Famciclovir (Famvir) for Cats


  • Drug Name: Famciclovir
  • Common names: Famvir, Ezovir, Fambir, Famcino, Famcir, Fanclomax, Fanle, Microvir, Oravir, Pentavir, Penvir, Pixin and Virovir
  • Drug Type: Virostatic agent
  • Used For: Feline herpesvirus (FHV-1)
  • Species: Humans, cats, horses
  • Administered: Tablet

Famciclovir (fam-sye-clow-veer) is a prescription antiviral medication used in the treatment of feline herpesvirus, a common viral infection that produces upper respiratory infections (URI’s). The drug is not approved for use in cats by the Food and Drug Administration but is routinely prescribed extra-label.

The drug is virustatic and not virucidal, which means it inhibits viral replication but does not kill the virus or eliminate the infection. Once a cat is infected with the feline herpesvirus, he or she will have it for life, after the initial infection, the cat recovers, but the virus lies dormant in the body evading the immune system, during periods of stress or corticosteroid treatment, the virus may re-activate.

Mechanism of action

Famciclovir is converted to penciclovir which targets virus-infected cells where it is rapidly converted into penciclovir-triphosphate (mediated via virus-induced thymidine kinase) which inhibits herpesvirus DNA polymerase via competition with deoxyguanosine triphosphate (dGTP), a nucleoside triphosphate, and a nucleotide precursor used in cells for DNA synthesis.


Famciclovir is available in 125 mg, 250 mg and 500 mg tablets.

Feline herpesvirus type 1 (FHV-1) – Oral doses range from 40 – 90 mg per kilo every 8 to 12 hours.

Administer without food for optimal absorption. If the cat vomits or develops nausea (loss of appetite, drooling, lip-smacking), give with a small amount of food.


Famciclovir is contraindicated in cats who are hypersensitive to it or penciclovir.

Use with caution or at a lower dose in cats with kidney disfunction.

Side effects

Famciclovir is well-tolerated and side effects are not well documented. High doses may cause loss of appetite, increased thirst and reduction in white blood cells (neutropenia) in some cats. Side effects in humans include nausea, vomiting, diarrhea and headache.


The veterinarian may recommend a complete blood count and creatinine to monitor for neutropenia or renal dysfunction.

Missed dose

If you miss a dose, administer as soon as possible unless the next dose is due soon. If you miss more than one dose, consult your cat’s veterinarian. Do not give your cat two doses at once.

Overdose/acute toxicity

Little information is available regarding famciclovir overdose in cats, however, supportive treatment is recommended. Penciclovir can be removed by hemodialysis.

Always seek immediate veterinary attention if your cat has ingested a higher than the prescribed dose of famciclovir.

Drug interactions

Probenecid: May reduce the amount of penciclovir which is excreted by the kidneys and lead to an increase in penciclovir plasma levels.


Store at room temperature between 15-30 C.

Frequently asked questions

Is famciclovir effective against calicivirus?

One small study found that famciclovir was effective in treating two cats with calicivirus.

Is famciclovir available as a liquid or paste?

It is possible to compound famciclovir into an oral suspension or paste.

How long should I give my cat famciclovir?

Administer as prescribed by your veterinarian. Treatment duration recommendations vary between 2-4 weeks in most cases. Kittens may require treatment for two weeks.


Plumb, D.C. (2018). Veterinary drug handbook. Ames, Iowa: Iowa State Press.


  • Julia Wilson, 'Cat World' Founder

    Julia Wilson is the founder of Cat-World, and has researched and written over 1,000 articles about cats. She is a cat expert with over 20 years of experience writing about a wide range of cat topics, with a special interest in cat health, welfare and preventative care. Julia lives in Sydney with her family, four cats and two dogs. Full author bio

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