Vitamin B12 is a water-soluble vitamin that is naturally found in such foods as shellfish, meat and dairy products, but it can also be taken orally in pill form, injected intravenously or applied to the mouth and nose in gel form, according to the MayoClinic.com. Vitamin B12 helps make and maintain healthy nerves, red blood cells and DNA. The recommended dosage of vitamin B12 for adolescents and adults is 2.4mcg per day. Excess amounts of vitamin B12 can cause serious side effects, so it is important to speak with a medical professional before taking any vitamins.
Cardiovascular Problems
People who take too much vitamin B12 intravenously may experience cardiovascular problems, according to Drugs.com. Excess vitamin B12 can interfere with blood flow to the heart and lungs, resulting in congestive heart failure or peripheral vascular thrombosis, a condition that occurs when a blood clot forms in a blood vessel. Too much vitamin B12 can also cause pulmonary edema, a condition that arises when air sacs form in the lungs. Symptoms of a B12 overdose may include loss of breath and pain and numbness in the arms and legs.
Gastrointestinal Problems
An extremely rare side effect associated with excess vitamin B12 is gastrointestinal problems, according to Drugs.com. Elderly people and people with a history of multiple gastrointestinal surgeries may be especially vulnerable to gastrointestinal problems when taking higher then normal amounts of vitamin B12. Side effects of a vitamin B12 overdose may include nausea, upset stomach, indigestion, vomiting and mild diarrhea.
Skin Rashes
Some people may develop dermatological problems when they take extra vitamin B12, according to Drugs.com. Applying too much vitamin B12 gel in the nose or injecting B12 into the body can cause a variety of skin rashes. People who absorb too much vitamin B12 in the body may experience severe inflammatory facial acne, mild itchy skin, body rashes or folliculitis, a condition that occurs when hair follicles become infected and inflamed. One person experienced acne rosacea after ingesting 100mcg of B12 and 100mg of B6 on a daily basis. Skin rashes generally improve when the dosage is reduced or the vitamin discontinued.
Hypersensitivity
A rare complication associated with the overuse of vitamin B12 is hypersensitivity, according to Drugs.com. A small number of hypersensitive people may experience a mild allergic reaction or anaphylaxis, a life-threatening allergic reaction during a B12 injection due to the addition of impure ingredients to vitamin B12 solution. Anaphylaxis may occur soon after receiving the first dose of B12 or it may arise weeks, months or rarely years later. According to Medline Plus, symptoms of an allergic reaction may include nausea, abdominal pain or cramping, loss of breath, wheezing, dizziness, heart palpitations, hives and diarrhea.
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