Cidofovir Ophthalmic in Cats

Cidofovir ophthalmic is a prescription antiviral eye drop used in cats, most often for eye disease linked to feline herpesvirus-1, also called FHV-1. Your vet may prescribe it for conjunctivitis, keratitis, corneal ulcers with a herpes pattern, or recurrent eye flare-ups in cats with a known history of herpesvirus. In veterinary medicine, it is usually used as a compounded 0.5% ophthalmic solution rather than a mass-market pet product.

This medication is considered extra-label in cats, which means it is not FDA-approved specifically for veterinary ophthalmic use but is commonly prescribed when your vet believes it fits the situation. One reason cidofovir is used so often is convenience. Compared with some older antiviral eye medications that need very frequent dosing, cidofovir is often given every 12 hours, which can be easier for many pet parents and less stressful for cats.

Cidofovir is not a cure for feline herpesvirus. FHV-1 can remain latent in the body and flare again during stress or illness. The goal of treatment is to reduce viral replication in the eye, improve comfort, and help the surface of the eye heal. Some cats need only a short course during a flare, while others need a broader plan that may include lubricants, pain control, antibiotics for secondary infection, or oral antiviral medication depending on exam findings.

See your vet immediately if your cat is squinting hard, keeping the eye closed, has a cloudy eye, has a deep-looking ulcer, or seems painful. Eye disease can worsen quickly, and different causes can look similar at home. Your vet needs to confirm whether cidofovir is appropriate and whether other treatments should be added or avoided.

Cidofovir is an antiviral medication that interferes with viral DNA synthesis. In plain language, it gets in the way of the virus copying itself. That matters in feline herpesvirus eye disease because active viral replication contributes to inflammation of the conjunctiva and cornea, discharge, squinting, and delayed healing of the eye surface.

A practical advantage of cidofovir is that its active metabolites stay inside cells for a relatively long time. That long intracellular activity is why vets can often use it less frequently than some other topical antivirals. Merck lists a common feline ophthalmic dose as one drop every 12 hours, and experimental work in cats has shown twice-daily treatment can reduce both clinical disease severity and ocular viral load.

Even so, cidofovir is only one part of the picture. Herpesvirus flare-ups may be triggered by stress, concurrent illness, or irritation of the eye surface. If your cat also has a corneal ulcer, dry eye, symblepharon, secondary bacterial infection, or severe upper respiratory signs, your vet may recommend a combination plan rather than relying on one medication alone.

Your vet may also compare cidofovir with other antiviral options such as ganciclovir eye medication or oral famciclovir. The best choice depends on what part of the eye is affected, how severe the flare is, how often you can medicate safely at home, and whether your cat has had repeated episodes.

Cidofovir ophthalmic is generally well tolerated in cats, but mild local irritation can happen. Some cats show brief stinging, blinking, tearing, or discomfort right after the drop goes in. If your cat already has a very inflamed eye, even a well-chosen medication may seem uncomfortable for a few moments.

More recently, a Cornell retrospective study described an uncommon local ocular toxicity pattern in cats treated with topical 0.5% cidofovir. In that group, 6 of 140 treated cats, or about 4.3%, were suspected to develop a reaction. Reported signs included persistent blepharospasm, discharge, conjunctival redness, swelling, and progressive conjunctival dark pigment change. In those cases, lesions improved after the medication was stopped, and the authors suggested limiting duration to 3 weeks or less when possible.

Because cidofovir used in cats is compounded, your vet may also consider formulation quality, storage, and whether the bottle has become cloudy or contaminated. Do not use eye drops that look abnormal unless your vet or pharmacist tells you they are still safe. If the eye looks worse instead of better, that does not always mean the virus is winning. It can also mean the diagnosis needs to be revisited, the cornea has ulcerated, or the medication itself is causing irritation.

Contact your vet promptly if your cat develops worsening squinting, more discharge, a new brown or black discoloration on the conjunctiva, marked redness, swelling, or reduced appetite from eye pain. See your vet immediately if the cornea looks white, blue, or deeply indented, or if your cat cannot open the eye.

Cidofovir ophthalmic for cats is usually compounded as a 0.5% solution. Merck lists a common feline dose as 1 drop in both eyes every 12 hours for 5 days for ocular herpesvirus, but real-world treatment length often varies based on your vet’s exam findings, whether one or both eyes are affected, and how your cat responds. Some cats need a short course, while others need longer monitoring and a treatment adjustment if signs persist.

Use the medication exactly as your vet prescribes. Wash your hands, or wear disposable gloves if your vet or pharmacist recommends that. Do not let the dropper tip touch the eye, eyelids, fur, or your fingers. If your cat gets more than one eye medication, wait at least 5 to 10 minutes between products unless your vet gives different instructions. If some solution spills onto the fur, wipe it away so your cat is less likely to groom it off.

If you miss a dose, give it when you remember unless it is close to the next scheduled dose. Then skip the missed dose and return to the regular schedule. Do not double up. Because eye medications work best when given consistently, it helps to tie dosing to morning and evening routines.

Storage instructions can vary by compounding pharmacy, so follow the label on your bottle. Ask your vet or pharmacist how long the medication stays usable after dispensing and whether it should be refrigerated. If the solution becomes cloudy, develops particles, or passes its beyond-use date, contact your vet before using it again.

Cidofovir ophthalmic has fewer day-to-day interactions than many oral medications, but it still matters what else is going into your cat’s eye. VCA and PetMD both note that other eye medications should usually be separated by several minutes. That spacing helps reduce washout and gives each product time to contact the eye surface.

One important concern is topical ophthalmic corticosteroids. Wedgewood notes that the antiviral activity of cidofovir may be decreased by topical eye steroids. More importantly, steroids can be risky in cats with corneal ulcers or active herpesvirus keratitis because they may worsen infection or delay healing. That does not mean steroids are never used in feline eye disease, but they should only be used when your vet has examined the eye and decided they fit the diagnosis.

Your vet should also know about all oral medications, supplements, and health conditions, especially kidney disease, pregnancy status, or breeding plans. Although the serious kidney and reproductive toxicities associated with injectable cidofovir in humans have not been reported with routine ophthalmic use in cats, veterinary references still advise caution in pregnant animals and careful handling by pregnant people.

If your cat is not improving, do not add leftover eye medications from a previous problem. Antibiotics, steroid drops, pain medications, lubricants, and antivirals each have different roles, and the wrong combination can make an eye problem harder to treat. Your vet may need to stain the eye, measure tear production, or check for a corneal ulcer before changing the plan.