Vitamin B-12 is a water-soluble vitamin involved in several biological processes in the body. Because vitamin B-12 is water-soluble, eating more than your body needs simply causes the excess to be flushed from the system. As a result, overdosing on vitamin B-12 is extremely rare. However, some people may experience side effects from vitamin B-12, regardless of the dose. As with any vitamin or health supplement, you should consult your doctor before taking vitamin B-12.
Recommended Daily Dose
The recommended daily allowance of vitamin B-12 is 2.4 mcg per day for adults 14-years-old and older, reports the Office of Dietary Supplements of the National Institutes of Health. Because of the low risk of toxicity, no tolerable upper intake level has been set by the U.S. Food and Drug Administration. Doses up to 1,000 mcg per day have been used to treat cases of vitamin B-12 deficiency, with no increased risk of side effects, reports the Linus Pauling Institute at Oregon State University.
Side Effects
Regardless of the size of the dose, vitamin B-12 supplements may cause side effects. Commonly reported side effects include dry, itchy skin, rash, or diarrhea, reports MedlinePlus. In certain individuals, more serious side effects, including blood clots, may develop.
Allergic Reaction
In rare cases, a severe allergic reaction to vitamin B-12 may develop. Vitamin B-12 contains a metal cobalt ion, and people who are allergic to cobalt will often experience an allergic reaction to vitamin B-12. Symptoms of an allergic reaction including swelling of the throat, face, tongue or hands and difficulty breathing. If you experience any of these symptoms after taking vitamin B-12, contact emergency medical services immediately.
Interactions
Long-term use of a single B vitamin, such as B-12, can cause the other B vitamins to become unbalanced, explains the University of Maryland Medical Center. You should usually take a B complex vitamin instead of an individual vitamin, unless directed otherwise by your doctor. Vitamin B-12 can reduce the effectiveness of the antibiotic tetracycline. If you are prescribed tetracycline, you should always take it at a different time during the day from when you take vitamin B-12. People with a rare hereditary eye disease called Leber's disease should not take vitamin B-12, because of risks of serious damage the to optic nerve.
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