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Megaloblastic anemia - view of red blood cells
Megaloblastic anemia - view of red blood cells


B12 deficiency anemia is a low red blood cell count due to a lack of vitamin B12 .

Alternative Names:

Macrocytic anemia

Causes, incidence, and risk factors:

Vitamin B12 is essential for normal nervous system function and blood cell production. The main sources of vitamin B12 include meat, eggs, and dairy products. For vitamin B12 to be sufficiently absorbed by the body, it must bind to intrinsic factor , a protein released by cells in the stomach. The combination of vitamin B12 bound to intrinsic factor is absorbed in the final part of the small intestine.

Causes of vitamin B12 deficiency include:

  • Abdominal or intestinal surgery that affects intrinsic factor production or absorption
  • A diet low in vitamin B12 (for example, a strict vegetarian diet that excludes all meat, fish, dairy products, and eggs)
  • Chronic alcoholism
  • Crohn's disease
  • Infection with the fish tape worm
  • Intestinal malabsorption disorders
  • Pernicious anemia , which is caused by the destruction of intrinsic factor by the immune system

The risk factors are related to the causes.


Signs and tests:

A physical exam may show problems with reflexes or positive Babinski reflex.

The following tests may be done:


Treatment depends on the specific cause of B12 deficiency anemia.

Pernicious anemia requires lifelong vitamin B12 injections. People with anemia due to a lack of dietary vitamin B12 may be told to take vitamin supplements and follow a more balanced diet . Treatment may start with vitamin B12 injections.

Anemia caused by malabsorption is treated with vitamin B12 injections until the condition improves.

Support Groups:

Expectations (prognosis):

Treatment for this form of anemia is usually effective.

  • The signs and symptoms related to the central nervous system may be irreversible if treatment is not begun within 6 months of when these symptoms begin.
  • Vitamin B12 affects cells that form the outer surface of the body and line inner passageways (epithelial cells). Therefore, a lack of B12 may cause a falsely abnormal Pap smear .

Calling your health care provider:

Call your health care provider if you have any of the symptoms of anemia.


Anemia caused by a lack of vitamin B12 can be prevented by following a well-balanced diet. B12 injections can prevent anemia after surgeries known to cause vitamin B12 deficiency. Early diagnosis and prompt treatment can limit the severity and complications of this anemia.


Antony AC. Megaloblastic anemias. Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007: chap 170.

Review Date: 11/23/2008
Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; and Yi-Bin Chen, MD, Leukemia/Bone Marrow Transplant Program, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Call 911 for all medical emergencies. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997- A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.

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Phone: (603) 742-5252
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