Clinically there is no such thing as what is the worst side effect of taking entecavir, side effects vary from person to person and cannot be generalized. At present, the more common adverse reactions are diarrhea, nausea, vomiting, headache, dizziness, etc., generally will not affect the patient’s health and quality of life. Entecavir is a commonly used antiviral drug, which can effectively inhibit the replication of hepatitis B virus in patients, and is mainly used for the treatment of chronic hepatitis B. At present, entecavir is more common in the clinic. At present, the more common clinical adverse reactions of entecavir include diarrhea, nausea, vomiting, dizziness, headache, drowsiness, insomnia, hepatic discomfort, rash, etc. A very small number of patients may experience lactic acidosis. Entecavir has more adverse reactions, and patients should strictly follow the doctor’s instructions to use the drug correctly. It is contraindicated for those who are allergic to entecavir or any of the ingredients in the preparation. Pregnant and lactating women, children under 16 years of age should not use. If the above symptoms of adverse reactions are more serious, patients need to consult a doctor and choose whether to stop the drug or change the drug under the guidance of the doctor.