The hydrochloride salt of the L-valyl ester of the antiviral drug acyclovir. Orally administered, valacyclovir is rapidly converted to acyclovir which inhibits viral DNA replication after further conversion to the nucleotide analog acyclovir triphosphate by viral thymidine kinase, cellular guanyl cyclase, and a number of other cellular enzymes. Acyclovir triphosphate 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 herpes simplex virus (HSV) compared with varicellla-zoster virus (VZV) is due to its more efficient phosphorylation by HSV thymidine kinase.