--I'd like to point out two things.
Acid blockers often contribute to B12 deficiency in those who are prone to it, especially as we get older. We need intrinsic factor to break the large B12 molecule out of food, and this works only in an acidic stomach environment. We tend to secrete less of it as we get older, and using meds that block acid can actually hasten/worsten a developing inability to break B12 out of food.
Second, many doctors still don't know they should not only be testing people for B12 deficiency, but if such is revealed (and the MMA and homocyseine tests are probably better indicators of this than a "borderline" Serum B12 level), megadose oral supplemenation (on the order of 1-5mg/day) is generally just as effective as injections--and cheaper. There is enough passive absorption of the megadose to begin to build B12 levels back up. Methylcobalamin forms are preferred if can get them, as the B12 is already in absorbable form. (Commercially prepared cyanocobalamin will work for many people, but why tax the system getting rid of the cyanide radical?)
Part of the problem is that "normal" ranges for serum B12 are set much too low in the US--usually at the 200 level. In Japan and many parts of Europe, the low normal boundary is set at 500.
Many of use here take B12 even with higher initial levels than that as a form of neuroprotection--there is no known toxicity level, and people have taken up to 36mg/day in Japanese studies without ill effect (the body tends to use what it needs and discard the rest). Most of us on the peripheral neuropathy board, for example, feel better with our serum B12 values in 4 digits.
Several people who frequent these boards are among the world's leading lay experts on this subject--one should search out posts by Rose, MrsD, JCCglutenfree, and Paul Golding for more info.