Aciclovir cream contains the antiviral aciclovir at 5% concentration, designed for topical application to cold sores on the lips and face. It works by inhibiting herpes simplex virus DNA polymerase, blocking viral replication in infected skin cells. When applied at the first sign of a cold sore (tingling, itching, or redness), it can reduce the severity and duration of the outbreak. It is most effective when started during the prodromal phase before a blister fully forms. Topical aciclovir treats the lesion but does not eliminate the dormant virus or prevent future outbreaks.