What’s in a name?

  • Vitamin B12:
    • Refers to a specific group of cobalt-containing corrinoids, also referred to as cobalamins.
    • Terms cobalamin and vitamin B12 are generally used interchangeably.
  • Adenosylcobalamin – active coenzyme form
  • Methylcobalamin – active coenzyme form
  • Hydroxocobalamin – Food cobalamin; also a synthetic form of vitamin B12.
  • Cyanocobalamin – stable synthetic pharmaceutical that has to be converted to adenosylcobalamin or methylcobalamin to become metabolically active.

What are the possible routes for delivering vitamin B12?


Parenteral vitamin B12Avoids potential problems with compliance or absorption.Requires visit to medical office.Generally preferred for initial therapy in patients with significant neurologic symptoms.
Oral vitamin B12 May be as effective as parenteral vitamin B12 in patients with normal absorption; less painful and usually lower cost than injection.May not be appropriate if absorption or compliance problematic.High-dose tablets of 1,000 mcg expected to deliver about 5-40 mcg of vitamin B12 assuming oral dose absorption rate of 0.5%-4%.
Intranasal vitamin B12Easy to use.N/AInitial dose: 1 spray (500 mcg) in 1 nostril once weekly;
administer ≥ 1 hour before or ≥ 1 hour after ingestion of hot foods or liquids.

Treatment protocols for patients without neurological involvement vary. Some examples include:

  • Initial therapy with vitamin B12:
    • 1,000 mcg intramuscularly 3 times weekly for 2 weeks
    • 1,000 mcg/day orally
    • 1,000 mcg intramuscularly per day or every other day for 1 week, followed by weekly injections up to 8 weeks, followed by every 3-4 weeks
  • Maintenance therapy with vitamin B12:
    • For irreversible cases (such as pernicious anemia, or after gastric surgery), continue treatment for life typically with vitamin B12 1,000-2,000 mcg/day orally or monthly parenteral cyanocobalamin 1,000 mcg (or hydroxocobalamin 1,000 mcg every 3 months).

Which of the following are potential risks of vitamin B12 replacement?

Correct. Hypokalemia may occur during vitamin B12 therapy as a result of increased red blood cell requirements during hematopoiesis.

Expected response to therapy:

  • Neurologic improvement within 1 week
  • Normalization of mean cell volume within 8 weeks

This patient was treated with vitamin B12 1,000 mcg intramuscularly 3 times weekly for 2 weeks followed by monthly injections of 1,000 mcg. As shown below, his complete blood count showed rapid improvement: