Well, if you are reading here and have a need great enough to
end up here looking for answers, logically you probably need
B12.

More and more people are turning up low in this nutrient.
Vegans, who eat a diet devoid of animal protein, need to supplement this. They like to claim that their own bacteria that live in their GI tract will make enough for them, but that often does not turn out to be true. These bacteria may be killed off with antibiotics.

Our liver stores up to 5 years of excess B12 that has been eaten.
And it follows that if you have liver damage, less of this can be relied upon.

Lifestyle today is critical when it comes to B12. The over the counter availability of acid blocking drugs, impairs the utilization of B12. Acid normally breaks up proteins, and makes the B12 available to intrinsic factor which grabs it and facilitates its absorption in the intestine. (nature has developed this strategy because B12 is normally only in foods in tiny microgram amounts.) Previously to the appearance of these drugs, B12 deficiencies were found in people with hereditary errors who cannot make intrinsic factor, or who had their stomach removed completely or partially, or who have autoimmune disease which damaged the cells making intrinsic factor, OR who are getting older and no longer making adequate stomach acid-- called achlorhydria.

Now babies are born deficient (if Mom was on a vegan diet and did not take vitamins) and many in the general population are deficient because they take Zantac or Prilosec daily for years.

Some other drugs also upset B12 levels. One common drug which is no longer available orally here in US is Vioform (clioquinol). This drug was taken off the market in Japan and US for causing blindness and neurological damage. It is still found in creams for treating fungal infections however. But its tragedy in Japan led to aggressive use of B12 in that country, and its more aggressive interpretation of adequate blood levels. So we see Japanese papers using B12 for MS in enormous doses, and we see a higher cut off for "normal" in blood testing there as well.

Other drugs that reduce B12 are:
colchicine
most antibiotics (long term use)
some retroviral drugs for HIV
metformin
birth control pills
acid reducing drugs

One misconception doctors still have is that anemia must be present to have a real deficiency. This has been shown in papers to not be the case. Sometimes one blood test will hint at low B12 before anemia becomes severe, and that is MCV.
But many people can be low and not anemic.

The link I gave in the first post here, suggests various testing
http://www.aafp.org/afp/20030301/979.html
It has a chart suggesting testing and treatment for those at 400 and below.

Testing for MMA, and homocysteine are still considered better indicators than serum B12 levels. Rose's website has far more detail on this subject.

Since B12 has no upper toxic limit set by researchers, one can be flexible in using oral supplements safely.
I think that people very low in testing, need to bring that level up as quickly as possible.
There are 5mg (5000mcg) tablets available orally for about 30 cents a day. Take one of these daily on an empty stomach for 2 to 3 months and get retested. If you show levels above 1000, that is a good start. You can continue with this if you want, and feel you need it. Or you can reduce to 1 or 2 mg a day, thereafter. Most test ranges are not kept up to date, and most labs have a ceiling test of 2000. They will report this as HIGH and some doctors --who are trained to fear anything HIGH-- think this is "bad". There is no evidence anywhere in the research that high serum levels of B12 are harmful in any way.
Should you become ill however, or develop some new symptom, it is best to contact your doctor for evaluation.

I personally think 1mg of B12 daily is enough for most people once they bring up their blood work to normal.

People with MS and Fibro may have to take larger doses for a longer period of time. Research has shown that the fluid called the CSF is low in B12 for these patients. Spinal taps are not routinely given, but for MS patients who have them as diagnostic tools, ask for the B12 measurement when the test is done. Others may be offered a spinal tap, especially in rapidly progressing or long lasting PNs. Ask for the B12 measurement if you have one.

The next post is going to discuss the differences in cyanocobalamin and methylcobalamin, as well as some others.