At present, there are four kinds of vitamin B12 in the world API market. They are hydroxycobalamin, Mecobalamin, cyanobalamin and cobalamin adenosine. This is because the central ion in B12 molecule has different functional groups, so it is called different names. Hydroxycobalamin is called hydroxycobalamin because the central ion of B12 is connected with hydroxyl group. Among the four B12 molecules, hydroxycobalamin has the best water solubility and slow metabolism in urine, which is also known as long-acting cobalamin. Although the absorption rate is not comparable to that of cyanocobalan and Mecobalamin, its concentration in the eye is very high, which can greatly relieve visual fatigue and nourish the optic nerve. Therefore, it has a certain curative effect on amblyopia caused by poisoning. In addition, hydroxycobalamin can quickly combine with free cyanide to form nontoxic cyanobalamin in aqueous solution, so its other therapeutic effect is the antidote of cyanide poisoning.