Entecavir (ETV), sold under the brand name Baraclude, is an antiviral medication used in the treatment of hepatitis B virus (HBV) infection. In those with both HIV/AIDS and HBV antiretroviral medication should also be used. Entecavir is taken by mouth as a tablet or solution. Common side effects include headache, nausea, high blood sugar, and decreased kidney function. Severe side effects include enlargement of the liver, high blood lactate levels, and liver inflammation if the medication is stopped. While there appears to be no harm from use during pregnancy, this use has not been well studied. Entecavir is in the nucleoside reverse transcriptase inhibitors (NRTIs) family of medications. It prevents the hepatitis B virus from multiplying by blocking reverse transcriptase. Entecavir was approved for medical use in 2005. It is on the World Health Organization's List of Essential Medicines. Available as a generic medication in the United States. Medical uses Entecavir is mainly used to treat chronic hepatitis B infection in adults and children two years and older with active viral replication and evidence of active disease with elevations in liver enzymes. It is also used to prevent HBV reinfection after liver transplant and to treat HIV patients infected with HBV. Entecavir is weakly active against HIV, but is not recommended for use in HIV-HBV co-infected patients without a fully suppressive anti-HIV regimen as it may select for resistance to lamivudine and emtricitabine in HIV. The efficacy of entecavir has been studied in several randomized, double-blind, multicentre trials. Entecavir by mouth is effective and generally well tolerated treatment. Pregnancy and breastfeeding No adequate and well-controlled studies exist in pregnant women. Side effects The majority of people who use entecavir have little to no side effects. The most common side effects include headache, fatigue, dizziness, and nausea. Less common effects include trouble sleeping and gastrointestinal symptoms such as sour stomach, diarrhea, and vomiting. Serious side effects from entecavir include lactic acidosis, liver problems, liver enlargement, and fat in the liver. Laboratory tests may show an increase in alanine transaminase (ALT), hematuria, glycosuria, and an increase in lipase. Periodic monitoring of hepatic function and hematology are recommended. Mechanism of action Entecavir is a nucleoside analog, or more specifically, a deoxyguanosine analogue that belongs to a class of carbocyclic nucleosides and inhibits reverse transcription, DNA replication and transcription in the viral replication process. Other nucleoside and nucleotide analogues include lamivudine, telbivudine, adefovir dipivoxil,