Emtricitabine S-oxide is a potential impurity found in commercial preparations of emtricitabine. Emtricitabine is a nucleoside analog and an inhibitor of HIV-1 and hepatitis B reverse transcriptase with antiviral activity. It inhibits replication of HIV-1 (ECs = 10-20 nM) and hepatitis B (ECs = 10-40 nM) in various cell lines. Emtricitabine inhibits duck hepatitis B viral DNA synthesis when administered pre- and post-viral inoculation in primary duck hepatocytes. In vivo, emtricitabine (3-30 mg/kg) reduces viremia and hepatic woodchuck hepatitis virus (WHV) replication in a dose-dependent manner in a woodchuck model of chronic WHV infection. Formulations containing emtricitabine have been used in the treatment of HIV infection.