Acyclovir is a synthetic purine nucleotide analogue that is used in a variety of infections caused by herpes simplex virus and can be used in primary and recurrent skin, mucosal, and external genital infections, as well as HSV infections occurring in immunodeficient patients. Acyclovir is the drug of choice for the treatment of HSV encephalitis, reducing morbidity and decreasing mortality, preferentially over acepromazine, and can also be applied to infections such as herpes zoster, EBV, and varicella in immunodeficient individuals with concurrent varicella. It can be applied topically to the skin, and acyclovir has low skin absorption. It is effective in herpes simplex virus encephalitis, herpes keratitis, and external genital unity infections, AIDS, cytomegalovirus infections, and chronic viral hepatitis B.