Vitamin B12 is required for the proper function and development of the brain, nerves, blood cells, and many other parts of the body. It is one of the most important vitamins to have in your diet every day to help keep your body active and healthy. B12 helps support adrenal function, maintain a healthy nervous system, are necessary for key metabolic processes, and are important to DNA synthesis. Vitamin B12 is taken by the mouth to treat and prevent vitamin B12 deficiency, a condition in which vitamin B12 levels in the blood are too low.