The biguanide metformin (dimethylbiguanide) is an oral antihyperglycaemic agent used in the management of non-insulin-dependent diabetes mellitus (NIDDM). It reduces blood glucose levels, predominantly by improving hepatic and peripheral tissue sensitivity to insulin without affecting the secretion of this hormone. Metformin also appears to have potentially beneficial effects on serum lipid levels and fibrinolytic activity, although the long term clinical implications of these effects are unclear. Metformin possesses similar antihyperglycaemic efficacy to sulphonylureas in obese and nonobese patients with NIDDM. Additionally, interim data from the large multicentre United Kingdom Prospective Diabetes Study (UKPDS) indicated similar antihyperglycaemic efficacy for metformin and insulin in newly diagnosed patients with NIDDM. Unlike the sulphonylureas and insulin, however, metformin treatment is not associated with increased bodyweight. Addition of metformin to existing antidiabetic therapy confers enhanced antihyperglycaemic efficacy. This may be of particular use in improving glycaemic control in patients with NIDDM not adequately controlled with sulphonylurea monotherapy, and may serve to reduce or eliminate the need for daily insulin injections in patients with NIDDM who require this therapy. The acute, reversible gastrointestinal adverse effects seen with metformin may be minimised by administration with or after food, and by using lower dosages, increased slowly where necessary. Lactic acidosis due to metformin is rare, and the risk of this complication may be minimised by observance of prescribing precautions and contraindications intended to avoid accumulation of the drug or lactate in the body. Unlike the sulphonylureas, metformin does not cause hypoglycaemia. Thus, metformin is an effective antihyperglycaemic agent which appears to improve aberrant plasma lipid and fibrinolytic profiles associated with NIDDM. Possible long term clinical benefits of this drug with regard to cardiovascular mortality and morbidity are not yet established but are being assessed in a major ongoing study. Since metformin does not promote weight gain or hypoglycaemia it should be considered first-line pharmacotherapy in obese patients with NIDDM inadequately controlled by nonpharmacological measures. Metformin appears similarly effective for the pharmacological management of NIDDM in nonobese patients.