Brivudine (trade names Zostex, Mevir, Brivir, among others) is an antiviral drug used in the treatment of herpes zoster ("shingles"). Brivudine is an analog of the nucleoside thymidine. The active compound is brivudine 5'-triphosphate, which is formed in subsequent phosphorylations by viral (but not human) thymidine kinase and presumably by the nucleoside-diphosphate kinase. Brivudine 5'-triphosphate works because it is incorporated into the viral DNA, but then blocks the action of DNA polymerases, thus inhibiting viral replication. Brivudine is used for the treatment of herpes zoster in adult patients. It is taken orally once daily, in contrast to aciclovir, valaciclovir, and other antivirals. A study has found that it is more effective than aciclovir, but this has been disputed because of a possible conflict of interest on part of the study authors. The drug is contraindicated in patients undergoing immunosuppression (for example because of an organ transplant) or cancer therapy, especially with fluorouracil (5-FU) and chemically related (pro)drugs such as capecitabine and tegafur, as well as the antimycotic drug flucytosine, which is also related to 5-FU. It has not been proven to be safe for children and pregnant or breastfeeding women. The drug is generally well tolerated. The only common side effect is nausea (in 2% of patients). Less common side effects (<1%) include a headache, increased or lowered blood cell counts (granulocytopenia, anemia, lymphocytosis, monocytosis), increased liver enzymes, and allergic reactions. Brivudine is approved for use in a number of European countries including Austria, Belgium, Germany, Greece, Italy, Portugal, Spain, and Switzerland.