The use of metformin is contraindicated in patients with factors predisposing to lactic acidosis, including impaired renal function. The precise renal thresholds for the safe use of metformin remain uncertain. Improved clinical outcomes with metformin have been reported in observational studies of patients with diabetes and renal impairment (estimated glomerular filtration rate [eGFR] 45 to 60 mL/min). On the basis of these studies, the US Food and Drug Administration (FDA) revised its labeling of metformin, which previously had identified metformin as contraindicated in women and men with serum creatinine levels ≥1.4 mg/dL (124 micromol/L) and ≥1.5 mg/dL (133 micromol/L), respectively [5]. The use of metformin is contraindicated in patients with an eGFR