Entecavir is used to treat long-term hepatitis B infection. Hepatitis B is an infection of the liver caused by the hepatitis B virus. Long-term infection can cause liver damage, rarely liver cancer, and liver failure. Entecavir helps to decrease the amount of the hepatitis B virus in your body. It is unknown if this medication lowers your chance of getting liver cancer or liver damage. Entecavir is an antiviral that belongs to a class of drugs known as hepatitis B virus nucleoside reverse transcriptase inhibitors.