Valaciclovir, also spelled valacyclovir, is an antiviral medication used to treat outbreaks of herpes simplex or herpes zoster (shingles). It is also used to prevent cytomegalovirus following a kidney transplant in high risk cases. It is taken by mouth. Common side effects include headache and vomiting. Severe side effects may include kidney problems. Use in pregnancy appears to be safe. It is a prodrug, which works after being converted to aciclovir in a person's body. Valaciclovir was patented in 1987 and came into medical use in 1995. It is on the World Health Organization's List of Essential Medicines. It is available as a generic medication. In 2020, it was the 119th most commonly prescribed medication in the United States, with more than 5million prescriptions. Medical uses Valaciclovir is used for the treatment of HSV and VZV infections, including: Oral and genital herpes simplex (treatment and prevention) Reduction of HSV transmission from people with recurrent infection to uninfected individuals Herpes zoster (shingles): the typical dosage for treatment of herpes is 1,000 mg orally three times a day for seven consecutive days. Prevention of cytomegalovirus following organ transplantation Prevention of herpesviruses in immunocompromised people (such as those undergoing cancer chemotherapy) Chickenpox in children (ages 2–18) It has shown promise as a treatment for infectious mononucleosis and is preventively administered in suspected cases of herpes B virus exposure. Bell's palsy does not seem to benefit from using valaciclovir as its only treatment. Adverse effects Common adverse drug reactions (≥1% of people) associated with valaciclovir are the same as for aciclovir, its active metabolite. They include: nausea, vomiting, diarrhea and headache. Infrequent adverse effects (0.1–1% of patients) include: agitation, vertigo, confusion, dizziness, edema, arthralgia, sore throat, constipation, abdominal pain, rash, weakness and/or renal impairment. Rare adverse effects (<0.1% of patients) include: coma, seizures, neutropenia, leukopenia, tremor, ataxia, encephalopathy, psychotic symptoms, crystalluria, anorexia, fatigue, hepatitis, Stevens–Johnson syndrome, toxic epidermal necrolysis and/or anaphylaxis. Pharmacology Valaciclovir is a prodrug, an esterified version of aciclovir that has greater oral bioavailability (about 55%) than aciclovir. It is converted by esterases to the active drug, aciclovir, and the amino acid valine via hepatic first-pass metabolism. Aciclovir is selectively converted into a monophosphate form by viral thymidine kinase, which is more effective (3000 times) in phosphorylation of aciclovir than cellular thymidine kinase. Subsequently, the monophosphate form is further phosphorylated into a disphosphate by cellular guanylate kinase and then into the active