Metformin (dimethylbiguanide) is an antihyperglycemic agent with a variety of extrapancreatic actions that counter insulin resistance in patients with type II diabetes. Recent studies have emphasized the drug's ability to curb excessive hepatic glucose production, reduce fatty acid oxidation, and improve the glucose-fatty acid cycle. Because the actions of metformin are modest in potency and do not raise insulin concentrations, the drug does not produce overt hypoglycemia. Lack of weight gain, improvements in the dyslipidemia, and potential benefits in terms of vascular parameters are useful advantages of the drug. The long-term glycemic control achieved with metformin is similar to that achieved with a sulfonylurea, and the different modes of action of these two types of drugs enable them to exert an additive effect when prescribed in combination.