Metformin is one of the oldest oral treatments to reduce hyperglycaemia in people with diabetes. Gastrointestinal side effects are common, and metformin should be used with caution in patients with renal impairment because of the slight risk of lactic acidosis. In the United Kingdom Prospective Diabetes Study (UKPDS) patients treated with metformin had a significant reduction in myocardial infarction and mortality that was not demonstrated in patients treated with sulphonylureas or insulin. The fact that metformin significantly reduces cardiovascular events plus reduces weight has meant that metformin is the drug of first choice in guidelines for the treatment of type 2 diabetes. There are no longer concerns about using metformin in patients with chronic heart failure, other than in patients with associated renal failure, or during episodes of acute left ventricular failure when metformin should be temporarily stopped.