A synthetic fluoro derivative of thiacytidine with potent antiviral activity. Emtricitabine is phosphorylated to form emtricitabine 5'-triphosphate within the cell. This metabolite inhibits the activity of human immunodeficiency virus (HIV) reverse transcriptase both by competing with the natural substrate deoxycytidine 5'-triphosphate and by incorporation into viral DNA causing a termination of DNA chain elongation (due to the lack of the essential 3'-OH group).