Famciclovir (INN; /ˌfæmˈsaɪkloʊˌvɪər/) is a guanosine analogue antiviral drug used for the treatment of various herpesvirus infections, most commonly for herpes zoster (shingles). It is a prodrug form of penciclovir with improved oral bioavailability. Famciclovir is marketed under the trade name Famvir (Novartis).