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Anemia - B12 deficiency



Anemia is a condition in which the body does not have enough healthy red blood cells. Red blood cells provide oxygen to body tissues.

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

See also:

Alternative Names

Macrocytic anemia


Your body needs vitamin B12 to make red blood cells. In order to provide vitamin B12 to your cells:

  • You must eat enough foods that contain vitamin B12, such as meat, poultry, shellfish, eggs, and dairy products.
  • Your body must absorb enough vitamin B12. Your body uses a special protein, called intrinsic factor, released by cells in the stomach. The combination of vitamin B12 attached to intrinsic factor is absorbed in the last part of the small intestine.

A lack of vitamin B12 in the diet may be due to:

  • Eating a vegetarian diet
  • Poor diet in infancy
  • Poor nutrition during pregnancy

A number of problems can make it difficult for your body to absorb enough vitamin B12:

  • Chronic alcoholism
  • Crohn's disease, celiac disease, infection with the fish tapeworm, or other problems that make it difficult for your body to digest foods
  • Pernicious anemia, which occurs when your body destroys cells that make intrinsic factor. Intrinsic factor is needed to absorb vitamin B12.
  • Surgeries that remove certain parts of your stomach or small intestine, such as some weight-loss surgeries
  • Taking antacids and other heartburn medicines for a long period of time

People with mild anemia may have no symptoms, or symptoms may be very mild.

More typical symptoms of vitamin B12 deficiency anemia include:

Symptoms from nerve damage caused by vitamin B12 deficiency that has been present for a longer time include:

  • Confusion or change in mental status (dementia) in severe cases
  • Depression
  • Loss of balance
  • Numbness and tingling of hands and feet
Signs and tests

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

The following tests may be done:

Esophagogastroduodenoscopy (EGD) to examine the stomach, or enteroscopy to examine the small intestine may be done.

Bone marrow biopsy is done only when the diagnosis is not clear.

Support Groups

Expectations (prognosis)

Treatment for this form of anemia is usually effective.

Any damage to nerves may be permanent, especially if treatment is not started within 6 months of when the symptoms begin.

Calling your health care provider

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


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.


Treatment depends on the cause of B12 deficiency anemia.

  • Pernicious anemia requires lifelong vitamin B12 replacement, most often using injections. Some patients can get enough supplementation with high-dose tablets of oral vitamin B12.
  • 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 poor digestion and absorption is treated with vitamin B12 injections until the condition improves. These shots are given every day and then every week at first, and then every month.

Many people may need these shots once a month for the rest of their life. The shots may no longer be needed after Crohn's disease, celiac disease, or alcoholism is properly treated.


You can prevent anemia caused by a lack of vitamin B12 by following a well-balanced diet. Vitamin 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. In: Hoffman R, Benz Ej, Shattil SS, et al., eds. Basic Principles and Practice. 5th ed. Philadelphia, Pa: Elsevier Churchill Livingstone;2008:chap 39.

Kaferle J, Strzoda CE. Evaluation of macrocytosis. Am Fam Physician. 2009;79:203-208.

Related Taxonomy

Review Date: 1/31/2010
Reviewed By: 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.
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