Indication used to treat recurrent cold sores on the lips and face from various herpesvirus invections.mechanism of action penciclovir has in vitro activity against herpes simplex virus types 1 (hsv-1) and 2 (hsv-2). in cells infected with hsv-1 or hsv-2, viral thymidine kinase phosphorylates penciclovir to a monophosphate form. the monophosphate form of the drug is then converted to penciclovir triphosphate by cellular kinases. the intracellular triphosphate of penciclovir is retained in vitro inside hsv-infected cells for 10-20 hours, compared with 0.7-1 hour for acyclovir. in vitro studies show that penciclovir triphosphate selectively inhibits viral dna polymerase by competing with deoxyguanosine triphosphate. inhibition of dna synthesis of virus-infected cells inhibits viral replication. in cells not infected with hsv, dna synthesis is unaltered. resistant mutants of hsv can occur from qualitative changes in viral thymidine kinase or dna polymerase. the most commonly encountered