Metformin is a biguanide that can used alone or in combination with sulfonylureas or insulin in the treatment of non-insulin-dependent diabetes mellitus (NIDDM). Since biguanides do not increase pancreatic insulin secretion, they are referred to as antihyperglycemic agents, as opposed to hypoglycemic agents. Biguanides reduce hyperglycemia by increasing, insulin sensitivity, decreasing glucose absorption, and inhibiting hepatic gluconeogenesis. Advantages of metformin include achieving glycemic control without exacerbating weight gain or hyperinsulinemia and beneficially affecting serum cholesterol concentrations. Although metformin has the potential to cause lactic acidosis, the incidence is significantly lower compared with phenformin. Risk factors for lactic acidosis include renal serum creatinine > 1.5 mg/dL and cardiovascular, pulmonary, and hepatic disease. Metformin should be temporarily discontinued prior to surgery and before administration of radiologic intravenous contrast, and in patients with sepsis, severe gastrointestinal disease, trauma, and acute cardiovascular events.