RESUMO
Lactic acidosis is a feared complication of metformin therapy. In our article we describe 2 case reports of patients treated with metformin, who developed this complication. In the first case, which was fatal, cummulation of lactate was a result of acute kidney failure caused by diarrhea. In the second patient, lactic acidosis developed in the terrain of preexisting chronic kidney disease, when dyspepsia and decreased fluid intake caused progression into acute kidney failure. In this case, treatment of lactic acidosis was successful. Death of the first patient was probably caused by the presence of serious comorbidities and other complications which developed early after her addmission to intensive care unit. Lactic acidosis can be prevented by strict avoidance of metformin use in case of contraindications and interruption of its use during intercurrent disease.