Vitamin B12, also called cobalamin, is one of 8 B vitamins. All B vitamins help the body convert food (carbohydrates) into fuel (glucose), which is used to produce energy. These B vitamins, often referred to as B complex vitamins, also help the body use fats and protein. B complex vitamins are needed for healthy skin, hair, eyes, and liver. They also help the nervous system function properly.

All B vitamins are water soluble, meaning that the body does not store them.

Vitamin B12 is an especially important vitamin for maintaining healthy nerve cells, and it helps in the production of DNA and RNA, the body's genetic material. Vitamin B12 works closely with vitamin B9, also called folate or folic acid, to help make red blood cells and to help iron work better in the body. Folate and B12 work together to produce S-adenosylmethionine (SAMe), a compound involved in immune function and mood.

Vitamins B12, B6, and B9 work together to control blood levels of the amino acid homocysteine. High levels of homocysteine are associated with heart disease. However, researchers are not sure whether homocysteine is a cause of heart disease or just a marker that indicates someone may have heart disease.

It is rare for young people to be deficient in vitamin B12, but it is not uncommon for older people to be mildly deficient. This may be because their diets are not as healthy, or because they have less stomach acid, which the body needs to absorb B12. Low levels of B12 can cause a range of symptoms including:

• Fatigue
• Shortness of breath
• Diarrhea
• Nervousness
• Numbness
• Tingling sensation in the fingers and toes

Severe deficiency of B12 causes nerve damage.

Others at risk for B12 deficiency include:

• Vegans and vegetarians who do not eat dairy or eggs, since vitamin B12 is found only in animal products
• People with problems absorbing nutrients due to Crohn disease, pancreatic disease, weight loss surgery, or medications
• People who are infected with Helicobacter pylori, an organism in the intestines that can cause an ulcer. H. pylori damages stomach cells that make intrinsic factor, a substance the body needs to absorb B12
• People with an eating disorder
• People with HIV
• People with diabetes
• The elderly

Folic acid (vitamin B9), especially when taken in high doses, can mask the symptoms of a vitamin B12 deficiency. The danger is that without symptoms, someone with a vitamin B12 deficiency may not know it, and could run the risk of developing nerve damage. You should talk to your doctor first if you plan to take more than 800 mcg of folic acid, to make sure you do not have a B12 deficiency.

Vitamin B12 is used for:

Pernicious anemia

Pernicious anemia is a type of anemia that happens when stomach cells are not able to make intrinsic factor. Without intrinsic factor, your body cannot absorb vitamin B12. Symptoms include:

• Weakness
• Pale skin
• Diarrhea
• Weight loss
• Fever
• Numbness or tingling sensation in the hands and feet
• Loss of balance
• Confusion, memory loss, and moodiness

Vitamin B12 supplements in high doses, either given as injections or orally, are prescribed to treat pernicious anemia. Pernicious anemia can be a dangerous condition and should always be treated by a doctor.

Heart disease

Many studies suggest that people with high levels of the amino acid homocysteine are almost two times more likely to develop coronary artery disease and 2.5 times more likely to have a stroke than those with normal levels. B complex vitamins, especially vitamins B9, B6, and B12, help lower homocysteine levels. However, researchers do not know whether high homocysteine levels actually cause heart disease.

People who are concerned about heart disease should try to get enough B vitamins from healthy foods. However, in some cases doctors recommend taking B vitamins to lower homocysteine levels. If you are worried about heart disease, ask your doctor whether taking a B vitamin supplement would be right for you.

Age-related macular degeneration (AMD)

One large study found that women who took 1,000 mcg of vitamin B12, along with 2500 mcg of folic acid and 500 mg of vitamin B6 daily, reduced the risk of developing AMD, an eye disease that can cause loss of vision.

Fatigue

Fatigue is one of the symptoms of a vitamin B12 deficiency. One small study suggested that some people who were not deficient in B12 might gain more energy from B12 shots. However, more research is needed. One preliminary study indicated that people with chronic fatigue syndrome might benefit from B12 injections. More research is needed.

Breast cancer

Although there is no evidence that vitamin B12 alone reduces the risk of breast cancer, population studies have shown that women who get more folate in their diet have lower risk of breast cancer. Vitamin B12 works with folate in the body, so it may help reduce breast cancer risk. Another preliminary study suggested that postmenopausal women who had the lowest amounts of B12 in their diet had an increased risk for breast cancer.

Male infertility

Studies suggest that vitamin B12 supplements may improve sperm counts and sperm's ability to swim. More research is needed.

Vitamin B12 is found only in animal foods. Good dietary sources include:

• Fish
• Shellfish
• Dairy products
• Organ meats, particularly liver and kidney
• Eggs
• Beef
• Pork

Vitamin B12 can be found in multivitamins (including children's chewable and liquid drops), B complex vitamins, and individual supplements. Vitamin B12 is available in intranasal forms (administered through the nose), as well as oral tablets, capsules, soft gels, and lozenges. Vitamin B12 is also sold under the names cobalamin and cyanocobalamin.

