Pieter Cohen, an internist in Massachusetts, got interested in dietary supplements several years ago, when some of his patients came to see him with unexplained — and serious — symptoms. Some went to the hospital with chest pain, or even kidney failure. Others lost their jobs because of positive drug tests. Eventually, after getting them to open up, Cohen realized they all had something in common: They were taking weight-loss pills and other dietary supplements.

An estimated 85,000 supplement products are sold in stores and online. Most of these are vitamins and minerals which, though unlikely to offer real health benefits, are fairly harmless. The supplement industry — which rakes in some $32 billion a year from American consumers — claims that the vast majority of supplements are safe. But that’s really an impossible claim, because most supplements don’t go through any kind of rigorous scientific scrutiny.

Unlike prescription drugs, dietary supplements do not have to be approved by the Food and Drug Administration (FDA) before they’re sold. That means that the public typically finds out about a product’s risks thanks to anecdotal reports from patients and doctors like Cohen.

Cohen’s experience with his patients spurred him to investigate the ingredients in a range of supplements. What he and others have found is alarming, particularly because about two-thirds of American adults say they’ve tried supplements, and half use them regularly. Most people think of supplements as “natural” ingredients found in plants. But it turns out that a lot of supplements — 560 products identified so far — are tainted with synthetic pharmaceutical compounds, including stimulants, steroids and antidepressants.

For example, last year Cohen published a study showing that a workout supplement called Craze — then sold in GMC stores, as well as Walmart’s website and Amazon — contained a synthetic stimulant called N,α-DEPEA that is a chemical cousin of methamphetamine. Yes, meth. The drug wasn’t in small quantities, either; Cohen’s group found between 21 and 35 milligrams per serving. (The bottle’s label said that the chemical was a natural product of the dendrobium orchid, a claim that, given Cohen’s findings, is almost certainly false.)

Cohen says that he told the FDA about the Craze findings six months before he published the paper. “They did nothing, zero,” he says. “I was really frustrated by the lack of action.”

After the paper came out (and after USA Today published an expose on the criminal past of the founder of Craze’s manufacturer, Driven Sports), the company announced that it had stopped production of Craze. Many months later, in April, the FDA finally sent a warning letter to Driven Sports.

By that time Cohen had heard that the company had replaced Craze with a new product, called Frenzy, sold outside of the U.S. He purchased some of it online and found an ingredient on the label that he had never seen before: “AMP citrate”. After digging into it a bit more, Cohen’s group found that more than a dozen supplements contained this ingredient — also known as 4-amino-2-methylpentane citrate, 1,3- dimethylbutylamine citrate, 4-amino-2-pentanamine, and 4-AMP. Frenzy and one other supplement described the chemical as an extract of Pouchong tea, another dubious claim.

In a study published last month, Cohen’s team performed a chemical analysis of 14 supplements containing this ingredient. The scientists found that 12 of them contained 1, 3-dimethylbutylamine (DMBA). DMBA is chemically similar to DMAA, a stimulant designed by Eli Lilly decades ago as a competitor to amphetamine. In 2006 DMAA started showing up in supplements, and by 2010 it was bringing in $100 million in annual sales. But the FDA banned DMAA last year, after it had been linked to dozens of health problems and five deaths.

Cohen informed the FDA about his new DMBA findings, but isn’t optimistic that they’ll take action anytime soon.

It may be, as industry groups argue, that these unfortunate events are the result of a few bad apples — shady companies that don’t represent the industry as a whole. The thing is, because the industry gets so little oversight, there’s no way to know that for sure. In fact, even when dangerous products are pulled from the shelves, they often reappear on the market later with the same scary ingredients.

A couple of weeks ago, Cohen and his collaborators published a study in the Journal of the American Medical Associationpublished a study in the Journal of the American Medical Association looking at 27 supplements that the FDA had formally recalled and were still being sold by their manufacturers under exactly the same name. The scientists bought the products long (8-52 months) after they had been recalled. Eighteen of the supplements contained a “pharmaceutical adulterant”, the study found, and 17 of those contained the same tainted drug that the FDA had warned them about before.

“What we’re talking about here is experimental pharmaceuticals—designer drugs — that have entered the mainstream supplement market,” Cohen says. “The FDA hasn’t caught up with the seriousness of this as a public health issue.”