ZOVIRAX Cold Sore Cream contains the active ingredient aciclovir. Aciclovir belongs to a group of medicines called antivirals. They are used for treating infections caused by the Herpes simplex virus. ZOVIRAX Cold Sore Cream is used to treat cold sores on the lips and face. Cold sores are caused by Herpes simplex virus infection. ZOVIRAX Cold Sore Cream does not get rid of Herpes simplex virus infection from your body, so it cannot prevent cold sores from coming back. It does, however, reduce the severity and duration of cold sore symptoms when they appear.