Famciclovir
Purine-derived, synthetic, acyclic purine nucleoside analog
Also known as: Famvir · AV 42810 · BRL 42810 · famciclovirum
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Feline herpes virus (FHV-1) | 125 mg (total dose) | PO | 2-3 times a day | — | 🌎 NA |
| Adjunctive treatment of FHV-1 rhinotracheitis | 31.25 mg (1/4 of one 125 mg tablet) | PO | q12h | 14 days | 🌎 NA |
| Chronic, recurrent and/or severe herpes viral infection | Kittens: 1/8th of one 125 mg tablet; adult cats: 1/4 of one 250 mg tablet | PO | once daily (q24h) | Kittens: 2 weeks; adult cats: 3 weeks | 🌎 NA |
| Feline herpes virus (FHV-1) | 1/4 of one 125 mg tablet | PO | twice daily (q12h) | 10-14 days; may continue once daily for up to 30 days | 🌎 NA |
| Adjunctive treatment (with interferon and lysine) of herpes virus-associated ulcerative facial dermatitis & stomatitis | 125 mg | PO | q12h | — | 🌎 NA |
| Management of feline herpesvirus (FHV-1) infections | 90 mg/kg | PO | q12h | — | 🌍 EU |
- Feline herpes virus (FHV-1): Author's choice. Wide range of doses used.
- Adjunctive treatment of FHV-1 rhinotracheitis: Has not been evaluated for longterm therapy.
- Chronic, recurrent and/or severe herpes viral infection: Dose range is variable!
- Management of feline herpesvirus (FHV-1) infections: Dose recommendations have varied because of complex and non-linear pharmacokinetics in cats.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Famciclovir is an oral antiviral medication primarily used in veterinary medicine to treat feline herpesvirus-1 (FHV-1) infections.
- It is a prodrug that is converted in the body to its active antiviral form, penciclovir.
- Because cats have very low levels of hepatic aldehyde oxidase (the enzyme responsible for this conversion), they often require significantly higher doses compared to humans or other species to achieve therapeutic concentrations.
- It appears to be well-tolerated for short-term use (2-3 weeks) in cats, though long-term safety data is still limited.
- Clinical Pearl: While relatively expensive, the availability of generic formulations has made this a more accessible option for severe or refractory FHV-1 cases.
Mechanism of action
Famciclovir is a prodrug that undergoes biotransformation to become active: Famciclovir → hepatic aldehyde oxidase → Penciclovir
Once inside a virus-infected cell: Penciclovir → viral thymidine kinase → Penciclovir monophosphate → cellular kinases → Penciclovir triphosphate
- The triphosphate form competes with deoxyguanosine triphosphate to inhibit herpes virus DNA polymerase.
- This selectively and effectively halts herpes viral DNA synthesis and replication.
- Viral resistance can occur via mutation.
Safety & warnings
Contraindications
- Known hypersensitivity to famciclovir or penciclovir
Adverse effects
- Cats: Anorexia and polydipsia (noted at high doses of 90 mg/kg PO q8h)
- Humans: Nausea, vomiting, diarrhea, headache, neutropenia, renal failure
- Gastrointestinal signs (vomiting, diarrhea, anorexia)
Precautions
Caution in Renal Impairment: Famciclovir is primarily excreted by the kidneys. Dosage adjustments are highly recommended for patients with renal dysfunction (e.g., CrCl < 40 mL/min in humans).
- Pregnancy/Nursing: FDA Category B in humans. Animal studies up to 1,000 mg/kg/day showed no embryo/fetal effects. Excreted in the milk of rats; clinical significance for nursing offspring is unclear.
Drug interactions
Can reduce the amount of penciclovir excreted by the kidneys, leading to increased penciclovir plasma levels
Monitoring
- Clinical efficacy (resolution of ocular/respiratory signs)
- Adverse effects (most likely GI upset, anorexia, polydipsia)
- Consider occasional CBCs and serum creatinine to monitor for neutropenia or renal dysfunction if using the drug chronically
- Haematology
- Biochemistry
- Urinalysis (especially in cats with concurrent disease or on long-term therapy)
Pharmacokinetics
Absorption
In humans, well absorbed orally but undergoes extensive first-pass metabolism. Food decreases peak levels but does not impact clinical efficacy. In cats, pharmacokinetics are complex; after 62.5 mg PO, penciclovir peak levels were less than the in vitro IC-50 for FHV-1.
Distribution
Penciclovir (active metabolite) is only marginally bound to plasma proteins.
Metabolism
Converted to penciclovir. In humans, requires oxidation by hepatic aldehyde oxidase. Cats possess little hepatic aldehyde oxidase, necessitating higher dosages.
Elimination
Excretion is primarily via renal mechanisms.
Overdose
Little information is available regarding acute toxicity. Supportive treatment is recommended. Penciclovir can be removed by hemodialysis.
Available products
Formulations
- Oral Tablets (film-coated): 125 mg, 250 mg, 500 mg
- 125 mg tablet
- 250 mg tablet
- 500 mg tablet
- 100 mg/ml oral paste
Veterinary
- 100 mg/ml oral paste (often compounded or specific to certain markets)
Human-labeled
- Famciclovir Oral Tablets (film-coated) 125 mg, 250 mg, & 500 mg: Famvir (Novartis); generic
- Famvir 125 mg tablets
- Famvir 250 mg tablets
- Famvir 500 mg tablets
Regulatory status
POM (Prescription Only Medicine)
POM (Prescription Only Medicine)
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Famciclovir tablets should be stored at room temperature (15-30°C).
Client information
- Administration: You can give this medication with or without food. If it causes stomach upset, giving it with a small treat or meal may help.
- Expectations: Feline herpesvirus (FHV-1) is a lifelong condition. This medication helps control the active flare-up and reduce symptoms (like sneezing, eye discharge, and ulcers), but it does not cure the virus. Recurrences may still happen during times of stress.
- Monitoring: Watch for signs of decreased appetite or increased thirst. Since experience with this drug in cats is still growing, please report any unusual side effects to your veterinarian immediately.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.