Acyclovir is an antiviral drug that decreases the severity and length of these outbreaks. It helps the sores to heal faster, keeps new sores from forming, and decreases pain or itching. It also helps reduce how long pain remains after the sores heal. However, it is not a cure for these infections for the viruses that cause these infections continue to live in the body even between outbreaks. Acyclovir also decreases the risk of the virus spreading to other parts of the body for people who have weak immune system.