Cidofovir in Cats

Cidofovir is a prescription antiviral medication your vet may use as a compounded eye drop for cats with ocular disease linked to feline herpesvirus-1, often called FHV-1. In practice, it is most often used for cats with conjunctivitis, keratitis, squinting, eye discharge, or recurring herpes flare-ups that affect the surface of the eye. It is not an antibiotic, so it does not treat bacterial infections by itself.

One reason cidofovir is used in cats is convenience. Compared with some other topical antivirals, it can often be given less frequently because it stays active inside cells for longer. That can make treatment more realistic for pet parents caring for cats who resist frequent eye medication. Because there is no widely available veterinary-labeled ophthalmic product, cidofovir is usually prepared by a compounding pharmacy from the injectable human drug.

Cidofovir is an off-label medication in cats, which is common in veterinary medicine. Off-label does not mean inappropriate. It means your vet is using a human medication in a way supported by veterinary references and clinical experience. The best candidate, treatment length, and follow-up plan depend on whether your cat has active herpesvirus disease, a corneal ulcer, secondary bacterial infection, chronic scarring, or another eye condition that can look similar.

See your vet immediately if your cat has severe eye pain, keeps the eye closed, has a cloudy cornea, has a visible ulcer, or suddenly seems unable to see. Eye disease can worsen quickly, and some cats need stain testing, pressure testing, or referral to a veterinary ophthalmologist before a medication plan is chosen.

Cidofovir works by interfering with viral DNA polymerase, an enzyme DNA viruses need to copy themselves. In plain language, it slows viral replication so the virus has a harder time multiplying in the tissues of the eye. That is why it is used for feline herpesvirus eye disease rather than for routine irritation, allergies, or bacterial conjunctivitis.

A practical advantage of cidofovir is that it accumulates inside cells and has a relatively long intracellular effect. That longer activity is why veterinary references commonly list less frequent dosing than with some other antiviral eye medications. For many pet parents, twice-daily treatment is easier than giving drops every four to six hours, and that can improve consistency.

Cidofovir does not remove herpesvirus from the body. Cats infected with FHV-1 often remain lifelong carriers, and flare-ups can return during stress, illness, or immune changes. The goal of treatment is usually to reduce active viral disease, improve comfort, and help the eye heal, not to cure the carrier state.

Because several eye diseases can overlap, your vet may pair cidofovir with other treatments based on exam findings. Some cats also need lubricating drops, pain control, oral antiviral medication such as famciclovir, or treatment for a secondary bacterial infection. The right plan depends on what your vet sees on the cornea and conjunctiva that day.

Topical cidofovir is generally used because systemic cidofovir can cause serious toxicity, especially kidney injury. In cats, the ophthalmic form is intended to limit whole-body exposure, but that does not mean side effects are impossible. Mild eye irritation after dosing, temporary squinting, tearing, or redness may occur. Some cats also resent the drops enough that administration becomes stressful.

If a cat licks excess medication from the face or chews the bottle, toxic effects are possible. That is why your vet may advise blotting away runoff after each dose and storing the medication securely. Pet parents who are pregnant or trying to become pregnant should ask your vet and pharmacist about handling precautions, because veterinary references note teratogenic concern with cidofovir exposure.

Cidofovir should be used carefully, or avoided, in certain cats. VCA notes caution in cats with kidney disease, breeding animals, or those with known hypersensitivity to the drug. Since compounded products can differ in beyond-use dates and storage directions, always follow the label from the dispensing pharmacy rather than assuming all bottles are handled the same way.

Call your vet promptly if your cat seems more painful, the eye becomes cloudier, discharge becomes thick or green, the cornea looks white or blue, or vision seems worse. Those changes may mean the underlying eye disease is progressing, a corneal ulcer is present, or another diagnosis is involved.

Cidofovir for cats is usually dispensed as a compounded 0.5% ophthalmic solution. Merck’s veterinary dosing table lists a common feline regimen as 1 drop in both eyes every 12 hours for 5 days for ocular herpesvirus, but real-world treatment plans vary. Your vet may prescribe it for one eye or both eyes, and the duration may be longer if flare-ups are recurrent or corneal disease is slow to improve.

Do not change the dose or stop early without checking in. Eye medications often need to be continued for the full course your vet recommends, even if the eye looks better after a few days. Stopping too soon can allow signs to rebound. If your cat is on more than one eye medication, ask your vet what order to use and how long to wait between products. PetMD advises waiting at least five minutes before applying another eye medication.

Good technique matters. Wash your hands, avoid touching the dropper tip to the eye or fur, and give the exact number of drops prescribed. If extra liquid runs onto the face, gently blot it away. Many cats do better when wrapped in a towel, placed on a non-slip surface, and rewarded right after treatment. Your veterinary team can demonstrate a low-stress method.

Because cidofovir is compounded, storage directions are not always identical from one pharmacy to another. Some bottles may need refrigeration, while others may not. Use the pharmacy label as the source of truth, check the beyond-use date, and request a refill early if your cat has chronic herpes flare-ups and the medication must be specially prepared.

There are no widely reported everyday drug interactions for topical cidofovir in cats that look like the long interaction lists seen with many oral medications. Still, that does not mean interactions are irrelevant. The biggest practical issue is how cidofovir fits into a broader eye treatment plan. If several drops or ointments are used too close together, one medication can dilute or wash away another.

Your vet may prescribe cidofovir alongside lubricants, antibiotic eye medications, anti-inflammatory therapy, or oral famciclovir depending on the diagnosis. That combination can be appropriate, but the timing matters. In general, eye drops are spaced apart, and ointments are often applied last. Ask your vet for a written schedule if your cat is receiving multiple eye products.

Compounded medications also add another layer. Different pharmacies may use different vehicles, bottle sizes, and storage instructions. Let your vet know about every prescription, over-the-counter eye product, supplement, or leftover medication you are considering. Never reuse old steroid eye drops unless your vet specifically tells you to. Steroids can be risky in some cats with corneal ulcers or active infection.

If your cat has kidney disease, is pregnant, is intended for breeding, or has had a prior reaction to antiviral medication, mention that before treatment starts. Even though ophthalmic cidofovir is topical, your vet still needs the full medical picture to decide whether it is the right option or whether another antiviral plan makes more sense.