Natamycin 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.

Natamycin is a topical antifungal eye medication used in horses, most often as a 5% ophthalmic suspension. In the U.S., the best-known brand is Natacyn. It belongs to the polyene antifungal class and is used on the eye surface rather than by mouth or injection.

In equine medicine, natamycin is most often discussed for keratomycosis, also called fungal keratitis. That is a fungal infection of the cornea, and it can become vision-threatening quickly in horses. Because horse eyes are large, exposed, and prone to trauma, fungal corneal infections are a well-recognized emergency in equine practice.

Natamycin works best on surface and stromal fungal infections of the cornea. It reaches useful levels in the corneal stroma after topical use, but it does not penetrate well through an intact corneal epithelium and is not expected to treat deeper intraocular infection by itself. That is why your vet may pair it with other eye medications, oral medications, lavage systems, or surgery depending on how deep and aggressive the infection is.

Natamycin is used in horses mainly for fungal infections of the eye, especially fungal keratitis. In the human product label, natamycin is indicated for fungal blepharitis, conjunctivitis, and keratitis caused by susceptible organisms. In horses, vets most commonly use it as part of treatment for corneal fungal ulcers and other forms of equine keratomycosis.

It is often chosen when your vet suspects or confirms fungi such as Fusarium, Aspergillus, or Candida. Susceptibility can vary by region and by organism, so many equine ophthalmology references recommend corneal cytology, culture, and sensitivity testing when possible. That helps your vet decide whether natamycin is a good fit or whether another antifungal, such as voriconazole or miconazole, may be more appropriate.

Natamycin is rarely the whole plan by itself. Horses with fungal corneal disease often also need pain control, anti-collagenase support, atropine, systemic anti-inflammatory medication, and close rechecks. In deeper, melting, or perforation-risk ulcers, your vet may recommend subpalpebral lavage placement for frequent dosing or surgical support to protect the eye and preserve vision.

Natamycin dosing in horses should always come from your vet, because frequency depends on the depth of the ulcer, the fungus involved, whether the cornea is melting, and how the eye responds over time. A commonly referenced ophthalmic concentration is natamycin 5% suspension.

For fungal keratitis, the FDA-approved human label recommends 1 drop in the affected eye every 1 to 2 hours initially, then decreasing to 6 to 8 times daily after the first 3 to 4 days if the eye is improving. Therapy is generally continued for 14 to 21 days or until active infection resolves, with gradual tapering. Merck Veterinary Manual lists natamycin 5% for keratomycosis at every 2 to 6 hours in animals, while noting that it does not penetrate an intact corneal epithelium well.

In real equine cases, dosing may be much more intensive early on. Some horses need medication around the clock, and many are treated through a subpalpebral lavage system so drops can be delivered safely and consistently. If your horse misses doses, the infection can worsen fast, so ask your vet exactly how to give it, whether to shake well before use, and when to call if the eye looks more painful, cloudy, or swollen.

Do not stop treatment early because the eye looks a little better. Fungal corneal disease can relapse or continue deeper in the tissue. Your vet may adjust the schedule based on repeat exams, fluorescein staining, cytology, or culture results.

Most side effects with natamycin are local eye effects rather than whole-body effects, because systemic absorption after topical ophthalmic use is expected to be minimal. Still, horse eyes are sensitive, and any worsening signs deserve prompt follow-up.

Possible side effects reported with natamycin ophthalmic use include eye irritation, eye pain, tearing, redness, swelling, foreign-body sensation, discomfort, and changes in vision. The suspension can also stick to ulcerated areas of the cornea, which may be expected but can make the eye look messy or coated. Rarely, allergic reactions or hypersensitivity can occur.

See your vet immediately if your horse develops more squinting, increased cloudiness, a white or yellow plaque, more discharge, worsening corneal blue haze, a suddenly more painful eye, or signs that the ulcer is getting deeper. Those changes may reflect progression of the fungal infection rather than a medication side effect, but either way they need urgent re-evaluation.

Because fungal keratitis in horses can threaten vision quickly, the biggest risk is often undertreatment or delayed reassessment, not the drop itself. If there is no clear improvement after several days, or if your vet warned you the ulcer is deep or melting, expect close monitoring and possible changes to the treatment plan.

There are no widely documented major systemic drug interactions for topical ophthalmic natamycin in horses, largely because absorption into the bloodstream is expected to be very low. That said, interaction risk in practice is less about classic whole-body drug interactions and more about how multiple eye medications are coordinated.

Natamycin is commonly used alongside other ophthalmic and systemic medications in horses with fungal keratitis. These may include atropine, serum or anti-collagenase therapy, systemic NSAIDs, topical antibiotics, and sometimes other antifungals. Your vet may space medications a few minutes apart so one drop does not immediately wash out another.

The most important caution is not to add or continue steroid-containing eye medications unless your vet specifically instructs you to do so. Corticosteroids can worsen corneal ulcer infections, especially fungal disease, in many cases. Also tell your vet about every medication, supplement, and compounded eye product your horse is receiving, because treatment plans for equine eye disease are often adjusted as culture results and corneal depth change.

If your horse is on several eye medications, ask your vet for a written schedule. That can make a demanding treatment plan much easier to follow and helps reduce missed doses or accidental double-dosing.