Is cyanocobalamin toxic?
Potassium cyanide is added to the cobalamins produced by bacteria to give rise to cyanocobalamin. CN is strongly bonded to Co and will not easily dissociate, thereby making this form of B12 inactive. Therefore, it has been argued that cyanocobalamin is not a good vitamin supplement and other variants such as hydroxycobalamin or methylcobalamin should be used. Displacement of CN from the complex would be energy expensive and even if the reaction is efficient the amount of cyanide released would not be in the toxic concentration. The CN-Co bond is so strong that hydroxycobalamin is used as an antidote for cyanide poisoning; it forms cyanocobalamin by displacing OH with CN. 

 Source: Wikipedia and the cross references.