B12 absorption appears to be the major issue. There is an extensive discussion here of why and the possible solutions.
” Make your own B-12 intra-nasal supplement. It is cheap, easy and best done behind closed doors. Obtain your doctor’s OK before trying this procedure. Take any B-12 tablet (between 100 to 1,000 mcg) and grind it into a powder between two tablespoons. Add water, just a few drops at a time, to make a soft paste. With a “Q-Tip,” its generic equivalent, or your clean pinkie finger, gently swab the paste inside your nose up to a comfortable level. D ...
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