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 <30 mL/min, and the initiation of metformin is not recommended in patients with an eGFR between 30 and 45 mL/min. For patients taking metformin whose eGFR falls below 45 mL/min, the benefits and risks of continuing treatment should be assessed, whereas metformin should be discontinued if the eGFR falls below 30 mL/min. For patients with eGFR between 30 and 60 mL/min, we typically reduce the metformin dose by half (no more than 1000 mg per day), although there are no data to support this approach. source UpToDate