A synthetic analogue of 2′-deoxyguanosine, entecavir is a nucleoside reverse transcriptase inhibitor with selective antiviral activity against hepatitis B virus. It is phosphorylated intracellularly to the active triphosphate form, which competes with deoxyguanosine triphosphate, the natural substrate of hepatitis B virus reverse transcriptase, inhibiting every stage of the enzyme’s activity, although it has no activity against HIV. Enteclavir is used for the treatment of chronic hepatitis B. It has a role as an EC 2.7.7.49 (RNA-directed DNA polymerase) inhibitor and an antiviral drug.