Metformin is a very commonly used metformin hypoglycemic drug in clinical practice, but some patients may develop lactic acidosis after applying metformin, which is a very rare but very serious side effect of metformin. Since metformin can inhibit lactic acid, which is produced as glucose, it can have a hypoglycemic effect, but in the process, it may cause lactic acid to accumulate, resulting in an increase in serum lactate levels, which can then develop into lactic acidosis. Patients with lactic acidosis may experience nausea, abdominal pain, decreased appetite, dizziness and drowsiness; in severe cases, they may experience headache, weakness of the limbs, cyanosis of the lips, decreased blood pressure and increased heart rate; they may also experience impaired consciousness, shock or even coma. Patients should go to the hospital promptly for treatment if the above conditions occur.