Methylcobalamin (or hydroxycobalamin) is a man-made form of vitamin B12 used to treat deficiency of this vitamin. B12 helps your body use fat and carbohydrates for energy and make new protein. It is also important for normal red blood cells and nerves. Most people get enough vitamin B12 in their diet, but a deficiency may occur in people over 40 on certain medications or with health conditions like poor digestion or intestinal problems, low stomach acid, infection, or cancer. Serious vitamin B12 deficiency may result in anemia, fatigue, or nerve problems. Even borderline low B12 levels have been associated with cognitive decline. There is no known toxicity with use of B12.