Adefovir is a prescription medicine used to treat (chronic) infections with hepatitis B virus. Adefovir works by blocking reverse transcriptase, an enzyme that is crucial for the hepatitis B virus (HBV) to reproduce in the body. It is approved for the treatment of chronic hepatitis B in adults with evidence of active viral replication and either evidence of persistent elevations in serum aminotransferases (primarily ALT) or histologically active disease. The main benefit of adefovir over lamivudine (the first NRTI approved for the treatment of hepatitis B) is that it takes a much longer period of time before the virus develops resistance to it. Adefovir dipivoxil contains two pivaloyloxymethyl units, making it a prodrug form of adefovir. It is used for treatment of hepatitis Band herpes simplex virus infection. It is a failed treatment for HIV.