Sitavig (acyclovir buccal tablet) is a prescription antiviral medication used to treat cold sores (herpes labialis) in adults. It works by releasing acyclovir slowly through the inner lip, targeting the herpes simplex virus at the site of infection to reduce healing time and symptom severity. Applied as a single dose, Sitavig is most effective when used at the first sign of a cold sore. Common side effects may include headache or application-site irritation, and it should be used under medical guidance.