This topic is very much related and partially linked to issues already discussed in the essay about homocysteine. Many of us suffer from deep deficiencies of B12 (interestingly, there is no consensus in regards to the normal level of this vitamin in the bloodstream – Japanese scientists claim it is above 600 pg/mL while 200 pg/ml is the norm in Europe). The claim that the deficiency in B12 causes anxiety and depression has been indeed very popular worldwide.
B12 terminates homocysteine but this is not its only function – it aids methylation (especially in the nervous system). In cases of levels falling below 300 pmol/l or 400 pg/MmL supplementation of high doses of B12 is recommended. Forget injections – taking a supplement in a pill is perfectly fine, assuming the dose is high enough (it is true that a fraction of us cannot digest the vitamin but even in this case about 1% of the dose will enter the bloodstream). It means that some patients who though are doomed to taking injections till their dying breath can switch to a vitamin pill.
It is vital to take the active form of B12 – methylocobalamin as it aids the correct transformation of it in the body. If you aim at nourishing the brain, not blood, taking additional iron and/or folic acid is not necessary. Initially, take at least 5000 mcg daily for the first few months – it will mimic effects of an injection.