Oxytetracycline Polymyxin B in Cats

See your vet immediately if your cat has a painful eye, keeps the eye closed, has a cloudy cornea, has eye trauma, or seems to be losing vision. Oxytetracycline polymyxin B is a prescription antibiotic eye ointment used in cats for certain superficial bacterial eye infections. It is most commonly sold under the brand name Terramycin and is applied directly to the eye rather than given by mouth.

Your vet may use this medication for conjunctivitis, blepharitis, keratitis, or corneal ulcers when bacterial infection is present or likely. In cats, eye disease is not always bacterial. Viral disease, especially feline herpesvirus-1, is a very common cause of conjunctivitis and keratitis, and some cats need antiviral treatment, pain control, or other testing instead of an antibiotic alone. That is why a red or goopy eye should not be treated as a routine problem at home.

This medication can be a practical option because it covers a range of gram-positive and gram-negative bacteria and comes as an ointment that stays on the eye surface longer than many drops. Still, it is not the right fit for every eye problem. Your vet may recommend it as part of a conservative plan for a mild surface infection, a standard plan after an eye exam and stain test, or an advanced plan that includes culture, antiviral therapy, or referral if the eye is painful or not improving.

Oxytetracycline polymyxin B combines two antibiotics with different strengths. Oxytetracycline interferes with bacterial protein production, which stops susceptible bacteria from growing and surviving. Polymyxin B damages bacterial cell membranes, with especially useful activity against certain gram-negative organisms. Together, they broaden the medication's surface antibacterial coverage.

In the labeled veterinary product, each gram contains oxytetracycline hydrochloride equivalent to 5 mg of oxytetracycline and 10,000 units of polymyxin B sulfate in an ophthalmic ointment base. The ointment base helps the medicine stay in contact with the eye surface. That can be helpful in cats, where frequent eye dosing is often challenging for pet parents.

Even so, this medication does not treat every cause of feline eye disease. Cats commonly develop conjunctivitis from herpesvirus, Chlamydia felis, Mycoplasma, irritation, allergies, eyelid problems, or corneal injury. In some of those cases, an antibiotic ointment may help with secondary bacterial infection but will not address the main cause. If the eye is not improving within the timeline your vet expects, the treatment plan may need to change.

Most cats tolerate oxytetracycline polymyxin B reasonably well, but mild local irritation can happen. You may notice brief stinging, redness, itching, swelling around the eye, or your cat blinking more right after the ointment is placed. Because ointments are thick, temporary blurry vision for a few minutes is also possible, which can make some cats act annoyed after dosing.

A more serious concern is hypersensitivity. The product label warns that some cats have had life-threatening allergic reactions after ophthalmic antibiotic application. Signs can include facial swelling, itching, weakness, vomiting, or trouble breathing within about four hours of use. If that happens, stop the medication and contact your vet immediately.

There is also a treatment-related risk that matters over time. Using antibiotics when they are not needed, or using them longer than directed, can allow resistant bacteria or fungal organisms to overgrow. If discharge worsens, the eye becomes more painful, the cornea looks cloudy, or a new problem appears during treatment, your vet may want to recheck the eye rather than continue the same ointment.

Always follow your vet's instructions, because the right schedule depends on the diagnosis and how severe the eye problem is. The labeled Terramycin directions state that the ointment is applied topically to the eye 2 to 4 times daily. PetMD also notes a common administration amount of about a 1/4-inch strip into the affected eye, but your vet may adjust frequency, duration, or whether one or both eyes should be treated.

Wash your hands before and after use. Avoid touching the tube tip to your cat's eye, fur, or skin so the medication stays clean. Gently pull down the lower eyelid, place the ointment into the pocket between the lid and the eye, and let your cat blink to spread it. If your cat uses more than one eye medication, eye drops are usually given before ointments, and doses are often spaced 5 to 10 minutes apart.

Do not stop early because the eye looks better after a day or two. Surface infections and ulcers can relapse if treatment is cut short. If you miss a dose, give it when you remember unless it is almost time for the next one. Do not double up. Contact your vet if you cannot medicate safely, if your cat fights hard enough to risk injury, or if the eye is not clearly improving on the schedule your vet gave you.

Oxytetracycline polymyxin B is used on the eye surface, so whole-body drug interactions are usually less of a concern than with oral medications. The more common issue is how it fits into a larger eye treatment plan. If your cat is using multiple ophthalmic medications, timing matters. Eye drops are generally applied before ointments, and medications are usually separated by 5 to 10 minutes so one product does not immediately dilute or block the next.

The bigger clinical interaction is with the diagnosis itself. A cat with herpesvirus keratitis may need an antiviral such as cidofovir, ganciclovir, or another medication chosen by your vet. A cat with chlamydial infection may need systemic antibiotics because topical treatment alone may not be enough. A painful eye with a corneal ulcer may also need stain testing, pain relief, an e-collar, and close rechecks. In those cases, the ointment may be only one part of care.

Tell your vet about every medication and supplement your cat receives, including other eye products, steroid drops, and over-the-counter rinses. Some eye medications are not appropriate if a corneal ulcer is present, and using the wrong product can delay healing or make the eye worse. If your cat seems more painful after starting treatment, your vet should reassess the plan.