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More than half of U.S. adults take dietary supplements, a category of products that include over-the-counter vitamins, minerals, botanicals, amino acids, and enzymes. But the supplement industry is only loosely regulated by the Food and Drug Administration (FDA), and a new study is shedding light on just how many of these products contain unapproved and potentially dangerous prescription-strength drugs.

Even though the FDA only inspects a small percentage of the supplements sold in the U.S. every year, the agency issued warnings about 776 different products that contained “undeclared, unapproved pharmaceutical ingredients” between 2007 and 2016, according to the study published this week in JAMA Network Open.

Experts have been warning about these problems in the pharmaceutical industry for years, but this is the first time FDA warnings have been quantified to show the extent of the problem.

More than 98% of the FDA’s warnings over this period pertained to supplements marketed for sexual enhancement, weight loss, or muscle building. Most commonly, the FDA found traces of the drugs sildenafil (the generic name for Viagra); sibutramine (a weight-loss drug that was taken off the market after it was linked to heart attacks and strokes); and synthetic steroids or steroid-like ingredients.

Other hidden drugs include antidepressants, laxatives, and stimulants. Some of those drugs have not been approved—or have been banned—by the FDA, and have been linked to serious side effects including suicidal thoughts, abnormal bleeding, and seizures.

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In addition to those top three categories, the investigation also found unapproved ingredients in 14 supplements marketed for joint pain, muscle pain, osteoporosis, bone cancer, sleep issues, gout, or prostate health.

Overall, the FDA sent warnings to 146 different supplement companies. More than 150 of the products cited in these warnings (about 20%) contained more than one unapproved ingredient, and 28 of them were cited in two or three warnings more than six months apart. In other words, even after these companies were warned once, they continued selling adulterated products—often with new unapproved substances detected the second or third time around.

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The study authors concluded that the drugs found in dietary supplements “have the potential to cause serious adverse health effects,” either from accidental misuse, overuse, or interaction with other medications, supplements, or existing health conditions. They also point out that hidden pharmaceutical ingredients may be one reason why supplement use is associated with 23,000 emergency department visits and 2,000 hospitalizations in the United States each year.

The study also shows that the number of tainted supplements reported by the FDA has increased over the years, and that they include both products purchased via international mail shipments as well as those for sale in the United States. (Just today, a weight-loss supplement was recalled due to detection of a hidden drug.) “Adulteration with active pharmaceutical ingredients does not happen by accident,” the authors wrote in their paper, “and poses a serious public health risk as consumers unknowingly ingest these drugs.”

Pieter Cohen, MD, assistant professor of medicine at Harvard Medical School, wrote an editorial that accompanies the new study. The number of adulterated products highlighted in this study was “no surprise at all,” he tells Health.

“We’ve known for years that companies have been putting drugs into supplements, and we thought there might be 100 or 200 of those kinds of products,” he says. “As time has gone by, every year more and more products have been seen.”

Dr. Cohen says the new study also highlights just how few of these products were ultimately recalled by the FDA. Even if they were, it may not have made much of a difference: His own research shows that following FDA recalls, supplements with unapproved ingredients are often still available for purchase, and consumers remain unaware of their potential dangers.

The U.S. government isn’t currently doing enough to guarantee supplement safety, Cohen wrote in his editorial, and Congress should reform the Dietary Supplement Health and Education Act of 1994 to require stricter testing and enforcement policies.

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In the meantime, he says, it’s largely up to consumers to be informed about the potential risks and benefits of supplements. He agrees that products marketed for sexual enhancement, weight loss, and muscle building pose the greatest risk in terms of hidden drug ingredients. Supplements used for more general health and wellness (like multivitamins or fish-oil capsules, for example) are less likely to contain dangerous drugs, he says—but that doesn’t mean they don’t have their own problems with inconsistent and misleading ingredients.

“They might not be accurately labeled in terms of the dosage you’re getting, or the labels might be misrepresenting the health benefits,” he says. Dr. Cohen says he regularly recommends vitamins and minerals to his patients who have (or are at risk of) deficiencies, but—with the exception of multivitamins and calcium/vitamin D supplements—he tells them to look for single-ingredient products.

“If your doctor tells you to take iron, look for a product that’s just iron,” he says. “If you decide to take black cohosh, look for something that’s just labeled black cohosh—not a mix of ingredients.” He also recommends staying away from supplements that claim to have specific health benefits, since those claims don’t have to be backed up by research.

That’s one of the most important things to understand about supplements, he says. “When you buy a product that says it will help you lose weight or improve your workout, there is no requirement that there’s any evidence in humans that it will actually work,” he says. “That’s where consumers have to be wise about their choices, and should consult with their doctor about what they really need.”