Vitamin B12, an essential water-soluble vitamin, is found in fish, shellfish, dairy products and meat, according to MayoClinic.com. Many supplements contain vitamin B12. Unlike most water-soluble vitamins, vitamin B12 is stored in large amounts within the body. If your vitamin B12 level increases significantly -- perhaps due to excessive dosing with supplements -- you may experience vitamin B12 toxicity.
Function
Supplements are most commonly used to prevent vitamin B12 deficiency and to treat any diseases caused by low vitamin B12 levels, according to Medline Plus. Doctors use vitamin B12 to treat a condition called pernicious anemia. Vitamin B12 is essential for the growth and production of red blood cells and nerves. A deficiency can cause abnormalities in the function and shape of your blood cells; deficiency can also lead to nervous system symptoms, such as tingling and numbness in the extremities.
Overdose
Doses as high as 1 mg per day have been used to treat patients suffering from vitamin B12 deficiency, according to the Linus Pauling Institute. Even at these doses, vitamin B12 does not cause any significant side effects. However, massive doses -- greater than 2 mg --or combination of vitamin B12 with other vitamins, such as vitamin B6 or folic acid, can cause unintended adverse side effects.
Symptoms of Toxicity
Symptoms from high doses of vitamin B12 include diarrhea, blood clots, itching and, at increased doses, allergic reactions, warns Medline Plus. Pregnant or breast-feeding women should take no more than 2.8 mcg per day. Higher doses can cause discomfort and other side effects to both the mother and the child.
Normal Dosages
Vitamin B12 toxicity is rare; treatment focuses on reducing your vitamin B12 intake. Most commonly, doctors attempt to reduce the daily intake to recommended values. Teenagers and adults should get 2.4 mcg of vitamin B12 per day, according to MayoClinic.com. Pregnant and breast-feeding women can tolerate up to 2.8 mcg each day. Adults aged 50 and older may have a difficult time absorbing vitamin B12 and may require supplementation. Senior citizens require 25 to 100 mcg of vitamin B12 per day.
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