Metformin is safe in patients with adequate renal function. The current prescribing information for metformin is that its use is permitted with a serum creatinine ≤ 1.4 mg/dL in women and ≤ 1.5 mg/dL in men which is equivalent to a GFR ≥ 60 ml/min (5). It is excreted by the kidneys and therefore can potentially accumulate in patients with renal failure. Metformin works by inhibiting gluconeogenesis—a process that utilizes lactic acid to produce glucose. Thus, with accumulation of this medication, it could predispose patients to lactic acidosis. The incidence of lactic acidosis with metformin is extremely low. A systematic review of 347 randomized trials and prospective cohort studies representing 70,490 patient-years of metformin use and 55,451 patient-years in the non-metformin group revealed no cases of lactic acidosis. Almost half of the studies allowed inclusion of patients with a serum creatinine above 1.5 mg/dL, and 97% of the studies allowed inclusion of patients with at least one contraindication to metformin therapy (6). Another trial evaluated the risk of lactic acidosis with metformin in 393 patients with renal insufficiency, with serum creatinine levels of 1.5 to 2.5 mg/dl (mean level 1.8 mg/dl) and found no cases of lactic acidosis over four years of the trial duration (7).