Famciclovir is an oral prodrug antiviral agent of the guanine nucleoside analogue class. It is rapidly converted in vivo to its active metabolite, penciclovir, which selectively inhibits viral DNA replication in herpesvirus-infected cells. It is a cornerstone therapy for herpes zoster (shingles) and recurrent episodes of genital herpes in immunocompetent patients, and for the suppression of recurrent genital herpes. It is also used for herpes labialis (cold sores) and for the management of herpesvirus infections in immunocompromised patients.
Adult: Herpes Zoster: 500 mg every 8 hours for 7 days. Recurrent Genital Herpes: 125 mg twice daily for 5 days OR 1000 mg twice daily for 1 day. Suppression of Genital Herpes: 250 mg twice daily. Herpes Labialis: 1500 mg as a single dose.
Note: May be taken with or without food. Swallow tablet whole with a glass of water. For optimal results in treating an episode, initiate therapy at the earliest sign or symptom (e.g., prodrome of tingling, itching, burning).
Famciclovir is a prodrug that is rapidly converted to penciclovir. Penciclovir is phosphorylated by viral thymidine kinase (in infected cells) to penciclovir monophosphate, and subsequently by cellular kinases to penciclovir triphosphate. This active form competitively inhibits viral DNA polymerase, thereby inhibiting viral DNA synthesis and replication.
Pregnancy: Pregnancy Category B. Animal studies have shown no direct harm, but there are no adequate and well-controlled studies in pregnant women. Use only if the potential benefit justifies the potential risk to the fetus. Consult an obstetrician.
Driving: May cause dizziness, somnolence, or confusion. Patients should be cautioned about operating machinery or driving until they know how the drug affects them.
| Probenecid | Increases plasma concentration of penciclovir by reducing its renal tubular secretion. May increase risk of side effects. | Moderate |
| Other nephrotoxic drugs (e.g., Aminoglycosides, NSAIDs, Cyclosporine) | Increased risk of renal dysfunction. Monitor renal function closely. | Moderate |
| Digoxin | Famciclovir may slightly increase digoxin levels. Monitor digoxin levels. | Mild |
| Theophylline | May slightly decrease theophylline clearance. Clinical significance is likely minimal. | Mild |