Valaciclovir is an antiviral medicine used to treat genital herpes by slowing the herpes simplex virus, helping sores heal faster and reducing pain and recurrent outbreaks. Take it exactly as prescribed and start treatment as soon as symptoms begin for the best results. Common side effects include headache, nausea, stomach pain and dizziness. Drink plenty of fluids during treatment and remember that it does not completely prevent transmission to others.