Vitamin B12 is a water-soluble vitamin that is bound to protein in ingested food. Tissue storage capacity of water-soluble vitamins is limited. Water-soluble vitamins are excreted in urine daily. This prevents overdosing. When high doses of vitamin B12 are taken orally, only a small percentage can be absorbed, which may explain the low toxicity.
No Sufficient Evidence of Vitamin B12 Toxicity
No toxic or adverse effects associated with a large intake or overdose of vitamin B12 from food sources or supplements in healthy people have been documented. Approximately 56 percent of a 1mcg oral dose of vitamin B12 is actually absorbed. Absorption actually decreases when the intake amount of the vitamin is increased. There was no sufficient evidence for the Food and Nutrition Board to set a tolerance upper intake level for vitamin B12.
Side Effects of Oral and Injectable Vitamin B12 Supplements
No side effects are generally associated with recommended or increased doses of oral vitamin B12 supplements. Rare allergic reactions to vitamin B12 injections have been reported. It's unclear if the allergic response is to the vitamin or the preservatives and other substances in the solution.
Contraindications for Taking Vitamin B12
Vitamin B12 is contraindicated for patients with early Leber's disease, which is a hereditary optic nerve atrophy. Vitamin B12 can cause severe and sudden optic atrophy, which is a degeneration of the optic nerve.