The absorption of vitamin B12 (cobalamin, cyanocobalamin) is dependent upon a number of anatomical and physiological factors and is also necessary for a number of biological reactions/functions.  This article will focus on the type of patients or medical conditions that may require the supplementation of vitamin B12 (see the table below).1-6  More importantly, it is necessary to recognize the reason that some of these patients require supplementation, including those using vitamin B12 in different dosage forms other than those to be taken by mouth.  

While having megaloblastic anemia, hyperhomocysteinemia, and/or neuropathy are not medical conditions that influence vitamin B12 absorption, they are clearly indications for the use of vitamin B12 supplements when deficiency is established.  Furthermore, it is likely that many of these medical conditions will likely require the use of vitamin B12 supplements that bypass the gastrointestinal tract.  This would be especially true where there is insufficient intrinsic factor produced and/or where the ileum of the small of the intestine is no longer present or able to absorb the vitamin B12/intrinsic factor complex.  Currently, the only prescription options include cobalamin delivered intranasally and delivered parenterally (as intramuscular or deep subcutaneous injection).4,7  The intranasal products appear to be well tolerated and are marketed under the brand names, CaloMist and Nascobal.  The initial and maintenance dose and frequency of administration vary based on the degree of deficiency and medical situation needing treated.

References:

1. Institute of Medicine. Food and Nutrition Board. Dietary Reference Intakes: Thiamin, Riboflavin, Niacin, Vitamin B6, Folate, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington, DC: National Academy Press, 1998.
2. National Institutes of Health. Office of Dietary Supplements.  Dietary Supplement Fact Sheet: Vitamin B12.  Bethesda, Maryland.  05/26/2010. 
3. Hurwitz A, Brady DA, Schaal SE et al. Gastric acidity in older adults. JAMA 1997;278:659-62.  
4. Butler CC, Vidal-Alaball J, Cannings-John R et al. Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency: a systematic review of randomized controlled trials. Fam Pract 2006;23:279-85.  
5. Sumner AE, Chin MM, Abraham JL et al. Elevated methylmalonic acid and total homocysteine levels show high prevalence of vitamin B12 deficiency after gastric surgery. Ann Intern Med 1996;124:469-76.  
6. Brolin RE, Gorman JH, Gorman RC et al. Are vitamin B12 and folate deficiency clinically important after roux-en-Y gastric bypass? J Gastrointest Surg 1998;5:436-42.  
7. Suzuki DM, Alagiakrishnan K, Masaki KH et al.  Patient acceptance of intranasal cobalamin gel for vitamin B12 replacement therapy. Hawaii Med J 2006;65:311-4.

• Pharmacology: The Mechanism of Vitamin B12 Absorption
• Pharmacology: Medications Known to Decrease Vitamin B12 Levels
• Pharmacology: Proton Pump Inhibitor Induced Reductions in Vitamin B12 Levels
  

Vitamin B12, Cobalamin, Cyanocobalamin, Medical Conditions that Require Vitamin B12