Cyancobalamin

"Cyanocobalamin is an especially common vitamer of the vitamin B₁₂ family. It is the most famous vitamer of the family, because it is, in chemical terms, the most air-stable. It is the easiest to crystallize and, therefore, easiest to purify after it is produced by bacterial fermentation, or synthesized in vitro."

"Vitamin B₁₂ is the name for a whole class of chemicals with vitamin B₁₂ activity, and cyanocobalamin is only one of these. Cyanocobalamin usually does not even occur in nature, and is not one of the forms of the vitamin that are directly used in the human body (or that of any other animal). However, animals and humans can convert cyanocobalamin to active (cofactor) forms of the vitamin, such as methylcobalamin.^[1] This process happens by equilibration, as cyanocobalamin slowly loses its cyanide in surroundings that contain no cyanide."