Adverse reactions to excessive quantities of vitamin B12 typically do not occur. The vitamin is nontoxic, water soluble and safe enough in large doses that a maximum daily limit does not exist. Although consuming large amounts of vitamin B12 generally does not lead to overdose, any amount of vitamin B12 can have in adverse effects under certain conditions.
Sensitivity and Allergies
A vitamin B12 overdose occurs after consuming large quantities of vitamin B12. Adverse reactions can occur if you are sensitive or allergic to the vitamin supplement's components, according to the Mayo Clinic and eMedTV. Vitamin B12 supplements usually contain cobalt and cobalamin, and if you are sensitive or allergic to those or other ingredients in the supplements, you may experience symptoms such as rashes, itchiness, hives, difficulty breathing, wheezing and swollen lips, throat or mouth. Other potential side effects include a swollen sensation throughout your entire body, diarrhea and blood clots, particularly in the legs. Shock and acne are other potential side effects, according to Encyclopedia.com.
Angioplasty and vitamin B12 do not mix. Angioplasty is a procedure that unblocks arteries using a thin, balloon-tipped tube. Doctors insert the tube into the artery and inflate the balloon to restore blood flow. Your risk for adverse effects from oral doses of vitamin B12 are high if you take them after this procedure, notes the Mayo Clinic. The angioplasty procedure includes an intravenous dose of vitamin B12, as well as vitamin B6 and folic acid. Consuming additional vitamin B12 after the procedure can cause your arteries to re-harden.
Vitamin B12 supplements pose a risk if you have Leber's disease, which is a heredity atrophy of the optic nerve. The vitamin can speed up the atrophy, or wasting away, of this main nerve in your eye. If you are pregnant, your body requires additional B12 during pregnancy, but you should pay attention to your intake. Insufficient evidence exists to determine whether high doses of vitamin B12 are harmful during pregnancy, and the Mayo Clinic reports that oral supplement doses that remain within the recommended dietary allowance are likely to be safe.
Recommended Dietary Allowance
The RDA of vitamin B12 for people 14 and older is 2.4 mcg per day, obtained either through supplements or diet. The most effective doses for those over 50 come from supplements or food items fortified with vitamin B12. Up to 30 percent of the adults over 50 are unable to absorb vitamin B12 from food items efficiently. The RDA is slightly increased to 2.6 mcg and 2.8 mcg for pregnant and lactating women, respectively. No RDA exists for children under 14, although adequate intake levels have been defined. Adequate intake for children 9 to 13 is 1.8 mcg. Children 4 to 8 should have 1.2 mcg, and those1 to 3 need 0.9 mcg. Infants need even less B12: the adequate intake level is 0.4 mcg for babies 0 to 6 months of age and 0.5 for those 7 to 11 months.