Vitamin B12 is a water soluble with a key role in the normal functioning of the and , and for the formation of . It is one of the eight . It is normally involved in the of every of the body, especially affecting synthesis and regulation, but also synthesis and energy production.

Vitamin B12 is the name for a class of chemically-related compounds, all of which have vitamin activity. It is structurally the most complicated vitamin. of the basic structure of the vitamin can only be accomplished by , but conversion between different forms of the vitamin can be accomplished in the human body. A common synthetic form of the vitamin, , does not occur in nature, but is used in many pharmaceuticals, supplements and as food additive, due to its stability and lower cost. In the body it is converted to the physiological forms, and , leaving behind the , albeit in minimal concentration. More recently, , methylcobalamin and, adenosylcobalamin can also be found in more expensive pharmacological products and food supplements. The utility of these is presently debated.

Historically, vitamin B12 was discovered from its relationship to the disease , which is an autoimmune disease that destroys parietal cells in the stomach that secrete intrinsic factor. Intrinsic factor is crucial for the normal absorption of B12, therefore, a lack of intrinsic factor, as seen in pernicious anemia, causes a vitamin B12 deficiency. Many other subtler kinds of , and their biochemical effects, have since been elucidated.