Famciclovir (INN) () is a guanine 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 (Schering-Plough).

On August 24, 2007, the United States Food and Drug Administration approved the first generic version of famciclovir. Generic Famciclovir Tablets (125 mg, 250 mg, 500 mg) are manufactured by Kfar-Sava, Israel-based TEVA Pharmaceuticals.

Famciclovir is indicated for the treatment of herpes zoster (shingles). Treatment of herpes simplex virus 2 (genital herpes) and herpes labialis (cold sores) in immunodeficient patients. Suppression of recurring episodes of herpes simplex virus 2. Treatment of recurrent episodes of herpes simplex in HIV patients.

Famciclovir comes as an oral tablet in 125mg, 250mg, and 500mg dosage forms. To treat herpes zoster, Famciclovir is taken 500mg every 8 hours (three times a day) for 7 days. To treat genital herpes it is taken twice a day for 1 day. To treat herpes labialis (cold sores), famciclovir is given as a single 1500mg oral dose. It is also sometimes given in one single large dose as opposed to several days of scheduled small doses. Among other side effects, Famciclovir may cause an upset stomach. Take famciclovir with food or milk.

Side effects: mild to extreme stomach upset, headaches, mild fever

Several studies in humans and mice provide evidence that early treatment with Famciclovir soon after the first infection with herpes can significantly lower the chance of future outbreaks of herpes. Use of Famciclovir in this manner, known as post-exposure prophylaxis, has been shown to reduce the amount of latent virus in the neural ganglia.The effects of antiviral therapy on the distribution of herpes simplex virus type 1 to ganglionic neurons and its consequences during, immediately following and several months after treatment"[1]"Famciclovir and Valaciclovir Differ in the Prevention of Herpes Simplex Virus Type 1 Latency in Mice: a Quantitative Study"[2]"Persistence of Infectious Herpes Simplex Virus Type 2 in the Nervous System in Mice after Antiviral Chemotherapy"[3]" A review of human subjects treated for five days with Famciclovir 250 mg three times daily during their first herpes episode found that only 4.2 percent experienced a recurrence within six months after the first outbreak, a five-fold decrease compared to the 19 percent recurrence in acyclovir-treated patients.Observation May Indicate A Possible Clinical Effect On Latency"[4]" Despite these promising results, early Famciclovir treatment for herpes in this or similar dosage regimes has yet to find mainstream adoption. As a result, some doctors and patients have opted for off-label use. One suggested regime is Famciclovir at 10–20 mg/kg per day for 5–14 days, with treatment to commence as soon as possible after the first herpes infection(not the first symptoms or outbreak), and the most effective time for initiating treatment to be five days or less after the first herpes infection.