The most serious and life-threatening side effect seen in patients using metformin is metabolic acidosis. If there is a history of diabetes in patients with high anion gap metabolic acidosis who present to the emergency department, metformin use should be questioned. In our case, metformin-related lactic acidosis was detected in a 67-year-old female patient who applied to the emergency department with a complaint of palpitations. The patient's kidney function tests were within normal range and he did not describe excessive drug intake. Despite the treatments administered in the emergency room, the patient's blood gas lactate level continued to increase and his acidosis deepened. After other causes of lactic acidosis were excluded, the patient was consulted with internal medicine and transferred to the intensive care unit. With this case report, we tried to draw attention to the fact that the patient group for which metformin will be used in the treatment should be carefully selected and that lactic acidosis, which is a fatal side effect, should be considered in the differential diagnosis and treatment should be started in those using this drug.