Although absorption of dietary vitamin B12 requires gastric intrinsic factor and an intact terminal ileum, pharmacologic doses of the vitamin can be absorbed by passive diffusion throughout the small intestine.1(pp96-97) Indeed, an oral dose of 2000 μg per day of cyanocobalamin was found to be effective treatment for clinically significant vitamin B12 deficiency even in subjects with classic pernicious anemia.2 Ease of administration and low cost make oral vitamin B12 a more attractive option than traditional intramuscular therapy. However, over-the-counter vitamin B12 preparations are not subject to regulation and standardization.
When one subject was noted to be using vitamin B12 tablets labeled as a “timed release” preparation, pharmacies in the New Haven, CT, area were surveyed. In the 9 pharmacies visited, 10 of 12 brands of tablets containing 1000 μg or 2000 μg of vitamin B12 were found to be “timed release” preparations. Dissolution times were indicated on the labels of 4 of these preparations and ranged from 3 hours to 6 hours. Only “timed release” preparations were available in 5 of 6 major chain discount pharmacies visited.
The most recent study of therapy with oral vitamin B12 used two 1000-μg tablets of the Nature's Bounty brand (Bohemia, NY) which is not labeled as a “timed release” preparation.2 Thus, the effectiveness of “timed release” tablets has not been established. Since prompt treatment of vitamin B12-deficient subjects is required to prevent progressive, irreversible neurologic and cognitive impairment,1(pp287-290),3 caution is warranted in the use of oral vitamin B12 therapy particularly with “timed release” tablets, and particularly in the setting of pernicious anemia or disorders of the terminal ileum.
