The physiological functions of vitamin B12 determine that vitamin B12 deficiency can cause megaloblastic anemia, hyperhomocysteinemia and neurological damage. For diabetic patients, the role of vitamin B12 is as follows: i. Prevention and control of anemia. Metformin is widely used clinically in the treatment of diabetes as the first-line drug recommended by various guidelines for type 2 diabetes. Long-term application of metformin can cause vitamin B12 deficiency and megaloblastic anemia, and the application of vitamin B12 can prevent megaloblastic anemia. Prevention and control of hyperhomocysteinemia, hyperhomocysteinemia is one of the important risk factors for the development of cardiovascular and cerebrovascular diseases. The application of vitamin B12 can prevent and treat diabetic neuropathy, which is one of the most common chronic complications of diabetes.