Cyanide or a form of Cyanide, which in tiny quantities don't do any damage but it does increase stress in the liver. I couldn't find the article I originally read about this but a quick Google search confirmed it's in a certain form of b12 found in 99% of multis it's one of the signs to look out for a good multi. http://www.greenmedinfo.com/blog/99-vitamin-b12-market-contains-cyanide
cyanocobalamin is totally safe, its used in injection IM, most of the articles with this 'problem' are just trying to sell you expensive versions of the same thing.
https://en.m.wikipedia.org/wiki/Cyanocobalamin "In cases of cyanide poisoning the patient is given hydroxocobalamin, which is a precursor to cyanocobalamin. The hydroxocobalamin binds with the cyanide ion and forms cyanocobalamin, which can then be excreted by the kidneys" Science is a wonderful thing.