Tramadol serious side effects. Tell your doctor right away if any of these unlikely but serious side effects occur: mental/mood changes (such as agitation, confusion, hallucinations), severe stomach/abdominal pain, difficulty urinating, signs of your adrenal glands not working well (such as loss of appetite, unusual tiredness, weight loss). Get medical help right away if any of these rare but serious fainting, seizure, slow/shallow breathing, severe drowsiness/difficulty waking up.