Vitamin B12 Toxicity

Vitamin B12 is a popular supplement with dosages often ranging into thousands of percent of RDA, so it's only natural to wonder about the potential for vitamin B12 toxicity. If prolonged overdosing on vitamin A, D, E and others can cause significant health problems, who's to say B12 is any different?

First off, B vitamins are different from A, D, E and K in that they are water-soluble rather than fat-soluble. That means it is very difficult to overdose, as any excess vitamins, ie. whatever the body doesn't need, is simply flushed out. In other words, if you chug big doses of B12, the main impact is that you get extra expensive urine.

Even when injected directly into the body, it seems vitamin B12 toxicity is something of a non-issue. According to Jane Higdon, Ph.D. at Linus Pauling Institute, Oregon State University:

"Doses as high as 1 mg (1000 mcg) daily by mouth or 1 mg monthly by intramuscular (IM) injection have been used … without significant side effects. When high doses of vitamin B12 are given orally, only a small percentage can be absorbed, which may explain the low toxicity."

These are comforting words to be sure. All other sources reviewed concur in this assessment. Many respected institutes like University of Pittsburgh Medical Center simply conclude that "No symptoms of vitamin B12 toxicity have been reported."

What does Uncle Sam have to say on the topic of vitamin B12 toxicity? Surely, a cautious outfit like the Food and Nutrition Board, which decides the RDA, has a few words of warning? Not really, it turns out. When the RDA guidelines received their major overhaul in 1998, the Board concluded the risk of toxicity was so low they simply chose not to establish an "upper limit" recommendation (1).

So what are the official RDA guidelines on vitamin B12 intake, then? This fact sheet by the Office of Dietary Supplements, National Institutes of Health provides this chart:

While it should be clearly established that vitamin B12 toxicity in and by itself is not a big risk, there are other concerns that should be mentioned. Those who take different kinds of medication, notably proton pump inhibitors (like the common over-the-counter acid reflux drug Prilosec) have lower amounts of the stomach acid needed for proper B12 absorption from food.

The same applies to H2-receptor antagonists, another class of gastric acid inhibitors (Zantac, Tagamet etc). Thus, those who are taking this kind of medication for a prolonged period of time would be well advised to err on the side of caution and take B12 supplements regularly to avoid any risk of deficiency.

For those wishing to avoid artificial supplements, most forms of seafood (clams, mussels, crab and fish) are naturally rich in vitamin B12 and often come with a lot of other very beneficial nutrients.

1 Food and Nutrition Board, Institute of Medicine. Vitamin B12. Dietary Reference Intakes: Thiamin, Riboflavin, Niacin, Vitamin B6, Vitamin B12, Pantothenic Acid, Biotin, and Choline. Washington D.C.: National Academy Press; 1998:306-356

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