Acyclovir is an antiviral medication. It prevents the multiplication of viruses in human cells. This stops the virus from producing new viruses and clears up your infection. It treats infections caused by the herpes virus (herpes simplex), including: cold sores. genital herpes. It treats cold sores around the mouth (caused by herpes simplex), shingles (caused by herpes zoster), and chickenpox.This medication is also used to treat outbreaks of genital herpes. In people with frequent outbreaks, acyclovir is used to help reduce the number of future episodes.Acyclovir is an antiviral drug. However, it is not a cure for these infections. The viruses that cause these infections continue to live in the body even between outbreaks. Acyclovir decreases the severity and length of these outbreaks. It helps the sores heal faster, keeps new sores from forming, and decreases pain/itching. This medication may also help reduce how long pain remains after the sores heal. In addition, in people with a weakened immune system, acyclovir can decrease the risk of the virus spreading to other parts of the body and causing serious infections.