There are many causes of low B12. One common cause today is the chronic use of acid blocking heartburn drugs. These drugs reduce acid in the stomach, which then interferes with absorption of nutrients that require acid.

These are:
B12
folic acid
calcium
magnesium
iron
zinc
and some trace minerals.

Because of this effect B12 deficiency is more common than it ever was! Some studies suggest 40% of Americans may be low.

B12 (from cyanocobalamin) and folic acid are not active in the body. They need to be converted once absorbed to their active methyl forms. This step can fail if you inherited damaged methylation genes. Estimates today are that 10-30% of Americans cannot methylate these two crucial B vitamins.
This is called MTHFR polymorphism. Luckily there are OTC versions of them available at very reasonable prices so you can correct this problem easily. Methylcobalamin and methylfolate are their names. Methylfolate is sometimes called Metafolin.
Not easily found locally but online they are now available from many quality discount sources, like iherb.com, Puritan's Pride, Amazon and Swanson's to name a few.

The methylB12 needs to be taken on an empty stomach, and daily use can be as effective as injections. This is my B12 thread, with medical information explaining:
http://neurotalk.psychcentral.com/thread85103.html

MethylB12 is a cofactor in the synthesis of melatonin. For people with insomnia, this is a critical thing to understand, as it suggests methylation failure. Most doctors do not know this yet, because they do not keep up with new research.
You could use cyano every day, and if you cannot methylate it and change it to the active form, it will NOT do anything for you.
Taking oral is as effective as shots, and there are studies to prove that. So do read the link I gave and especially the links listed there that explain this so you can improve your status quickly. MethylB12 crosses the blood brain barrier, into the central nervous system, and is dependant on the gradient concentration in the serum. If blood serum is low, then little can enter the brain. So keeping your blood levels high, insures that the brain will have enough.

Eventually low B12 will lead to degeneration of the spinal cord, and paralysis, blindness and death if not treated.
http://en.wikipedia.org/wiki/Subacut...of_spinal_cord

It is quite possible that low B12 creates further difficulties for TBI patients, than for normals. Since it is so common, and doctors continue to follow outdated lab ranges for testing, patients in the US continue to deteriorate. The new low today is 400pg/ml. But doctors continue to accept "normal" in ranges below that, even in the face of deterioration of the patient!

That is why you need to be proactive on this subject and find out your lab results and medicate yourself if necessary.
If you are one of the 40% who are already low in B12, you are not going to HEAL a brain injury very well...and in fact you are already deteriorating from THAT before you even had the injury.

On the thread I gave you are links to medical papers illustrating that absorption of oral B12 is effective, if done properly. Presence of food in the intestine will block the micrograms needed for passive absorption (when intrinsic factor fails, due to damage to the stomach cells producing it, or when acid is removed from the stomach with heartburn drugs. Acid is necessary to enable absorption of B12 orally and from food). So it is critical to take oral B12 supplements on an EMPTY STOMACH, for best absorption.

Therefore you must learn how your body works, and work with it and not against it, to supply B12 on a daily basis to heal and maintain your nervous system. This will cost you pennies a day and is just about the least expensive insurance you can provide for yourself.