: matching the drug to the likely bacteria, the infection site, and the horse's kidney status. When possible, your vet may recommend culture and sensitivity testing before or during treatment to confirm that gentamicin is a sensible option.

For adult horses, a commonly cited and currently supported regimen is 6.6 mg/kg IV once every 24 hours for 3 to 5 days. This once-daily approach is reflected in equine antimicrobial references and current product information for injectable equine gentamicin. Because gentamicin has a narrow safety margin, your vet should calculate the dose from an accurate body weight rather than estimating.

As a rough example, a 500 kg horse at 6.6 mg/kg would receive 3,300 mg total per dose. With a 100 mg/mL injectable solution, that works out to about 33 mL IV once daily. That is only an example, not a home dosing instruction. Concentration, route, dilution, treatment length, and whether the horse needs fluids or lab monitoring all matter.

Foals and neonates are different. Current equine product guidance specifically notes that use in foals and neonates is not recommended because safety data are limited and kidney risk is a major concern. If your foal needs an aminoglycoside, your vet may use a different protocol, different monitoring plan, or a different drug entirely.

Before and during treatment, your vet may recommend hydration support, bloodwork, and kidney monitoring, especially if treatment will last more than a few days or your horse is sick enough to be at risk for acute kidney injury. Ask whether creatinine, chemistry values, urine output, and concurrent medications should be checked before the next dose.

The most important side effect with gentamicin in horses is nephrotoxicity, meaning kidney injury. Risk goes up in horses that are dehydrated, have endotoxemia or sepsis, already have kidney disease, or are receiving other medications that can reduce kidney perfusion or directly stress the kidneys. This risk is one reason your vet may want baseline bloodwork before treatment starts.

At home, pet parents may notice reduced appetite, dullness, decreased water intake, reduced urine output, or worsening illness despite treatment. Those signs are not specific to gentamicin, but they are reasons to contact your vet promptly. Some horses show no obvious early signs, which is why lab monitoring matters.

There are also isolated reports of ototoxicity, meaning ear or balance-related toxicity, although kidney effects are the better-recognized concern in horses. Rare product-related adverse reactions such as hypersensitivity-type responses have also been reported with some formulations. If your horse seems suddenly weak, unsteady, or significantly worse after dosing, call your vet right away.

See your vet immediately if your horse becomes markedly lethargic, stops eating, seems dehydrated, produces very little urine, develops severe diarrhea, or shows signs of colic or collapse during treatment.

The biggest interaction concern is additive kidney stress. Gentamicin should be used very carefully with other drugs that may affect kidney function or hydration status. In horses, that often includes NSAIDs such as flunixin meglumine or phenylbutazone, especially if the horse is dehydrated, systemically ill, or already has reduced kidney perfusion.

Other important concerns include combining gentamicin with other aminoglycosides such as amikacin or with drugs known for nephrotoxic or ototoxic potential, including furosemide and amphotericin B. These combinations are not always forbidden, but they do raise the need for a very clear medical reason and close monitoring.

On the other hand, gentamicin may be used intentionally with beta-lactam antibiotics such as penicillin or ampicillin because that combination can broaden coverage and may provide useful antibacterial synergy in some infections. The key point is that every combination should be chosen by your vet based on the infection, hydration status, kidney values, and treatment goals.

Tell your vet about every medication and supplement your horse is receiving, including anti-inflammatories, diuretics, ulcer medications, and any recent antibiotics. That full list helps your vet choose the safest treatment tier for your horse.