The most common form of vitamin B-12 (cobalamin) in supplements is cyanocobalamin and although this form includes a cyanide molecule, it is very safe. Why? Even at a very high dose, it would provide about a thousand times less cyanide than is toxic, and the cyanide is excreted in the urine.
B-12 is also commonly available in supplements as methylcobalamin. Supplements with both types of B-12 are included in the B Vitamin Supplements Review
and the Multivitamin Review
Some non-authoritative websites claim that methylcobalamin is better absorbed or more bioavailable than cyanocobalamin, but there is no clinical evidence supporting this claim. Other sites suggest that methylcobalamin supplements cannot yield one of the important, active metabolites of B-12, but this is not correct. More information about this, and another form of B-12, hydroxocobalamin, is found in the "Cobalamin (B-12)" section of the B Vitamin Supplements Review >>