Famciclovir is an antiviral medication primarily used to treat infections caused by certain viruses, including herpes simplex and varicella-zoster. It works by inhibiting the replication of viral DNA, helping to reduce the severity and duration of outbreaks. This medication is often prescribed for conditions such as genital herpes, cold sores, and shingles. Famciclovir is typically taken orally and is most effective when started at the first sign of an outbreak. Common side effects may include headache, nausea, and diarrhea.