Acyclovir is a synthetic purine analog derived from guanine. It exerts its effects on the herpes simplex virus (HSV) and varicella-zoster virus by interfering with DNA synthesis through phosphorylation by viral thymidine kinase and subsequent inhibition of viral DNA polymerase, thereby inhibiting viral replication. It is effective against HSV-1 and 2, varicella-zoster virus, Epstein-Barr virus, herpesvirus simiae, and cytomegalovirus. Acyclovir may be administered intravenously, orally, or topically.