More than two years after regulators found an amphetamine-like substance in nearly a dozen supplements marketed for aiding workout and weight loss, a new study has found that pills containing the chemical are still on the market.

The study, published Tuesday in the journal “Drug Testing and Analysis,” analyzed 21 supplements labelled as containing Acacia rigidula, a type of shrub native to Texas. Eleven of them were found to contain beta-methylphenylethylamine, or BMPEA, an isomer of amphetamine whose effect on humans has never been studied.

The Food and Drug Administration reported similar findings two years ago, in a little-publicized study published in the Journal of Pharmaceutical and Biomedical Analysis.

In a statement to Reuters, FDA spokeswoman said the agency’s review of products containing BMPEA “does not identify a specific safety concern at this time.” She added that ensuring supplement safety is their “first priority” and that the agency will consider taking regulatory action, if appropriate.

But Roy Gerona, a clinical chemist at University of California-San Francisco and a co-author of the study, said that it was dangerous for the FDA not to inform consumers about BMPEA, which bears a close resemblance to the powerful stimulant amphetamine. Though the effects of BMPEA aren’t known, other amphetamine stimulants are known to increase blood pressure, cause cardiovascular complications, suppress sleep and appetite and can be addictive.

Only three of the supplements in which the chemical was found indicated its presence on their labels, and none of them specified the amount of BMPEA in each pill, according to the study.

“When you’re taking these products you don’t know if they contain this compound, you don’t know the toxic effect and if it’s there you don’t know the amount at which you could potentially overdose,” Gerona said in a phone interview. “… It’s worrying that there hasn’t been any announcement.”

The study recommends that the FDA recall all products found to contain BMPEA and that the chemical be eliminated from other dietary supplements.

“Jet Fuel Superburn,” one of the drugs examined in the Drug Testing and Analysis study, was recalled by Canadian health authorities last year for posing “serious health risks.”

BMPEA is a little-studied chemical first synthesized in the 1930s. It has the same chemical formula as amphetamine, but a different structure. The compound has never been identified in or extracted from Acacia rigidula, indicating that it is likely being produced synthetically and added to the pills, researchers say.

Acacia rigidula is code in the industry for a potent synthetic stimulant,” lead author Pieter Cohen told Reuters. “They are using the name as a cover.”

Unlike pharmaceuticals, which must undergo a multi-stage testing and review process, supplements are only lightly regulated by the FDA. Federal guidelines require companies to ensure that their products are safe and accurately labeled, but the FDA has little power to enforce that rule.

This means that supplement manufacturers aren’t held to the same standards to prove that their products are safe, let alone effective, according to Gerona.

In some herbal supplements “the active ingredient really something synthetic … but it’s a useful ploy in order to deceive the public that whatever they’re taking is natural and therefore should not be harmful,” he said.

In the past five years, tainted supplements have been associated with kidney failure, hepatitis and other problems. A recent investigation by the New York attorney general’s office found that many common supplements don’t contain the active ingredient listed on their labels. And multiple studies have criticized the FDA’s loose regulation of the supplement industry, arguing that the policy contributes to health issues.

Speaking with the New York Times, Michael F. Jacobson, the executive director of the consumer advocacy organization Center for Science in the Public Interest, pointed out that top agency regulators have backgrounds in the supplement industry.

“To have former officials in the supplement industry become the chief regulators of that industry at the FDA is like the fox guarding the hen house,” he said.

But Daniel Fabricant, a former and current executive at the Natural Products Association who ran the FDA’s dietary supplement division from 2011 to 2014, said his experience in the industry was a benefit, not a bias. He cited the agency’s recall of an herbal supplement linked to an outbreak of liver disease in 2013, which he oversaw.

Last week, 14 state attorneys general asked Congress to launch an investigation of the herbal supplements industry.

“The FDA has long been aware of problems in the dietary and herbal supplement supply chain, from dubious ingredient sourcing to a failure to carry out proper testing on finished products,” the attorneys general said in their letter to the chairmen of the House and Senate subcommittees dealing with product safety and health.