Famciclovir is used to treat herpes zoster (shingles; a rash that can occur in people who have had chickenpox in the past). It is also used to treat repeat outbreaks of herpes virus cold sores or fever blisters in people with a normal immune system. Famciclovir is used to treat repeat outbreaks and to prevent further outbreaks of genital herpes (a herpes virus infection that causes sores to form around the genitals and rectum from time to time) in people with a normal immune system. Famciclovir is also used to treat returning herpes simplex infections of the skin and mucus membranes (mouth, anus) in people with human immunodeficiency virus (HIV) infection. Famciclovir is in a class of medications called antivirals. It works by stopping the spread of the herpes virus in the body. Famciclovir does not cure herpes infections and may not stop the spread of herpes virus to other people. However, it may decrease the symptoms of pain, burning, tingling, tenderness, and itching; help sores to heal; and prevent new sores from forming.