Terramycin ophthalmic ointment is FDA-approved for use in cats and has been on the market since 1953. It is generally safe when used as directed, but cats carry a specific and serious risk that dog owners don’t face: rare but potentially fatal allergic reactions to one of its ingredients. Understanding that risk, and knowing what to watch for, is essential before applying it.
What Terramycin Is and What It Treats
Terramycin is a veterinary eye ointment containing two antibiotics that work together. The first, oxytetracycline, is a broad-spectrum antibiotic that stops bacteria from growing. The second, polymyxin B, targets a different set of bacteria, particularly certain gram-negative species. Together, they cover a wide range of organisms that cause superficial eye infections.
The ointment is labeled for treating conjunctivitis (pink eye), corneal ulcers, inflamed corneas (keratitis), and eyelid inflammation (blepharitis) in cats, dogs, cattle, sheep, and horses. It’s applied as a thin strip directly into or around the affected eye, typically two to four times daily. Your vet will determine how long the course should last based on the type and severity of infection.
The Anaphylaxis Risk in Cats
This is the most important safety concern. Cats can develop severe, life-threatening allergic reactions (anaphylaxis) after topical application of antibiotic eye ointments, and Terramycin is among them. The culprit appears to be the polymyxin B component. In a documented review of 61 cats that experienced anaphylaxis from topical ophthalmic antibiotics, 13 of those cases (about 21%) involved oxytetracycline and polymyxin B, the exact combination in Terramycin.
The reaction is rare, but it can be fatal. Signs typically appear within minutes to a few hours of application and include:
- Facial swelling, especially around the eyes and head
- Itchiness concentrated around the face
- Vomiting or diarrhea with sudden onset
- Weakness or collapse, with pale gums and cold limbs
- Difficulty breathing or excessive drooling
- Incoordination, seizures, or loss of consciousness in the most severe cases
The drug’s official labeling warns that if your cat shows any of these signs within four hours of application, you should stop treatment immediately and contact a veterinarian. This risk is why Terramycin is restricted to use by or on the order of a licensed vet, even though it’s available over the counter in some retail settings.
Everyday Side Effects
Outside of the rare anaphylactic reaction, Terramycin is well tolerated by most cats. The ointment has a thick, petroleum-based consistency, so your cat may paw at the treated eye or blink excessively for a short time after application. Mild, temporary irritation at the application site is possible but uncommon. If you notice redness or swelling that worsens rather than improves over the first day or two of treatment, that warrants a call to your vet.
It Won’t Treat Viral Infections on Its Own
Many cat eye infections, especially in kittens and shelter cats, are caused by feline herpesvirus rather than bacteria. Terramycin has no antiviral activity, so it won’t eliminate a herpes infection. However, vets frequently prescribe it alongside antiviral treatment to prevent secondary bacterial infections from taking hold, particularly when corneal ulcers are present. If your cat’s eyes aren’t improving after several days on Terramycin alone, a viral cause is one likely explanation.
Benefits for Corneal Ulcers
Terramycin has a useful property beyond basic infection control. The oxytetracycline component blocks enzymes that break down corneal tissue, a process sometimes called corneal “melting.” It also stimulates growth factors that help the surface layer of the cornea migrate and heal. This makes it a particularly good choice when a cat has a corneal ulcer, since it fights infection and supports tissue repair at the same time. Vets sometimes use it specifically for these anti-collagenase properties, either alone or alongside other protective treatments like autologous serum.
How to Apply It Safely
Gently pull down your cat’s lower eyelid to create a small pocket, then squeeze a thin ribbon of ointment (about a quarter inch) into that space. Avoid touching the tip of the tube to the eye itself, as this can introduce new bacteria into the tube or scratch the cornea. Let your cat blink naturally to spread the ointment across the eye’s surface. Vision will be slightly blurry for a minute or two afterward, which is normal.
Most treatment plans call for application two to four times daily, spaced as evenly as possible. If you’re treating both eyes, use a clean section of the tube tip for each one. After the first application, watch your cat closely for at least an hour for any signs of an allergic reaction. If the first dose passes without trouble, the risk drops significantly for subsequent applications, though it doesn’t disappear entirely.
Store the tube at room temperature with the cap tightly closed. Once a tube has been opened and used, follow any expiration guidance from your vet. Using expired ophthalmic ointment risks both reduced effectiveness and increased irritation.
Signs the Infection Isn’t Responding
Bacterial eye infections that respond to Terramycin typically start showing improvement within two to three days: less discharge, reduced redness, and a more comfortable-looking eye. If your cat’s symptoms stay the same or worsen, the infection may be caused by a resistant organism, a virus, or a deeper problem like a foreign body or blocked tear duct. Kittens are especially vulnerable. Young cats with severe conjunctivitis can develop so much discharge that their eyelids seal shut, and if those eyes aren’t opened and treated promptly, permanent damage or blindness can result.