REPORTS of the clinical efficacy of acyclovir, a selective and specific inhibitor of herpesvirus replication, appeared in the literature a decade ago. Acyclovir has become the most widely prescribed and clinically effective antiviral drug available, and research on new indications is ongoing. This review will summarize our current understanding of the indications as well as the limitations of acyclovir therapy. Mechanism of Action and Spectrum of Antiviral Activity Acyclovir (9-[2-hydroxyethoxymethyl]guanine) is a selective inhibitor of the replication of herpes simplex virus (HSV) types 1 and 2 and varicella–zoster virus.1,2 It is converted by virus-encoded thymidine kinase to its monophosphate derivative,… © 1992, Massachusetts Medical Society. All rights reserved.