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THE MERCK MANUAL MEDICAL LIBRARY: The Merck Manual of Medical Information--Home Edition
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Vitamin B12(Cobalamins)


Vitamin B12 (cobalamins), with folate, is necessary for the formation and maturation of red blood cells and the synthesis of DNA (deoxyribonucleic acid), which is the genetic material of cells. Vitamin B12 is also necessary for normal nerve function. Unlike most other vitamins, B12 is stored in substantial amounts, mainly in the liver, until it is needed by the body. Usually, the body's stores of this vitamin would take about 3 to 5 years to exhaust.

People should not take high doses of vitamin B12 as a cure-all, but otherwise the vitamin does not appear to be toxic.

Vitamin B12 Deficiency

  • Anemia develops, causing paleness, weakness, fatigue, and, if severe, shortness of breath and dizziness.
  • A severe deficiency may cause tingling or loss of sensation in the hands and feet, muscle weakness, loss of reflexes, difficulty walking, confusion, and dementia.
  • The diagnosis is based on blood tests.
  • When high doses of vitamin B12 supplements are taken, most symptoms resolve.
  • Symptoms due to nerve damage, such as neuropathy or dementia in older people, may persist.

Vitamin B12 occurs in foods that come from animals. Normally, vitamin B12 is readily absorbed in the last part of the small intestine (ileum), which leads to the large intestine. However, to be absorbed, the vitamin must combine with intrinsic factor, a protein produced in the stomach. Without intrinsic factor, vitamin B12 moves through the intestine and is excreted in stool.

Because vitamin B12 is necessary for the formation of mature blood cells, deficiency of this vitamin can result in anemia. The anemia is characterized by abnormally large red blood cells (macrocytes) and white blood cells with abnormal nuclei. Anemia may not develop until 3 to 5 years after the deficiency begins because a large amount of vitamin B12 is stored in the liver.

Vitamin B12 deficiency can cause nerve damage (neuropathy) even when no anemia develops, particularly in people older than 60.


Vitamin B12 deficiency can result when people do not consume enough vitamin B12 or when the body does not absorb or store enough of the vitamin.

Inadequate Consumption: Vitamin B12 deficiency develops in people who do not consume any animal products (vegans) unless they take supplements. If a vegan mother breastfeeds her infant, the infant is at risk of vitamin B12 deficiency.

Inadequate Absorption: The most common cause of vitamin B12 deficiency is inadequate absorption. The following conditions can cause absorption to be inadequate:

  • Overgrowth of bacteria in part of the small intestine
  • Malabsorption disorders
  • Inflammatory bowel disease
  • Fish tapeworm infection
  • Surgery that removes the part of the small intestine where vitamin B12 is absorbed
  • Drugs such as antacids and metformin Some Trade Names
    (used to treat diabetes)
  • Lack of intrinsic factor
  • Decreased stomach acidity (common among older people)

Intrinsic factor may be lacking because abnormal antibodies, produced by an overactive immune system, attack and destroy the stomach cells that produce intrinsic factor—an autoimmune reaction. Intrinsic factor may be lacking because the part of the stomach where it is produced was surgically removed. Vitamin B12 deficiency due to lack of intrinsic factor causes a type of anemia called pernicious anemia.

Among older people, absorption may be inadequate because stomach acidity is decreased. Decreased stomach acidity reduces the body's ability to remove vitamin B12 from the protein in meat. The vitamin B12 found in vitamin supplements, however, can continue to be well absorbed even in people with decreased stomach acid.

Inadequate storage: Liver disorders may interfere with the storage of vitamin B12.


Anemia due to vitamin B12 deficiency develops gradually, allowing the body to adapt somewhat. Consequently, symptoms may be mild even when anemia is severe. Symptoms of anemia are paleness, weakness, and fatigue. If severe, anemia causes shortness of breath, dizziness, and a rapid heart rate. Occasionally, the spleen and liver enlarge. Younger adults who have pernicious anemia (due to lack of intrinsic factor) are more likely to develop stomach and other gastrointestinal cancers.

In people with nerve damage, the legs are affected earlier and more often than the arms. Tingling is felt in the feet and hands, and sensation in the legs, feet, and hands is lost. People become less able to tell where their arms and legs are (position sense) and to feel vibrations. Mild to moderate muscle weakness develops, and reflexes may be lost. Walking becomes difficult. Some people become confused, irritable, and mildly depressed. Advanced vitamin B12 deficiency may lead to delirium, paranoia, and impaired mental function, including dementia.


Usually, vitamin B12 deficiency is suspected when routine blood tests detect large red blood cells. If the deficiency is suspected, the level of vitamin B12 in the blood is measured. Usually, doctors also measure the blood level of folate to rule out folate deficiency, which can also result in large red blood cells.

If vitamin B12 deficiency is confirmed in an older person, no other tests are done because the cause, such as low stomach acidity, is usually not serious. In a younger person, other tests, including other blood tests, may be done to determine the cause. These tests (including the Schilling test) usually focus on intrinsic factor. Endoscopy (use of a flexible viewing tube to directly examine a body cavity) may be done to check for destruction of stomach cells that produce intrinsic factor.

Prevention and Treatment

Giving infants of vegan mothers vitamin B12 supplements from birth helps prevent the deficiency.

Older people with vitamin B12 deficiency benefit from taking vitamin B12 supplements because the deficiency usually results from difficulty absorbing the vitamin from meat. They can absorb the vitamin more easily from supplements than from meat.

Treatment of vitamin B12 deficiency or pernicious anemia consists of high doses of vitamin B12 supplements. If people have the deficiency but no symptoms, the vitamin may be taken by mouth. Blood tests are done periodically to make sure the vitamin B12 level returns to and remains normal. People who have symptoms due to nerve damage are usually given vitamin B12 by injection into a muscle. Injections, which may be self-administered, are given daily or weekly for several weeks until the vitamin B12 level returns to normal. Then injections are given once a month indefinitely, unless the disorder causing it can be corrected. Anemia usually resolves in about 6 weeks. But severe symptoms due to nerve damage—for example, dementia in older people—may not resolve.

Last full review/revision August 2007 by Larry E. Johnson, MD, PhD

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