Previous recommendation/black box warnings indicated that metformin should not be given to men with a serum creatinine greater than 1.5 mg/dL or women with a serum creatinine greater than 1.4 mg/Dl. The main concern here was the increased risk of developing lactic acidosis in patients with high serum levels of metformin due to decreased metformin excretion with decreased renal function. Current guidelines, based on the creatinine clearance (CC) rather than serum creatinine alone, are more liberal, reflecting greater clinical experience with the use of metformin in treating type 2 DM. They include normal use of metformin in patients with CC > 60 ml/min, and cautious use between CC of 45-60. At a creatine clearance of 30-45, patients doing well on metformin can continue taking it, but the dose should generally be reduced by one half. At this CC, patients should not be newly started on metformin. Metformin should not be used in patients with a creatinine clearance < 30 ml/min.