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Vitamin B-12

Other name(s):

cobalamin, cyanocobalamin, hydroxocobalamin

General description

Vitamin B-12 is a water-soluble vitamin. It’s needed for cell division. It’s only found in animal-based foods. Like the other B vitamins, B-12 plays an important role in energy production.

Vitamin B-12 is closely linked with folic acid (vitamin B-9). Vitamin B-12 and folic acid are needed for the synthesis of purines and pyrimidines. These are the building blocks of DNA. Generation of DNA is needed every time a cell divides.

Vitamin B-12 deficiency, like folic acid deficiency, causes megaloblastic anemia. This is abnormally large red blood cells and immature, abnormal white cells. This anemia is also called pernicious anemia. It occurs when the body is unable to absorb dietary vitamin B-12 due to not having enough of a protein (intrinsic factor) in the stomach. This issue is often hereditary.

Vitamin B-12 also helps transform homocysteine (an amino acid) into methionine (also an amino acid) in the body. This keeps homocysteine from building up. A high level of homocysteine increases the risk of heart attack (myocardial infarction). Vitamin B-12 keeps the myelin insulation sheath of the nerves. Not having enough B-12 can cause nerve damage (neuropathy). This causes numbness and abnormal feelings in your skin.

Vitamin B-12 is also a cofactor in carbohydrate metabolism.

Medically valid uses

Vitamin B-12 is used to treat pernicious anemia caused by lack of an intrinsic factor. Supplements are also used for vitamin B-12 deficiencies. This can happen from having a vegan diet, poor eating habits, or toxicity due to an overactive thyroid gland (thyrotoxicosis). It can also happen due to bleeding, cancer, liver or kidney disease, and infestation by the fish tapeworm (Diphyllobothrium latum).

Vitamin B-12 is used to treat certain inherited or genetic issues that affect metabolic functions. These include Immerslund-Graesbeck syndrome, homocystinuria, and cobalamin C, D, and F disease.

Unsubstantiated claims

Please note that this section reports on claims that have not yet been substantiated through studies.

Vitamin B-12 may increase energy and give you a sense of wellbeing. It may also help your immune system work well. It may also improve your memory and boost fertility.  

Recommended intake

Vitamin B-12 is measured in micrograms (mcg). It’s available as an oral tablet. It comes in strengths of 25–250 mcg. It’s also available as a nasal gel or sublingual pills. The RDA is the Recommended Dietary Allowance.



Infants (0–6 months)

0.3 mcg

Infants (6 months to 1 year)

0.5 mcg

Children (1–3 years)

0.7 mcg

Children (4–6 years)

1.0 mcg

Children (7–10 years)

1.4 mcg

Children and adults (11 years and older)

2.0 mcg

Pregnant women

2.2 mcg

Breastfeeding women

2.6 mcg



Food source

Nutrient content per 100 grams


85.9 mcg


32.2 mcg


18.8 mcg

Herring/ mackerel

9.44 mcg


8.3 mcg


5.6 mcg


1.8 mcg

Cheese, Swiss

0.8 mcg


0.77 mcg


Vitamin B-12 isn’t found in vegetables or fruits. Bacteria in the large intestine make about 10–15 mcg a day of B-12. The small intestine cannot absorb B-12. So about the same amount is lost through stool.

Vitamin B-12 is stable at room temperature. It doesn’t need to be refrigerated. Cooking doesn’t destroy it. Vitamin B-12 resists breakdown even at boiling point of water for several hours.

A diet deficient in animal proteins, milk, or dairy products may increase the need for vitamin B-12. People who eat vegan diets may need to take V-12 supplements. Breastfed infants of vegan-vegetarians also need supplements.

If you have a condition that causes you to not absorb enough B-12, you may need supplements. These can include gluten-induced enteropathy, celiac disease, sprue, gastrectomy, or fish tapeworm infestation.

Heavy alcohol use or alcoholism can increase the need for B-12. So can liver disease, hypothyroidism, and thalassemia.

Vitamin B-12 absorption in the intestinal tract may decrease with aging. So, people over 60 of age may need vitamin B-12 supplements.

Women who are pregnant or breastfeeding may need to take B-12 supplements. However, you should talk to your healthcare provider before doing so.  

Vitamin B-12 deficiencies tend to be caused by not getting enough B-12 in your diet. They can also be due to a reduced secretion or lack of intrinsic factor. This is a stomach secretion that helps the body absorb vitamin B-12.

Being deficient in B-12 can lead to pernicious anemia. In this condition, bone marrow is markedly changed. It contains megaloblasts (large, immature cells) in the marrow. The white blood cells look abnormal, with excess lobes on the nucleus. Degeneration also appears in the spinal-cord neurons. As a result, severe and irreversible neurological damage may occur.

Symptoms of pernicious anemia can include:

  • Weakness, fatigue, or vertigo

  • Lightheadedness

  • Dizziness

  • Ringing in your ears (tinnitus)

  • Fast heart rate (tachycardia)

  • Yellowish color to the skin (icterus or jaundice)

  • Sore tongue or a smooth "beefy" red tongue

  • Loss of appetite with weight loss

  • Diarrhea

  • Numbness, tingling, unusual feelings, or sensitivity in your hands or feet

  • Muscle weakness

  • Unstable gait (ataxia)

  • Irritability, memory loss, dementia, and psychosis

Side effects, toxicity, and interactions

There are no known side effects of B-12. There are also no known food or drug interactions linked with it.

You shouldn’t take vitamin B-12 if you’re sensitive to it or cobalt. In rare cases, allergic reactions can happen with the injectable form of B-12.

Additional information

Vitamin B-12 is absorbed in the lower part of the small intestine. In the bloodstream, it’s highly bound to plasma proteins. These are called transcobalamins. The half-life is about six days.



Online Medical Reviewer: Poulson, Brittany, RD, CDE
Online Medical Reviewer: Wilkins, Joanna, R.D., C.D.
Date Last Reviewed: 8/1/2016