Famciclovir for Cats: Uses, Dosage & 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.

Famciclovir is an antiviral medication used in cats most often to help manage illness caused by feline herpesvirus-1 (FHV-1). It is the generic form of the human medication Famvir. In cats, it is used off-label, which means your vet may prescribe it even though the label was not originally written for feline patients.

After your cat takes famciclovir by mouth, the body converts it into an active antiviral compound called penciclovir. This medication does not cure herpesvirus or remove it from the body. Instead, it helps slow viral replication, which can reduce the severity of flare-ups and support healing.

Cats are a little unusual in how they process this drug, so dosing is not a simple copy of human dosing. That is why your vet may choose a specific mg/kg dose, a set tablet size, or a compounded form based on your cat's weight, kidney function, and how severe the signs are.

Famciclovir is generally considered one of the more useful oral antiviral options for cats with herpesvirus disease, especially when eye signs are significant or when topical treatment alone is not enough.

Famciclovir is used most often for clinical signs associated with feline herpesvirus-1, especially conjunctivitis, squinting, eye discharge, corneal inflammation, and upper respiratory flare-ups. Your vet may recommend it when a cat has recurrent herpes-related eye disease or when signs are severe enough that supportive care alone is not likely to be enough.

It may be part of a treatment plan for cats with keratitis, corneal ulcers related to herpesvirus, chronic watery or mucoid eye discharge, sneezing, nasal discharge, and painful ocular flare-ups. In some cats, it is used during stressful periods when herpesvirus tends to reactivate.

Famciclovir is not an antibiotic, so it does not treat bacterial infections directly. Some cats need more than one treatment option at the same time, such as lubricating eye medication, topical antivirals, pain control, or antibiotics if a secondary bacterial infection is also present.

Because feline herpesvirus can become dormant and then flare again later, treatment goals are usually to reduce symptoms, shorten flare-ups, and improve comfort, not to permanently eliminate the virus.

Only your vet should decide the dose. Published veterinary references commonly list famciclovir for cats at about 40 mg/kg by mouth every 8 hours or 90 mg/kg by mouth every 12 hours, and some Merck references list 90 mg/kg by mouth every 8 to 12 hours until about 1 week after clinical signs resolve. In real practice, your vet may tailor that plan based on your cat's size, kidney values, severity of eye disease, and whether treatment is short-term or long-term.

Famciclovir is usually given two to three times daily. Tablets commonly come in 125 mg, 250 mg, and 500 mg strengths, so your vet may have you split tablets or use a compounded formulation if the needed dose is hard to give accurately. If your cat vomits after dosing, your vet may suggest giving it with a small amount of food.

Cats with kidney disease often need a longer dosing interval or dose adjustment, because famciclovir and its metabolites are cleared in part through the kidneys. Long-term use may also lead your vet to recommend periodic bloodwork to monitor tolerance.

If you miss a dose, contact your vet for guidance. In many cases, the advice is to give it when remembered unless the next dose is close, then return to the regular schedule. Do not double up unless your vet specifically tells you to.

Famciclovir is usually well tolerated in cats, but side effects can happen. The most commonly reported problems are gastrointestinal, including vomiting and diarrhea. Some cats may also show reduced appetite, weight loss, or seem less interested in food while taking it.

Other reported effects include increased thirst and increased urination. These signs matter more if your cat already has kidney disease, is older, or is taking other medications that can affect hydration or kidney function.

Call your vet promptly if your cat develops repeated vomiting, marked lethargy, poor appetite lasting more than a day, worsening eye pain, or signs that the original illness is getting worse instead of better. If your cat cannot keep medication down, your vet may want to adjust the plan rather than stopping treatment on your own.

An overdose or major dosing mistake deserves same-day veterinary advice. Bring the bottle or prescription label with you so your vet can confirm the strength and the amount actually given.

Famciclovir does not have a long list of well-defined feline drug interactions, but that does not mean interactions are impossible. The biggest practical concern is using it in cats that also have kidney disease, dehydration, or are taking other medications that may stress the kidneys. In those cases, your vet may adjust the dosing interval and monitor more closely.

Because famciclovir is metabolized by the liver and excreted by the kidneys, your vet should know about all prescription medications, over-the-counter products, supplements, and compounded eye medications your cat receives. That includes pain medications, antibiotics, appetite stimulants, fluids, and antiviral eye drops.

If your cat is on long-term famciclovir, your vet may recommend periodic bloodwork, especially if there are other chronic conditions in the picture. This is less about one specific interaction and more about making sure the whole treatment plan still fits your cat safely.

Never combine leftover human medications with your cat's prescription plan unless your vet has reviewed them. Antiviral drugs are not interchangeable in cats, and some related medications are much less safe for feline patients.