Clevudine, marketed as Levovir and Revovir, is a synthetic pyrimidine analogue. It is an antiviral drug used in the treatment of hepatitis B. In cells, clevudine is phosphorylated into its active metabolites, clevudine monophosphate and clevudine triphosphate. The triphosphate metabolite competes with thymidine for incorporation into viral DNA, causing DNA chain termination and inhibiting the function of the hepatitis B virus (HBV) DNA polymerase (reverse transcriptase). Clevudine has a long half-life and exhibits a significant reduction in covalently closed circular DNA (cDNA), so that the patient is less likely to relapse after cessation of treatment.