Vitamin B12, abbreviated as VB12, is also called cobalamin, one of the B vitamins, is a kind of cobalt-containing corrin complex organic compound, and the trivalent cobalt is located in the center of the corrin ring plane similar to porphyrin. It is the largest and most complex vitamin molecule found so far, and it is also the only vitamin that contains metal ions; its crystal is red, so it is also called red vitamin. Plants do not contain VB12, nor can they make VB12. Liver is the best source of VB12, followed by milk, meat, eggs, fish and so on.

VB12 is a coenzyme necessary for the synthesis of ribonucleic acid and deoxyribonucleic acid. The lack of VB12 in the body can cause pathological changes in the nervous system such as peripheral nerves and central encephalopathy. The prototype of the family of natural cobalt coordination compounds is called corrinoids. The analog of vitamin B12 differs only in the beta ligand of cobalt, called cobalamin. It is almost entirely synthesized by bacteria. Dietary sources include fish, meat, liver, and dairy products; there is little or no cobalamin in plants. Methylcobalamin and cobalamin are used as cofactors of enzymes, which are converted by the body into their biologically active forms. Severe deficiency can lead to megaloblastic anemia and/or nerve damage. The discovery of vitamin B12 should first be attributed to people's research work in the treatment of a pernicious anemia named after the British physician Addison.

The pernicious anemia was an incurable disease at the time. In 1926, Minot and Murphy discovered that the disease could be cured by ingesting a large amount of raw liver. For this reason, they won the 1934 Nobel Prize in Physiology and Medicine. Following the discovery of Minot and Murphy, people successfully cured this pernicious anemia with liver concentrate. In 1948, Riches in the United States and Smith and Parker in the United Kingdom separated a red crystal from liver concentrate and named it vitamin B12. In the same year, R. West of Columbia University confirmed that vitamin B12 injection can be used to effectively treat pernicious anemia. Then in 1955, D. Hodgkin et al. used X-ray analysis to determine the crystal structure of vitamin B12. He also won the 1964 Nobel Prize in Chemistry. In 1972, Woodward successfully completed the total synthesis of vitamin B12.