Aciclovir, also known as acyclovir, is an antiviral medication used primarily in the treatment of infections caused by the herpes simplex virus (HSV) and the varicella-zoster virus (VZV). It belongs to the class of drugs known as nucleoside analogues, which work by interfering with the replication of viral DNA.

Aciclovir is commonly used to treat conditions such as genital herpes, cold sores (herpes labialis), shingles (herpes zoster), and chickenpox (varicella). It is available in various formulations, including oral tablets, topical creams, ointments, and intravenous injections