As a clinical pharmacy specialist, you will be asked, is it ever necessary to give vitamin B12 by injection?.. Only rarely. Many patients have trouble absorbing B12 from food due to reduced gastric acidity or lack of intrinsic factor. We used to think that these patients needed B12 by injection, but in many cases supplements can be given orally instead. About 1% of the B12 in oral supplements is absorbed passively, even in patients without gastric acid or intrinsic factor.
Feel comfortable recommending ORAL B12 for treating mild to moderate deficiencies, or for maintenance therapy. The key is to give enough. Recommend 1000 to 2000 mcg/day of oral or sublingual B12 for mild to moderate deficiencies. Steer patients away from sustained-release B12 supplements, because their absorption might not be adequate. Recommend injectable B12 for initial treatment of more severe deficiencies, especially if there are neurologic symptoms.
Also recommend the injectable for patients who can't take or adhere to oral medications, or those who may not absorb it due to diarrhea, vomiting, inflammatory bowel disease, or bowel resection. Get our note, "Management of vitamin B12 deficiency", for more information.
REFERENCES
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Castelli MC, Friedman K, Sherry J, Brazzillo K, Genoble L, Bhargava P, Riley MG. Comparing the efficacy and tolerability of a new daily oral vitamin B12 formulation and intermittent intramuscular vitamin B12 in normalizing low cobalamin levels: a randomized, open-label, parallel-group study. Clin Ther. 2011 Mar;33(3):358-371.e2. Available at: https://www.sciencedirect.com/science/article/abs/pii/S0149291811001433?via%3Dihub
Andrès E, Fothergill H, Mecili M. Efficacy of oral cobalamin (vitamin B12) therapy. Expert Opin Pharmacother. 2010 Feb;11(2):249-56. Available at: https://www.tandfonline.com/doi/abs/10.1517/14656560903456053?journalCode=ieop20
Vidal-Alaball J, Butler CC, Cannings-John R, Goringe A, Hood K, McCaddon A, McDowell I, Papaioannou A. Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency. Cochrane Database Syst Rev. 2005 Jul 20;(3):CD004655. Available at: https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD004655.pub2/full