Last year I was diagnosed with B12 deficiency after months of complaining to doctors about my failing memory and tingling in my arms and feet. Several doctors as well as a nutritionist all thought it was just too much sugar and maybe an age related decline in memory. Then I started having burning pain in my feet and a new doctor finally ran some tests trying to get to the bottom of my crazy symptoms which revealed low B12 levels. Once I knew I was low in B12, it seemed really obvious to me, because it was a vitamin my son had also been low in and he started speaking after supplementation with sublingual B12.
If you have any type of neurological problems or age related decline, you should look into getting B12, methylmalonic acid and homocysteine levels checked. All three tests are needed to accurately determine B12 status. High levels of B12 when you are not taking supplements can also indicate problems with B12--sometimes it can pool in the blood and not be getting into the tissues. This was seen in some studies where the blood levels were high but the cerebral spinal levels were low.
Also, there are four different forms of B12--methyl B12 is supposed to be the best form. Sublingual forms of B12 are supposed to be better than the oral forms because they bypass the digestive system which needs intrinsic factor to be absorbed. Best to get a blood level and know your B12 status before supplementing with B12--it's of great importance to know your level. Read the Wall Street Journal Article on Tired And Confused? Vitamin B12 May Be Low for an overview of low B12 levels.