Metformin is currently used in many national and international guidelines for the diagnosis and treatment of diabetes as a first-line drug and a basic drug in combination for the control of hyperglycemia in patients with type 2 diabetes. The main pharmacological effect of metformin is to lower blood glucose by reducing hepatic glucose output and improving peripheral insulin resistance. The hypoglycemic efficacy of metformin was found to be a 1%-1.5% reduction in glycosylated hemoglobin and a reduction in body weight, with metformin showing a dose-dependent effect between the 500-2000 mg/d dose range. Metformin also reduces cardiovascular events and death in obese patients with type 2 diabetes. Metformin alone generally does not cause hypoglycemia, but the risk of hypoglycemia can be increased when metformin is used in combination with insulin or insulin agonists, and its main adverse effects are gastrointestinal reactions, which can be effectively reduced by starting with small doses and gradually increasing the dose. Metformin is contraindicated in patients with renal insufficiency, hepatic insufficiency, severe infection, hypoxia, or major surgery. In patients who are taking metformin, when eGFR is between 45-59 ml/min.1.73m^2, the dose can be reduced appropriately and continued, but close monitoring of renal function is required. Metformin should be temporarily discontinued when contrast examinations are done, such as when iodinated contrast is used. Long-term use of metformin requires attention to vitamin B12 deficiency.