Metformin is recommended as the treatment of choice in patients with type 2 diabetes mellitus because of its efficacy, general tolerability and low cost. Recent guidelines have extended the use of metformin to patients with Chronic Kidney Disease (CKD) up to stage III. However, in the recent literature, cases of MALA (metformin-associated lactic acidosis) are increasingly reported. MALA is the most dangerous side effect of the drug, with an incidence rate of 2-9 cases per 100000 person-years of exposure. We report on two patients with accidental metformin overdose, severe lactic acidosis and acute kidney injury. In both cases, the usual dose of metformin was inappropriate with respect to the level of kidney dysfunction (CKD stage III). As both patients met the criteria for renal replacement therapy in metformin poisoning, they were treated effectively with sustained low-efficiency dialysis until normalization of serum lactate and bicarbonate values. Clinical status and kidney function improved and both patients could be discharged from the hospital.