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Famciclovir (Famvir)
Revised: April 15, 2024
Published: April 12, 2018

(For veterinary information only)

The size of the tablet/medication is NOT an indication of a proper dose. Never administer any drug without your veterinarian's input. Serious side effects or death can occur if you use drugs on your pet without your veterinarian's advice. 

It is our policy not to give dosing information over the internet.

Brand name: Famvir

Available as 125 mg, 250 mg, and 500 mg tablets


Medications against bacteria have been available since World War II and many lives have been saved because of them. Unfortunately, medications agains viruses have been substantially more elusive. Only in recent years have meaningful medications been developed. Famciclovir is a human medication developed to treat herpesvirus infections in humans. It turns out to have activity against herpesvirus in cats. Famciclovir works by interfering with DNA replication in the herpesvirus. This prevents the virus from multiplying, but famciclovir cannot actually kill the virus, nor can it eliminate a herpes infection.

How this Medication is Used

If famciclovir is used at an appropriate dose, it will not only treat the viral respiratory symptoms, but it will get into the tears and treat the viral conjunctivitis as well. Twice daily oral dosing is sufficient, and a two to three-week course is typical. 

Famciclovir can be given with or without food. If a dose is accidentally skipped, pick it up at the next scheduled dose. Do not double up on doses.

Side Effects

Feline side effects have mostly been diarrhea, appetite loss, and increased thirst/urination. The most common side effect in cats is upset stomach (vomiting or diarrhea) which occurs in 17% of patients. Giving the medication with food can mitigate this side effect. Other side effects have been increased thirst (5% of feline patients) and drooling (which may related to bitter taste). 

Rarely a reduction in certain white blood cell levels can occur. A dose reduction has been recommended for cats with known insufficient kidney function. 

Humans report upset stomach and headache as the most common side effects. We do not know if this holds true for cats. Some cats have shown appetite loss.

The famciclovir tablet is somewhat large and a large cat may require more than one pill per dose. Compounding into a liquid product is possible but the bad taste of the medication is hard to overcome.

Concerns and Cautions

Famciclovir should be given on an empty stomach for best absorption but can be given with food if there is an issue with an upset stomach.

Dosages should be reduced in patients with reduced kidney function.

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