Abstract. Acyclovir is effective against herpes simplex virus (HSV) type 1 and type 2, varicella zoster virus (VZV) and Epstein-Barr virus (EBV). Acyclovir is taken up by the virus infected cell, converted to acyclovir triphosphate (by viral thymidine kinase) and this inhibits viral DNA synthesis by inhibiting viral DNA polyme rases and causing DNA chain termination. Like acyclovir, valaciclovir, ganciclovir, valganciclovir, famciclovir, penciclovir and idoxuridine get converted to their respective monophosphate, diphosphate and then triphosphate derivatives which are the active metabolites. The active metabolite inhibits viral DNA/RNA polymerases and interferes with viral replication [1].