Acyclovir is a synthetic purine nucleoside analogue. It is mainly used for various infections caused by herpes simplex virus. It can be used for initial or recurrent skin, mucous membrane, external genital infections and HSV infections in people with immunodeficiency. It is the first choice for the treatment of HSV encephalitis, and it is better than arabinosine to reduce morbidity and mortality. It can also be used for herpes zoster, Epstein-Barr virus, and immunodeficiency patients with chickenpox and other infections. Topically applied to the skin only, acyclovir is less absorbed by the skin.