Vitamin B12 is one of the largest and most complex vitamins. Part of this complexity arises from the fact that B12 is the only vitamin that contains a cobalt metal ion, which is the reason why the term "cobalamin" is used to refer to this vitamin. Several different synthetic and naturally occurring forms of vitamin B12 exist, which have slightly different properties.
Cobalamin and Cyanocobalamin
The term "cobalamin" is the most general term for vitamin B12. Cyanocobalamin was the first synthetic form of cobalamin, created in a laboratory. Most vitamin B12 supplements contain cyanocobalamin. Supplements containing cyanocobalamin are available in several forms including an injectable liquid, a nasal gel or a pill you can take orally. You can purchase cyanocobalamin by itself, in multivitamins or in B-complex vitamins.
Hydroxocobalamin is another synthetic form of vitamin B12. The process to create hydroxocobalamin was discovered later than the process to create cyanocobalamin; consequently, hydroxocobalamin is not used as commonly. Both forms have very similar properties, although they may cause slightly different side effects in different people. Hydroxocobalamin side effects include an acne-like rash, nausea and headaches, and some users have reported severe allergic reactions.
The body converts vitamin B12, given as either cyanocobalamin or hydroxocobalamin, into other active forms of cobalamin to fulfill different functions within the body. One example is methylcobalamin, a slightly modified form of cobalamin that interacts with an enzyme called methionine synthase, a critical enzyme involved in DNA synthesis.
Another active form of vitamin B12 is adenosylcobalamin. This form of cobalamin interacts with a different enzyme called methylmalonyl CoA mutase, a metabolic enzyme involved in breaking down fatty acids in the body. The body converts cobalamin into its different forms, depending on the current needs of the body. Bacteria living in the intestines or liver may create additional forms of cobalamin, although the functions, if any, of these forms are not well understood.