Vitamin B12 is an essential water soluble vitamin that is used by your body to manufacture DNA and to maintain red blood and nerve cells. Because excess water soluble vitamins are generally excreted by the body through the urine, overdose of vitamin B12 is rare. However, there are certain situations, such as if you plan to undergo angioplasty or if you have specific underlying conditions, in which too much vitamin B12 can be harmful.
Vitamin B12 Overview
Vitamin B12 is found primarily in animal-based foods like fish, poultry, dairy products and eggs. A water soluble vitamin, B12 is unusual because your body can store enough to last you several years. For this reason, vitamin B12 deficiency is rare. The B12 found in foods is released during the digestive process by acids in the stomach, where it then combines with what is called "intrinsic factor," so it can be absorbed by your bloodstream.
B12 and Angioplasty
According to the Mayo Clinic, you should be cautious about your vitamin B12 intake if you plan to go through angioplasty. Taking a large IV-based dose of the B-complex vitamins folic acid, B6 and B12 and then following up with daily supplementation has been linked to an increase in restenosis, or having to repeat the stent placement procedure. If you have any cardiac history, you should discuss your plans to take large doses of any vitamin with your physician.
Folic Acid and Vitamin B12
Occasionally, large doses of the B-complex vitamin folic acid can mask another underlying problem -- a deficiency of vitamin B12. According to the U.S. Department of Health and Human Services' Office of Women's Health, consuming more than 1,000 micrograms of folic acid per day may put you at increased risk of having a B12 deficiency masked. While vitamin B12 deficiencies are rare in healthy adults, be cautious about taking large doses of folic acid for this reason. If you are diagnosed with a B12 deficiency, follow your physician's recommendations for supplements to ensure you do not take too much vitamin B12.
When to Exercise Caution
For some people, large doses of vitamin B12 should be avoided. If you have a disease called Leber's disease, there is a chance that a large injection of vitamin B12 could result in damage to your optic nerve, and even eventually contribute to blindness. If you have an allergy to the element cobalt, you should also avoid large doses of vitamin B12 because cobalt is actually a component of B12. Aside from those situations, life-threatening reactions to large doses of B12 are rare. Contact your health practitioner immediately if you experience chest pain, shortness of breath, swelling or redness and pain in one of your extremities; as these symptoms could mean a reaction to B12.
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