Valacyclovir HCl is an antiviral prescription medicine approved by the U.S. Food and Drug Administration (FDA) to treat and/or prevent certain types of viral infections. It is commonly used to treat herpes virus infections, including herpes labialis (cold sores), herpes zoster (shingles), and genital herpes in adults. It is also used to treat chickenpox and cold sores in children. Valacyclovir works by slowing the growth and spread of the herpes virus to help the body fight the infection. It is important to take valacyclovir exactly as directed by a doctor and to inform the doctor about any medical conditions, allergies, or medications being taken. Some possible side effects of valacyclovir include headache, nausea, vomiting, upset stomach, and dizziness.