Acyclovir is an antiviral medication used to treat infections caused by HSV-1, including cold sores. It works by interfering with the virus’s ability to replicate, helping to reduce the severity and duration of symptoms. For cold sores, it can be taken orally (typically 400 mg five times a day for 5 days) or applied topically as a cream. Starting treatment at the first sign of symptoms, like tingling or itching, provides the best results. Acyclovir may also be used daily at a lower dose to prevent frequent outbreaks. While it doesn’t cure HSV-1, it helps manage and control the condition effectively.