Popular but Dangerous: 3 Vitamins That Can Hurt You

If you take a vitamin pill, these warnings probably apply to you.

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Adds Mullin, “In high-enough doses, vitamin C can cause kidney stones.” Any amount larger than 500 milligrams per day can be enough to cause a problem, he says. That’s only half a 1-gram packet of Emergen-C. “It rarely happens, but there have been case reports.”

Reasonable Doses. Sandquist recommends that healthy people abide by the Institute of Medicine’s “Tolerable Upper Intake Levels,” which indicates the maximum daily intake of a vitamin you should consume through a combination of diet and supplements. Taking more than that amount means the risks likely outweigh the benefits. The recommended amount is often less than the limit.

“When the IOM makes their recommendations, they look at all the available research,” she says, so its conclusions are more reliable than any single study, even one that gets a lot of publicity.

Because taking supplements is second nature for many consumers, vitamins are often overlooked as a potential culprit for symptoms like headaches or diarrhea, Sandquist says. It’s important to be conscious of what you’re consuming—in natural, supplement, and fortified forms—and to tell your doctor about every last one.

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“The best strategy is to follow the ‘choose my plate’ method,” she says, referring to the U.S. Department of Agriculture’s healthy food guide. If people do that, she says, “then they probably wouldn’t have to worry about a vitamin supplement unless they have a specific medical condition.”

[Next story: 4 Herbal Supplements Your Doctor Hates]