Avoid Cyanocobalamin form of B12 as its bioavailability is very poor. (fullhealthsecrets.com)
-- Vitamin B12 (must be sublingual Methylcobalamin and not Cyanocobalamin!) (fullhealthsecrets.com)
Most other B12 supplements use the synthetic form called Cyanocobalamin which the body needs to convert to Methylcobalamin before it can be used. (naturalhealthgoodies.com)