Valtrex is an antiviral drug. It slows the growth and spread of the herpes virus so that the body can fight off the infection. Valtrex will not cure herpes, but it can lessen the symptoms of the infection. Valtrex is used to treat infections caused by herpes viruses in adults and children. Illnesses caused by herpes viruses include genital herpes, cold sores, shingles, and chickenpox.