Risk of hypoglycaemia (higher with missed meals, alcohol, excessive exercise, elderly). Metformin may rarely cause lactic acidosis—seek urgent care for rapid breathing, severe weakness, abdominal pain, drowsiness. Use with caution in kidney or liver disease; periodic renal function monitoring needed. Temporarily stop metformin during severe dehydration, serious infection, surgery, or before/after iodinated contrast procedures as advised by doctor. Not for type 1 diabetes or diabetic ketoacidosis.