Risk of hypoglycemia (higher with missed meals, alcohol, intense exercise, elderly); carry fast-acting glucose. Metformin can rarely cause lactic acidosis—seek urgent care for rapid breathing, severe weakness, abdominal pain, hypothermia. Avoid/use with caution in kidney impairment; periodic renal function tests needed. Use caution in liver disease, heart failure, dehydration, severe infection, or hypoxia. Temporarily stop before/after iodinated contrast procedures and major surgery as advised by doctor.