Hydroxocobalamin, also known as hydroxycobalamin or vitamin B12a is a member of the class of compounds known as cobalamin derivatives. These organic compounds contain a corrin ring, a cobalt atom, and a nucleotide moiety. More specifically, cobalamins are characterized by a porphyrin-like corrin nucleus that contains a single cobalt atom bound to a benzimidazolyl nucleotide and a variable residue (R) group. The variable R group gives rise to the four most commonly known cobalamins: cyanocobalamin, methylcobalamin, 5-deoxyadenosylcobalamin, and hydroxycobalamin. Hydroxocobalamin is found in all living organisms ranging from microbes to plants to animals. However, it is only synthesized in microbes and plants. As humans cannot make hydroxocobalamin, it is required in the diet, making it an essential nutrient (i.e. a vitamin).  Vitamin B12 is more abundant in animal products, especially meat, fish, eggs and dairy products although it is found in all fruits and vegetables in lower concentrations. In the serum, hydroxocobalamin and cyanocobalamin are believed to function as storage or transport forms of vitamin B12, whereas methylcobalamin and 5-deoxyadenosylcobalamin are the active forms of the coenzyme required for cell growth and replication. Hydroxocobalamin is often called vitamin B12 and is the natural form of vitamin B12, however, vitamin B12 refers to a group of cobalamins that are available in the human body in a variety of mostly interconvertible forms. Together with folate, cobalamins are essential cofactors required for DNA synthesis in cells where chromosomal replication and division are occurring. Cyanocobalamin is usually converted to hydroxocobalamin in the serum, whereas hydroxocobalamin is converted to either methylcobalamin or 5-deoxyadenosyl cobalamin. Cobalamins circulate bound to serum proteins called transcobalamins (TC) and haptocorrins. As a supplement hydroxocobalamin is used to treat vitamin B12 deficiency including pernicious anemia. It is also used in the prevention and treatment of vitamin B12 deficiency arising from alcoholism, malabsorption, tapeworm infestation, celiac disease, hyperthyroidism, hepatic-biliary tract disease, persistent diarrhea, ileal resection, pancreatic cancer, renal disease, prolonged stress, vegan diets, macrobiotic diets, and other restrictive diets. Other uses include treatment for cyanide poisoning, Leber's optic atrophy, and toxic amblyopia. Hydroxocobalamin is practically insoluble in water. Hydroxocobalamin was first isolated in 1949. It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system. Hydroxocobalamin is available as a generic medication. The wholesale cost in the developing world is about 0.12 To 0.84 USD per dose. Commercially, it is made from one of a number of different types of bacteria.