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Famciclovir for veterinary use

by Barbara Forney, VMD

Basic Information

Famciclovir is a prodrug that is used to treat herpes virus infections in humans and cats. Famciclovir is converted into penciclovir through a two-step process in the blood and in the liver. Penciclovir inhibits herpes virus DNA synthesis. The half-life of penciclovir in a herpes-infected cell is considerably longer than the half-life in plasma. Famciclovir/penciclovir is virostatic, not virucidal.

Oral famciclovir is well-absorbed by humans. The absorption and blood levels are much more variable in the cat. This is thought to be due to either poor intestinal absorption or a lack of the appropriate hepatic enzyme for the conversion of famciclovir to penciclovir.


Despite the complex pharmacokinetics of famciclovir/penciclovir, recent studies have demonstrated that the oral administration of famciclovir "improved outcomes for systemic, ophthalmic, clinicopathologic, virologic, and histologic variables in cats experimentally infected with FHV-1." (Thomasy et al. 2011) Earlier work by Malik et al. in 2009 showed improvement in ocular, sinus, and dermatologic manifestations in cats with clinical FHV-1 that were treated with famciclovir.

Side Effects

  • Famciclovir appears to be well-tolerated in cats. Side effects in humans are mostly related to the GI tract.


  • Famciclovir should be used with caution and at a reduced dose in patients with poor renal function.
  • Work in laboratory animals has shown that famciclovir is excreted in maternal milk. It is not known if this is of clinical significance in the cat.

Drug Interactions

  • Concurrent use of famciclovir and probenecid may increase blood levels of penciclovir.


  • Overdose with penciclover can be treated with hemodialysis.
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Therapeutic Class


May Be Prescribed by Vets for:
Herpes virus infection

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About the Author

Dr. Barbara Forney

Barbara Forney, VMD

Dr. Barbara Forney is a veterinary practitioner in Chester County, Pennsylvania. She has a master's degree in animal science from the University of Delaware and graduated from the University of Pennsylvania School of Veterinary Medicine in 1982.

She began to develop her interest in client education and medical writing in 1997. Recent publications include portions of The Pill Book Guide to Medication for Your Dog and Cat, and most recently Understanding Equine Medications published by the Bloodhorse.

Dr. Forney is an FEI veterinarian and an active member of the AAEP, AVMA, and AMWA.