A study of consumer data in 2016 found that 71 percent of U.S. adults take some type of dietary supplement. The appeal of supplements is obvious. We know vitamins are necessary for health, so why not make sure we’re covering our bases?

The darker side of dietary supplements is that many of the alleged health benefits are coming straight from people who stand to make big money from vitamin sales — and research doesn’t always agree with the claims of vitamin manufacturers and retailers.

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Here’s a look at four vitamins you may want to think twice about.

1.) Vitamin E. According to the National Institutes of Health, the recommended daily allowance (RDA) of vitamin E for adults is 15 milligrams (or 22.4 IU). RDAs are based on what will meet the nutritional needs of 97-98 percent of healthy people, but supplements are widely available, from 200 to 1,000 IU per pill. When it comes to vitamin E, the research is clear that you can have too much of a good thing.

A large review of the research by John Hopkins University found that people who took more than 400 IU daily faced a 4-6 percent increased risk of death. Another large study found that, despite health claims to the contrary, vitamin E did not decrease prostate cancer risk in study participants. In fact, men who supplemented with vitamin E were slightly more likely to develop prostate cancer than men who didn’t use the supplement.

2.) Vitamin C. Pricey vitamin C supplements marketed for the prevention and treatment of the common cold and the flu have popped up in every grocery store and drug store. They come as pills, lozenges, and powdered drinks and typically contain 500-1,000 mg of vitamin C, at least five to 10 times the RDA of vitamin C for adults.

Some people use supplements daily to prevent colds, and others use high doses at the beginning of a cold to shorten its duration. But a review of the evidence shows that vitamin C only impacts the common cold in one way — if you’re already taking daily vitamin C, your colds may be a little bit shorter. Despite the claims of manufacturers, people who supplement daily don’t get fewer colds, and starting supplementation at the beginning of a cold doesn’t affect cold symptoms.

Related: Why You’ll Definitely Catch Your Child’s Cold

If you choose to take a daily vitamin C supplement to shorten your colds, keep in mind that megadoses can contribute to kidney stones. (go to page 2 to continue reading) [lz_pagination]