It is the active ingredient in the cold sore medications Denavir (NDC 0135-0315-52), Vectavir and Fenivir.

Cellular (human) kinases then add two more phosphate groups, producing the active penciclovir triphosphate.

This activated form inhibits viral DNA polymerase, thus impairing the ability of the virus to replicate within the cell.

First, cellular thymidine kinases phosphorylate the parent form significantly less rapidly than does the viral thymidine kinase, so the active triphosphate is present at much higher concentrations in virally infected cells than in uninfected cells.

The structure and mode of action of penciclovir are very similar to that of other nucleoside analogues, such as the more widely used aciclovir.