Abstract : Numerous patients with type 2 diabetes have renal impairment, especially in the elderly population. Metformin, the first choice oral glucose-lowering agent, is classically contraindicated in case of chronic kidney disease of stages 3-5 (creatinine clearance < 60 ml/min/1.73 m²), because of a risk of accumulation of the biguanide that may lead to lactic acidosis. Hence numerous patients with some degree of renal impairment are being treated with metformin in clinical practice, apparently without any harm. In contrast, several observational studies have shown that they may clinically benefit from this therapy, including with a significant reduction of all-cause mortality when compared to patients not receiving metformin. Thus, an increasing number of physicians plea for revisiting the official criteria of contraindication to the use of metformin in case of renal insufficiency. The present paper discusses this controversy and insists upon the mandatory cautions to be taken when using metformin in a diabetic patient with moderate (stage 3) chronic kidney disease (metformin being contraindicated in case of severe renal impairment - stages 4-5).