Vitamin B-12 helps form red blood cells and assists the body in acquiring energy from foods sources. Foods containing B vitamins include poultry and other meats, fish, eggs, dairy, beans and vegetables. Most Americans get enough vitamin B-12 from their diets, though 1.5 to 15 percent of people are deficient due to absorption problems, according to the National Institutes of Health. Vitamin B-12 is a water-soluble vitamin, which means that it is excreted daily in urine. Even when high doses of the vitamin are taken, only a small percentage is absorbed in the tissues. As such, side effects that may occur as a result of taking high dosages of this vitamin are minimal.
Allergic Reactions
MedlinePlus reports that although vitamin B-12 is safe for most people when taken orally or by injection, allergic reactions can occur in some users. The results may range from mild skin irritation and itching to anaphylaxis -- a potentially life-threatening condition that causes symptoms such as abdominal cramps, lightheadedness or fainting, vomiting or increased heart rate. Most drug allergies respond readily to treatment; however, in the case of anaphylaxis, immediate medical attention is crucial.
Diarrhea
In some users, vitamin B-12 can cause diarrhea, according to MedlinePlus. Diarrhea can be both uncomfortable and dangerous if too much fluid is lost, as dehydration can occur. The National Digestive Diseases Information Clearinghouse describes diarrhea as loose, watery stools that occur more than three times in a day. Other symptoms of diarrhea include abdominal cramps, bloating, nausea and an urgent need to have a bowel movement.
Polycythemia Vera
Taking vitamin B-12 may unmask symptoms of polycythemia vera, which is a disease involving too many red blood cells and the formation of abnormal blood clots. Vitamin B-12 does not cause this disease, but it can make the symptoms more noticeable.
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