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Vitamin B12 is a large complex compound the body relies upon for DNA synthesis, red blood cell production, electrical nerve conductance and healthy metabolism. A lack of dietary consumption is more of a concern -- especially for vegetarians -- than taking too much of it. Vitamin B12 toxicity is very low, but allergic reactions to synthetic supplements are possible, so use caution and consult your physician if you have concerns.
Vitamin B12 status is measured by blood tests, and the the National Institutes of Health says healthy B12 blood serum levels should be at least 250 picograms per milliliter in adults. Deficiency in fairly common in the United States, as somewhere between 2 percent and 15 percent of Americans are thought to be affected. Chronic B12 deficiency leads to a variety of problems, including anemia, debilitating fatigue, muscle weakness, loss of balance, depression, constipation, loss of appetite, reduced cognition and other neurological issues such as numbness and tingling in the hands and feet. To prevent deficiency issues, the recommended daily allowance of B12 for adults is 2.4 micrograms.
The toxicity of B12 is low enough that it doesn't have a tolerable upper intake level set by the U.S. Food and Nutrition Board. No toxic or adverse side effects are associated with large intakes of natural B12 from food, termed cobalamin, and only rare allergic reactions are associated with mega-dosing synthetic supplements such as cyanocobalamin. Cyanocobalamin doses as high as 1,000 micrograms a day by mouth or 1,000 micrograms by intramuscular injection have been used in studies to treat anemia without significant side effects. When taken orally, the absorption rate of B12 in the stomach is very low -- typically less than 1 percent -- and dependent on gastric production of intrinsic factor, which may partially explain the vitamin's low toxicity.
MayoClinic.com notes that people who are sensitive or allergic to cobalamin, cobalt or any other ingredients in the supplements should avoid vitamin B12 supplements. Typical allergic reactions include itching, hives, difficulty breathing, swelling in or around the mouth, vasculitis, increased heart rate, diarrhea and stomach upset. Elevated heart rate might be a result of the vitamin's natural potential to increase blood volume because it stimulates red blood cell production within bone marrow. Do not attempt vitamin B12 therapy without consulting your health care provider.
The most common type of B12 supplement is cyanocobalamin, a synthetic compound used primarily because of its low cost. The body converts cyanocobalamin into methylcobalamin and adenosylcobalamin, which are the bioactive forms of B12. The conversion process creates small amounts of cyanide, but not enough to be considered a health concern at doses up to 1,000 micrograms via injection. Less common and more expensive synthetic forms of B12 include hydroxocobalamin and methylcobalamin, which don't lead to cyanide production and are likely better absorbed within the intestines.