Why cyanocobalamin?
It' s manmade and may be less effective?
All B12 is man made. The only natural B12 is found in meat and dairy products. Before man made B12 was created, Pernicious Anemia patients were kept alive by eating half a pound of mostly raw liver
per day.
Cyanocobalamin is the most shelf stable of the versions of B12. It doesn't need refrigeration. But it should not be taken by anyone with a family history of Leber's Optic disease. Because the cyanide molecule in this form is already bound to cobalamin, it will not poison you, however it's not a good choice if you have a higher than normal level of cyanide exposure, such as from heavy smoking.
Hydroxocobalamin is not quite a shelf stable as cyanocobalamin. This form is also the form used to treat cyanide poisoning. Cyanide binds to cobalamin naturally.
Methylcobalamin is less shelf stable than the others. It will turn into hydroxocobalamin with time or if not stored properly.
Adenosylcobalamin is the least shelf stable version and is virtually impossible to make into a supplement.
It is widely believed (though not actually proven) that hydroxocobalamin lasts longer in the body than cyanocobalamin. Both versions are converted by the body into both methylcobalamin and adenosclcobalamin as the body needs them.
One crucial thing to remember about all of the versions of B12 on the market is that
all of the forms have to travel the same conversion pathways to become active B12 in the body. Methylcobalamin is not simply absorbed as is and immediately utilized. It actually seems to require one additional conversion step than hydroxocobalamin does.
Right now it is not known why any particular version seems to work better for some people than for others. More research is desperately needed.