Entecavir monohydrate is a carbocyclic nucleoside anti-hepatitis B drug developed by Bristol-Myers Squibb, which was approved by FDA in the United States in March 2005. Entecavir is a selective HBV DNA polymerase inhibitor and blocks all three stages of HBV replication. Clinical studies have demonstrated that Entecavir monohydrate has strong anti-HBV activity, significantly superior to lamivudine and remains effective in patients who have failed lamivudine treatment. Although the safety and tolerability of Entecavir monohydrate is similar to that of lamivudine, the incidence of clinical resistance is much lower than that of lamivudine. So Entecavir monohydrate will be the first choice for hepatitis B treatment.