Entecavir is an antiviral medication used primarily for treating chronic hepatitis B virus (HBV) infection. It works by inhibiting the viral DNA polymerase enzyme, which is essential for the hepatitis B virus to replicate and produce new viruses. By blocking this enzyme, entecavir reduces the amount of the virus in the body and helps prevent liver damage. It is taken orally and is effective in adults and children aged two years and older. Entecavir is known to have a high genetic barrier to resistance and is generally well tolerated. It helps improve liver function and slow the progression of liver disease caused by HBV.