Metformin is a biguanide extensively used in the treatment of type 11 diabetes mellitus. Between the nocive effects of the metformin emphasizes tha lactic acidosis because of its low frecuency but potential severity. The diagnosis of the poisoning due to metformin is based on the coexistence of lactic acidosis and one or more of the risk factors. The development of lactic acidosis in metformin poisoning is associated to a range of 50-80% of mortality.