The biguanide metformin is an oral anti-hyperglycemic agent used in the treatment of patients with non-insulin-dependent diabetes mellitus (NIDDM). Metformin is an important addition to the drug therapy options available for those patients because it reduces blood glucose levels predominantly by decreasing hepatic glucose production and release and also by increasing peripheral tissue sensitivity to insulin; it does not stimulate insulin secretion from the beta cells in the pancreas. Metformin also has a potentially beneficial effect by reducing serum lipid levels. Its glycemic control is similar to that of the sulfonylureas and is effective as monotherapy or in combination with sulfonylureas or insulin. Unlike sulfonylureas and insulin, it does not cause a gain in body weight, and when used as monotherapy, it does not cause hypoglycemia. The most common side effects associated with metformin are mild, transient, gastrointestinal symptoms, which are usually self-limiting. These side effects can be minimized by initiating metformin therapy at a low dose and gradually titrating upward, and by taking metformin with meals. Lactic acidosis caused by metformin is rare, and the risk of this complication may be diminished by the observance of prescribing precautions and contraindications that avoid accumulation of metformin or lactate in the body. In patients who are not getting the desired effect with sulfonylureas, it is useful to combine sulfonylureas with metformin therapy. Metformin should be considered a first-line agent, particularly in obese and/or hyperlipidemic NIDDM patients.