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Am Fam Physician. 2006;73(1):65

Clinical Question

Can vitamin B12 deficiency be treated orally?

Evidence-Based Answer

In patients with vitaminB12 deficiency, two oral regimens have been shown to achieve neurologic and hematologic response in the short term. Effective dosages were (1) 2,000 mcg daily or (2) 1,000 mcg daily for 10 days, then weekly and monthly.

Practice Pointers

Vitamin B12 deficiency causes macrocytic anemia, fatigue, loss of appetite, and mood disturbances. It also can cause serious neurologic and neuropsychiatric illness such as paresthesias, ataxia, and memory loss. The process of vitamin B12 absorption can be impaired at the level of the stomach, where intrinsic factor is produced, or at the level of the terminal ileum, where intrinsic factor bound to vitamin B12 is absorbed. Medications such as metformin (Glucophage) or antacids also can impair absorption. A small amount of vitamin B12 is absorbed by passive diffusion without intrinsic factor.

Traditionally, vitamin B12 replacement has been administered intramuscularly because absorption through the gastrointestinal tract is deficient. However, this route is less convenient for patients than oral medication and may not be covered by health insurance. Because a small amount of vitamin B12 is absorbed by passive diffusion, megadoses of oral vitamin B12 medication have been used as a cost-saving and more comfortable alternative to intramuscular B12 administration.1

Vidal-Alaball and colleagues searched for randomized controlled trials comparing oral with intramuscular administration of vitamin B12. They found two trials with a total of 93 patients who were followed for between 90 days and four months. All patients had vitamin B12 deficiency, defined as vitamin B12 level less than 244 pg per mL (180 pmol per L). Some patients had conditions that cause malabsorption from the gut, but no patients with inflammatory bowel disease or celiac disease were included in either study. One study used a daily oral dosage of 2,000 mcg, and the other an oral dosage of 1,000 mcg daily for 10 days, then weekly for four weeks, then monthly for life. In both studies, high-dose oral B12 was as effective as intramuscular injection at achieving neurologic and hematologic response.

Vitamin B12 is available over the counter in the form of tablets (100, 500, 1,000, or 5,000 mcg) and lozenges (50, 100, 250, or 500 mcg).

These are summaries of reviews from the Cochrane Library.

This series is coordinated by Corey D. Fogleman, MD, assistant medical editor.

A collection of Cochrane for Clinicians published in AFP is available at

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