You know those sexual enhancement dietary supplements for sale at gas stations and markets across the country? Beware, they might actually be viagra. Or steroids. Or an antidepressant. Many supposed dietary supplements for weight loss, erectile dysfunction, and muscle building may contain actual pharmaceuticals—but you likely have no way of knowing what's in them.

Between 2007 and 2016, the FDA issued warnings about unapproved pharmaceutical ingredients in 776 dietary supplements, according to a new report in JAMA Network Open. Of those, less than half received voluntary recalls. The authors compiled their data from the FDA’s own warning website. Known as the Tainted Products Marketed as Supplements List, it catalogs any time the FDA reports finding unapproved pharmaceutical ingredients in supplements. The hundreds of offending supplements the FDA found during that nine-year period traced back to 146 companies. These represent only a small fraction of the potentially hazardous supplements on the market.

The paper paints a damning picture of the growing supplement market, and should alarm anyone considering taking them, especially those for weight loss, muscle gain, or erectile dysfunction. And that’s a lot of people. More than half of US adults report taking dietary supplements, accounting for a $35 billion industry. A dietary supplement is defined by the FDA as any vitamin, enzyme, botanical, amino acid, or mineral that is not intended to cure or prevent a disease. A 2015 study found that supplement use sends 23,000 people to the ER each year in the US.

The paper published today is an analysis of what, if any, action the FDA has taken when it finds illegal substances in supplements. “The drug ingredients in these dietary supplements have the potential to cause serious adverse health effects owing to accidental misuse, overuse, or interaction with other medications, underlying health conditions, or other pharmaceuticals within the supplement,” write the authors, which include Jenna Tucker of the California Department of Food and Agriculture, Madhur Kumar of the Food and Drug Branch of the California Department of Public Health, and others. More than half of the supplements the FDA identified as dangerous are still for sale.

The authors report that the most common adulterants the FDA found in supplements were sildenafil (generic Viagra), sibutramine (generic Meridia for weight loss, which has been linked to increased risk of cardiovascular events and has been removed from the market in many countries), and synthetic steroids or steroid-like ingredients. They found that 20 percent of the products on the FDA’s Tainted list between 2007 and 2016 contained more than one unapproved ingredient. Even after receiving an FDA warning, some products continue to be sold. "This is alarming, especially considering that the FDA is only able to test a portion of products available on the market,” the report reads.

That gets at the heart of why this study matters so much, and where it fits into a growing body of work into the risks of supplements on store shelves today: The way the FDA regulates supplements doesn’t work.

“The FDA does not have control of this market,” says former principal deputy commissioner for the FDA Josh Sharfstein, who was not involved in the study. Sharfstein, who is now the vice dean for public health at Johns Hopkins, calls the study a good one, but cautions against blaming the FDA for its failures to sufficiently rein in the industry.

“Until the FDA has more authority to know what’s on the market,” he says, “there’s no way they are ever going to get control of it.”

Though the FDA is charged with regulating the supplement market, existing laws empower it to act only after supplements have hit store shelves. Unlike with prescription medications, dietary supplement makers are not required to apply for FDA approval or even register with the FDA before they sell their products. Experts say usually the FDA only investigates supplements after a consumer or physician alerts them to some kind of negative reaction the supplement has caused. “They have been set up with an impossible task of post-market surveillance of thousands upon thousands of products,” says Amy Eichner, Special Advisor on Drug Reference & Supplements at the US Anti-Doping Agency, where she works to warn athletes and military personal of the risks of dietary supplements. Eichner was not involved in the study.