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From 2007 to 2016, the Food and Drug Administration logged 776 dietary supplements as being adulterated with pharmaceuticals. Less than half of those products were recalled. That’s according to a study published today (October 12) in JAMA Network Open by researchers at the California Department of Public Health.

The Food and Drug Administration (FDA) regulates supplements differently from drugs—which have to be tested for safety and effectiveness before they go to market—but more like they do for food. That means that the FDA will find cases of adulterated supplements only after they are on the market. When the agency identifies spiked products, it logs them in a public database, which the researchers mined for this study.

The supplements most often adulterated were those marketed for sexual enhancement, weight loss, or muscle building, the study reports.

Such unlabeled and unauthorized inclusion of medicines can be dangerous. For example, 81 percent of sexual enhancement supplements listed in the database contained sildenafil, the active ingredient in Viagra. Some men cannot take erectile dysfunction drugs because their active ingredients interact dangerously with the nitrates in drugs for diabetes, high blood pressure, and high cholesterol and can dangerously lower blood pressure. Such restrictions might lead men to turn to supplements that claim to be plant-based as an alternative, not realizing that they’re still consuming the product they were trying to avoid, reports The Washington Post.

Not only did the supplements contain undisclosed pharmaceuticals, they also included products that should not be on the market in any form at all: those that had been withdrawn and that were never approved in the first place.

As Pieter Cohen, an internist affiliated with Harvard Medical School and Cambridge Health Alliance in Somerville, Massachusetts, describes in an editorial published alongside the study, “voluntary recalls are the most common approach used by the FDA to remove adulterated supplements from commerce.” If the company agrees to voluntarily recall the product, the FDA publicizes that recall. Yet fewer than half of the supplements in the database were recalled.

As Cohen puts it, “the FDA fulfilled its responsibilities less than half of the time.” He adds that in some cases, it’s possible the FDA did ask firms to pull their products from the market, but the companies refused.

Even in those situations, the FDA has other ways to force a firm to stop selling dangerous pills. Although the FDA can mandate a recall, it did not do so for any of these products, Cohen notes. “There’s just no way to interpret this other than the FDA is simply not doing its job,” Cohen tells Live Science.

So what should people do? “The advice I give my patients who want to use supplements is to only use single-ingredient supplements, and to not buy supplements that are riding a fad,” Cohen tells Gizmodo.