Vitamin B-12 is a vitamin, one of eight B vitamins which is important for the normal functioning of the brain and nervous system, and for the formation of blood. It is normally involved in the metabolism of every cell of the body, especially affecting DNA synthesis and regulation, but also fatty acid synthesis and energy production.

Vitamin B-12 is the name for a class of chemically-related compounds, all of which have vitamin activity. It is structurally the most complicated vitamin. Biosynthesis of the basic structure of the vitamin can only be accomplished by bacteria, but conversion between different forms of the vitamin can be accomplished in the human body. A common synthetic form of the vitamin, cyanocobalamin, 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, methylcobalamin and adenosylcobalamin, leaving behind the toxic cyanid, albeit in minimal concentration. More recently methylcobalamin and adenosylcobalamin can also be found in high quality supplements.

Historically, vitamin B-12 was discovered from its relationship to the disease pernicious anemia, which was eventually discovered to result from an effective lack of this vitamin due to problems with the mechanisms in the body which normally absorb it. Many other subtler kinds of vitamin B12 deficiency, and their biochemical effects, have since been elucidated.