Q: Can oral vitamin B
12 supplementation substitute for parenteral administration? If so, why do so many physicians cling to the latter
A: Cyanocobalamin has traditionally been administered via IM injection because its absorption depends on the patient's having intrinsic factor and an intact terminal ileum. Investigators in the 1950s and 1960s, however, found that a small but constant proportion (approximately 1%) of orally ingested B12 enters the bloodstream from the GI tract via passive absorption. Given that the recommended daily allowance is 2 µg, most patients get adequate levels from 100-250 µg, although 1,000 µg is preferable, as it produces consistently successful long-term results.
So why do many physicians continue with the more costly and painful injections? A 1991 JAMA editorial explored this issue and found several reasons — primarily that the literature has not encouraged use of oral B12 and that most practitioners are unaware that it is effective (JAMA. 1991;165:94-95). There have also been historical concerns regarding “unpredictable absorption,” as well as concerns over cost and compliance issues. The concerns regarding absorption of oral B12 have been laid to rest with more recent studies, and compliance has been noted to be good — in some cases better than with injections. Cost comparisons generally favor oral therapy as well, when the expenses of providing the injection and transportation to the physician's office are considered. Oral B12 has been effectively and safely used in Europe for more than 30 years and will likely increase in use in the United States with greater physician education and awareness.
