Acyclovir is a synthetic analog of the purine nucleoside, guanosine, with potent antiviral activity against herpes simplex viruses type 1 and 2, varicella-zoster virus and other viruses. After conversion in vivo to the active metabolite acyclovir triphosphate, acyclovir competitively inhibits viral DNA polymerase, incorporates into and terminates the growing viral DNA chain, and inactivates viral DNA polymerase. The greater antiviral activity of acyclovir against HSV compared to VZV is due to its more efficient phosphorylation by the thymidine kinase of HSV.