Acyclovir is an antiviral drug. It slows the growth and spread of the herpes virus so that the body can fight off the infection. Acyclovir lessens the symptoms of these infections and shortens the length of time you are sick. Acyclovir is used to treat infections caused by herpes viruses. Illnesses caused by herpes viruses include genital herpes, cold sores, shingles, and chicken pox. Acyclovir may also be used for purposes other than those listed in this medication guide. For more information on Acyclovir Zovirax, click here.