The Good shit'
Most drug stores sell Vitamin B12 in a form known as Cyanocobalamin. This form is the cheapest junk available. Its toxic, and requires conversion to a biologically active form.
Cyanocobalamin is not biologically active until converted to methylcobalamin, which also means releasing its cyanide. Cyanide can be toxic because it binds the iron (F3+) portion of cytochrome oxidases, preventing its reduction. This binding blocks electron transport and interrupts cellular respiration. Symptoms of sublethal cyanide toxicity include hypotension, tachypnea, and tachycardia.
Williams, H.L. et al. "Studies of cobalamin vehicle for the renal excretion of cyanide anion." Journal of Laboratoyr and Clinical Medicine, 1990, 116(1): 37-44.