Getting too many vitamins from food is almost unheard-of, but if you're taking individual supplements as a backup, use caution. "At megadoses of up to 10 times what's recommended, some nutrients behave very much like drugs," says Lora Sporny, a professor of nutrition at Columbia University.

Some examples: Too much calcium may inhibit iron absorption. Too much vitamin A may interfere with vitamin D function, leading to bone fractures. And vitamin E taken in amounts over 20 times the RDA can have an anticlotting effect — potentially dangerous if you're already taking aspirin or blood thinners.