Cyanocobalamin is the commercial synthetic form of B12 that was patented more than 40 yrs ago. For many years it was the only one. The body cannot utilize this form of B12 and has to
remove the Cyano portion (cyanide) and methylate it in order for the tissues to use it properly.

For many people cyano versions are adequate. It is the only version most doctors know about. It is in all their reference books, still.

But there are some people who:
1) do not have the genetic capability to methylate folic acid and B12 properly. These people will not improve with cyano version.

2) some people do not have the genetic ability to handle CYANIDE in any form. These patients may react to cyanocobalamin with worsening of symptoms (toxicity).

Luckily we have a very inexpensive oral version of methylcobalamin available without an RX (OTC) which can easily be used. But many doctors don't know it even exists.

1mg orally daily can cost patients about 9 cents a day.
Since it is easy to obtain and so inexpensive why take anything else? Many local stores do not have this yet, but it is very common online. Jarrow, Source Naturals and other reputable makers all have a version. Check out vitacost.com or iherb.com for examples.

There are alternate injectables too. When the cyano information started to become public, another form called hydroxycobalamin was marketed. Some doctors may use this, but it is more expensive. Methylcobalamin injections are available custom ordered at compounding pharmacies. This makes them the most expensive. This methyl version is being used in the Autism community.

Injections are invasive and more expensive, and really do not offer much incentive to ambulatory patients. They are still used for bedridden, or unconscious patients. But their days in the doctor's office are no longer necessary.

For people who really want to spend money on genetic testing, there are tests now that will tell if you are failing to methylate vitamins properly. These tests may not be covered by insurance (most likely not) and cost over $400. They are newly appearing in OTC outlets for less...more like $125. But since the methyl versions of the B12 and folate are available, it is not really absolutely necessary to test DNA at this time.

My observation after years on the net is that if people are having neurological symptoms severe enough and for long enough to appear here, they have some significant problems with their metabolic ability to handle some nutrients. Since these nutrients are now available in the activated forms for very little money, why use something that may not work? If methylcobalamin cost the same as RX cyano or more, I could see trying the cyano first. But the reverse is actually more true, so why bother with cyano at all?

There is another form of activated B12 called coenzymated.
It has another name dibencozide. This one is commonly available from Country Life. Its generic name is adenosylcobalamin and it is active also. But it tends to be more expensive. Some people like it better, and body builders tend to favor it for muscle building etc.
There is also an oral form of hydroxycobalamin. This has been suggested to be used to detoxify cyanide which can occur during acute smoke inhalation. This form is not as easy to find to purchase, is a bit more expensive, but is out there.

The actual chemistry of B12 and folic acid... and the entire methylation chemistry that they are involved with is really complex. I don't think going into it here is necessary for most people.

I can't find a Kegg module for B12... but here is the one for
methylation of folate:
http://www.genome.jp/kegg/pathway/map/map00670.html
Each box is one enzyme that can be affected by a genetic error/failure. The black arrows all come to folic acid to provide a carbon atom for it. Sort of like "all roads lead to Rome".
The methylation of B12 is complex too.
http://books.google.com/books?id=lZw...um=7#PPA122,M1
page 122 of this link gives some details for those who like this stuff