Contraindications include risk factors for metformin-associated lactic acidosis, such as congestive heart failure, hepatic insufficiency, impaired renal function (serum creatinine level greater than 1.5 mg per dL [132.6 ??mol per L] in men or greater than 1.4 mg per dL [123.8 ??mol per L ] in women), and any illness characterized by hypoxia or hypoperfusion.