Famciclovir (Famvir) is a nucleoside analogue drug that is approved for the treatment of shingles (varicella zoster virus) and herpes simplex viruses 1 and 2. It is also active against hepatitis B virus in the test tube.

Famciclovir was derived from another anti-herpes drug, penciclovir, which was not well absorbed when taken in tablet form. Famciclovir is well absorbed by mouth. Few side-effects have been reported, apart from occasional rash, stomach upsets and headache, although in laboratory tests an increased incidence of breast tumours was seen in female rats given high doses for the duration of their lives. An independent review committee concluded that humans are not considered at increased risk of breast tumours as a result of taking famciclovir.

Famciclovir is effective in treating herpes simplex and reducing viral shedding in HIV-positive patients^[1][2]. Combined with 3TC (lamivudine, Epivir), it also showed promising results as a treatment for hepatitis B infection in a preliminary 48-week study of HIV-negative patients^[3]. However, a study assessing the impact of adding famciclovir to a failing hepatitis B regimen containing 3TC in HIV-positive patients failed to show a long-term benefit in reducing hepatitis B levels^[4].

Famciclovir does not interact with any of the currently available protease inhibitors or non-nucleoside reverse transcriptase inhibitors (NNRTIs).

Famciclovir is manufactured by GlaxoSmithKline. It was formerly known by the codename BRL39123A.