Vitamin B12 is essential for healthy nerve and blood cells. It is also important for making DNA. In addition, the vitamin helps to break down homocysteine, an amino acid that can cause problems if it builds up to high levels. In particular, it is thought that high homocysteine may contribute to problems such as cardiovascular disease or blood clots.

In food, vitamin B12 is bound to protein and is released by stomach acid. Then it binds to a substance known as intrinsic factor (which is produced in the stomach) and is absorbed into the bloodstream through the small intestine. Any problems in either of these steps can result in a deficiency.

To overcome such problems in absorption, vitamin B12 injections (or, more recently, nasal sprays) are often used. Although it was once thought that injections were the only way to treat deficiencies due to low or absent intrinsic factor, it is now known that oral forms can be just as effective, although much higher doses are required. Small amounts of vitamin B12 can be absorbed after oral consumption, even without any intrinsic factor.