Vitamin B-12 is unusual in that it is only found in animal cells, since it is needed to make animal DNA, but not vegetable DNA. Furthermore, the stomach secretes a chemical, called intrinsic factor, which attaches to the Vitamin B-12 molecule and facilitates its absorption by the intestine. As we get older, the stomach secretes less intrinsic factor, so elderly people are more likely to develop a deficiency, which can lead to a reversible anemia, and irreversible neurological defects.
Studies in Sweden have shown that if you swallow enough Vitamin B-12, say 1 mg/day, then you will absorb sufficient vitamin to avoid a deficiency, even in the complete absence of intrinsic factor. So all that a patient on a proton pump acid inhibitor or an H-2 blocker to suppress stomach acid needs to do is to take 1mg/day of Vitamin B-12. The B-12 level should still be checked, but this OTC dose should be sufficient to prevent any problem.