Get emergency medical help if you have signs of an allergic reaction: hives; difficult breathing; swelling of your face, lips, tongue, or throat.
Some people using entecavir develop lactic acidosis, which can be fatal. Get emergency medical help if you have even mild symptoms such as:
unusual muscle pain;
feeling cold;
trouble breathing;
feeling dizzy, light-headed, tired, or very weak;
stomach pain, vomiting; or
fast or irregular heartbeats.
Entecavir may also cause severe liver symptoms. Call your doctor at once if you have: swelling around your midsection, nausea, stomach pain, loss of appetite, dark urine, clay-colored stools, jaundice (yellowing of the skin or eyes).
Common side effects may include:
nausea;
dizziness, tired feeling; or
headache.
This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.
 Entecavir side effects (more detail)