If your diet includes meat, fish or shellfish, milk, and other dairy products, you should be able to meet the recommended daily requirements without taking a vitamin B12 supplement. Vegetarians who do not eat any animal products should take a vitamin B12 supplement with water, preferably after eating. Elderly people may need larger amounts of vitamin B12 than younger people because the body's ability to absorb vitamin B12 from the diet declines with age.

If you are considering taking a B12 supplement, ask your health care provider to help you determine the right dose for you.

Daily recommendations for dietary vitamin B12 are listed below.

Pediatric

• Newborns to 6 months: 0.4 mcg (adequate intake)
• Infants 6 months to 1 year: 0.5 mcg (adequate intake)
• Children 1 to 3 years: 0.9 mcg (RDA)
• Children 4 to 8 years: 1.2 mcg (RDA)
• Children 9 to 13 years: 1.8 mcg (RDA)
• Teens 14 to 18 years: 2.4 mcg (RDA)

Adult

• 19 years and older: 2.4 mcg (RDA)*
• Pregnant women: 2.6 mcg (RDA)
• Breastfeeding women: 2.8 mcg (RDA)

*Because 10 to 30% of older people may not absorb B12 from food very well, people over 50 should meet their daily requirement through either foods fortified with vitamin B12 or a supplement containing B12.

Because of the potential for side effects and interactions with medications, you should take dietary supplements only under the supervision of a knowledgeable health care provider.

Vitamin B12 is considered safe and nontoxic, however taking any one of the B vitamins for a long period of time can result in an imbalance of other important B vitamins. For this reason, you may want to take a B complex vitamin that includes all B vitamins. Taking folic acid at high doses can hide a vitamin B12 deficiency. So these vitamins are often taken together. Talk to your doctor before taking more than 800 mcg of folic acid.

People with abnormal levels of red blood cells, or abnormalities in their red blood cells, should work with a doctor to determine whether B12 is appropriate for them. In some instances, B12 can be beneficial. But it can be harmful with other health conditions. For instance, B12 supplements can cause serious damage to the optic nerve in people with Leber's disease (a disease of the eye). Again, be sure to work with your doctor.

If you are currently being treated with any of the following medications, you should not use vitamin B12 supplements without first talking to your health care provider.

Medications that reduce levels of B12 in the body include:

• Anti-seizure medications -- including phenytoin (Dilantin), phenobarbital, primidone (Mysoline)
• Chemotherapy medications -- particularly methotrexate
• Colchicine -- used to treat gout
• Bile acid sequestrants -- used to lower cholesterol; include colestipol (Colestid), cholestyramine (Questran), and colsevelam (Welchol)
• H2 blockers -- used to reduce stomach acid; include cimetidine (Tagamet), famotidine (Pepcid AC), ranitidine (Zantac)
• Metformin (Glucophage) -- medication taken for diabetes
• Proton pump inhibitors -- used to reduce stomach acid; include esomeprazole (Nexium), lansprazole (Prevacid), omeprazole (Prilosec), rabeprazole (Aciphex)

Antibiotics, Tetracycline: Vitamin B12 should not be taken at the same time as tetracycline because it interferes with the absorption and effectiveness of this medication. Vitamin B12 should be taken at different times of the day from tetracycline. All vitamin B complex supplements act in this way and should be taken at different times from tetracycline. In addition, long-term use of antibiotics can lower vitamin B levels in the body, particularly B2, B9, B12, and vitamin H (biotin), which is considered part of the B complex vitamins.

Abularrage CJ, Sidawy AN, White PW, Aidinian G, Dezee KJ, Weiswasser JM, Arora S. Effect of folic Acid and vitamins B6 and B12 on microcirculatory vasoreactivity in patients with hyperhomocysteinemia. Vasc Endovascular Surg. 2007 Aug-Sep;41(4):339-45.

Adachi S, Kawamoto T, Otsuka M, Todoroki T, Fukao K. Enteral vitamin B12 supplements reverse postgastrectomy B12 deficiency. Ann Surg. 2000;232(2):199-201.

Alpert JE, Mischoulon D, Nierenberg AA, Fava M. Nutrition and depression: focus on folate. Nutrition. 2000;16:544-81.

Annibale B, Lahner E, Fave GD. Diagnosis and management of pernicious anemia. Curr Gastroenterol Rep. 2011;13(6):518-24.

Bauman WA, Shaw S, Jayatilleke E, Spungen AM, Herbert V. Increased intake of calcium reverses vitamin B12 malabsorption induced by metformin. Diabetes Care. 2000;13(9):1227-31.

Booth GL, Wang EE. Preventive health care, 2000 update: screening and management of hyperhomocysteinemia for the prevention of coronary artery disease events. The Canadian Task Force on Preventive Health Care. CMAJ. 2000;163(1):21-29.

