The medication enters your body in an inactive form. It only becomes active inside cells that are infected by the virus because it requires a specific viral enzyme called thymidine kinase to trigger its first chemical transformation. Healthy cells lack this enzyme, meaning the drug leaves them completely alone.

Once the virus inadvertently activates the drug, your body’s natural cellular enzymes complete the transformation, turning it into acyclovir triphosphate. This active compound selectively concentrates inside the infected cells at levels up to 100 times higher than in healthy tissue.