Bottiglieri T, Laundy M, Crellin R, Toone BK, Carney MW, Reynolds EH. Homocysteine, folate, methylation, and monoamine metabolism in depression. J Neurol Neurosurg Psychiatry. 2000;69(2):228-32.

Chatterjee S, Chowdhury RG, Khan B. Medical management of male infertility. J Indian Med Assoc. 2006 Feb;104(2):74, 76-7.

Christen WG, Glynn RJ, Chew EY, et al. Folic acid, pyridoxine, and cyanocobalamin combination treatment and age-related macular degeneration in women. Arch Intern Med. 2009;169:335-41.

Cuskelly GJ, Mooney KM, Young IS. Folate and vitamin B12: friendly or enemy nutrients for the elderly. Proc Nutr Soc. 2007;66(4):548-58.

Deutch B, Jorgensen EB, Hansen JC. n-3 PUFA from fish or seal oil reduce atherogenic risk indicators in Danish women. Nutr Res. 2000;20:1065-77.

Hoffman: Hematology: Basic Principles and Practice. 6th ed. Philadelphia, PA: Elsevier Saunders; 2012.

Kaptan K, Beyan C, Ural AU, et al. Helicobacter pylori -- is it a novel causative agent in vitamin B12 deficiency? Arch Intern Med. 2000;160(9):1349-53.

Ko SH, Ko SH, Ahn YB, et al. Association of vitamin B12 deficiency and metformin use in patients with type 2 diabetes. J Korean Med Sci. 2014; 29(7):965-72.

Konings EJ; Committee on Food Nutrition. Water-soluble vitamins. JAOAC Int. 2006 Jan-Feb;89(1):285-8.

Kris-Etherton P, Eckel RH, Howard BV, St. Jeor S, Bazzarre TL. Lyon diet heart study. Benefits of a Mediterranean-style, National Cholesterol Education Program/American Heart Association Step I dietary pattern on cardiovascular disease. Circulation. 2001;103:1823-25.

Lam JR, Schneider JL, Zhao W, Corley DA. Protein pump inhibitor and histamine 2 receptor antagonist use and vitamin B12 deficiency. JAMA. 2013; 310(22):2435-42.

Louwman MW, van Dusseldorp M, van de Vijver FJ, et al. Signs of impaired cognitive function in adolescents with marginal cobalamin status. Am J Clin Nutr. 2000;72(3):762-9.

Mazokopakis EE, Starakis IK. Recommendations for diagnosis and management of metformin-induced vitamin B12 (Cbl) deficiency. Diabetes Res Clin Pract. 2012;97(3):359-67.

Moore D, Jefferson J. Handbook of Medical Psychiatry. 2nd ed. Philadelphia, PA: Elsevier Mosby. 2004;Chapter 234.

National Academy of Sciences. Dietary Reference Intakes (DRIs): Recommended Intakes for Individuals, Vitamins. Accessed June 1, 2011.

Nutrients and Nutritional Agents. In: Kastrup EK, Hines Burnham T, Short RM, et al, eds. Drug Facts and Comparisons. St. Louis, MO: Facts and Comparisons; 2000:4-5.

Pawlak R, Lester SE, Babatunde T. The prevalence of cobalamin deficiency among vegetarians assessed by serum vitamin B12: a review of literature. Eur J Clin Nutr. 2014; 68(5):541-8.

Remacha AF, Sarda MP, Canals C, Queralto JM, Zapico E, Remacha J, Carrascosa C. Combined cobalamin and iron deficiency anemia: a diagnostic approach using a model based on age and homocysteine assessment. Ann Hematol. 2013;92(4):527-31.

Ryan-Harshman M, Aldoori W. Vitamin B12 and health. Can Fam Physician. 2008;54(4):536-41.

Schnyder G. Decreased rate of coronary restinosis after lowering of plasma homocysteine levels. N Engl J Med. 2001;345(22):1593-1600.

Sinclair S. Male infertility: nutritional and environmental considerations. Alt Med Rev. 2000;5(1):28-38.

Snowdon DA, Tully CL, Smith CD, Riley KR, Markesbery WR. Serum folate and the severity of atrophy of the neocortex in Alzheimer disease: findings from the Nun Study. Am J Clin Nutr. 2000;71:993-8.

Stucker M, Memmel U, Hoffmann M, et al. Vitamin B(12) cream containing avocado oil in the therapy of plaque psoriasis. Dermatology. 2001;203:141-7.

Triantafyllou NI, et al. Folate and vitamin B 12 levels in levodopa-treated Parkinson's disease patients: their relationship to clinical manifestations, mood and cognition. Parkinsonism Relat Disord. 2008;14(4):321-25.

Wang HX. Vitamin B12 and folate in relation to the development of Alzheimer's disease. Neurology. 2001;56:1188-94.

Yoshihara K, Kubo C. Overview of medical treatment and management of chronic fatigue syndrome. Nippon Rinsho. 2007 Jun;65(6):1077-81. Review.