A new study has found that there are still some weight loss supplements containing Acacia rigidula and the amphetamine-like compound available in the market. The U.S. Food and Drug Administration (FDA) has been aware of the untested substance since 2013, yet failed to act against regulating these products.

Acacia rigidula or "blackbrush" is a species of shrub or small tree native in Texas and Central Mexico. It gained popularity as a weight loss aid as it is known to suppress appetite and break down body fat. Many weight loss supplements list Acacia rigidula as one of their main ingredients, but some fail to disclose that they also contain a synthetic compound that is similar to the drug amphetamine.

Researchers at Harvard Medical University said that the products containing the untested substance are being sold under brand names JetFuel T-300, Fastin-XR and Black Widow that are manufactured by Hi-Tech Pharmaceuticals, Inc. of Norcross, Ga., according to USA Today.

Pieter Cohen, an assistant professor of medicine at Harvard Medical School and co-author of the new study, blames U.S. health regulators for failing to issue a public warning on the untested substance. Nine of the 21 dietary supplements tested by the agency in 2013 tested positive with the compound called beta-methylphenethylamine (BMPEA), which is chemically similar to amphetamine.

The researchers did a follow-up test on the same 21 dietary supplements and found out that almost half of it still contains BMPEA, which is not disclosed on the products' labels. To date, it is still unknown what kind of health issues the substance might bring, but since it is similar to amphetamine, the researchers are thinking that it that can increase one's risk of stroke, heart attack or even death. In addition, BMPEA is banned by the World Anti-Doping Agency, a foundation that lists substances and methods that are prohibited in sports.

"The FDA did a lot of hard work to figure out this brand-new designer stimulant was in supplements ... and then failed to inform the public. It's inexplicable and inexcusable," Cohen told LiveScience.

When asked why a public warning or product recall wasn't released, the FDA said that it doesn't see any safety concern yet regarding BMPEA.

"The FDA's first priority with regard to dietary supplements is ensuring safety... BMPEA does not identify a specific safety concern at this time, the FDA will consider taking regulatory action, as appropriate, to protect consumers," FDA spokeswoman JuliAnn Putnam told Reuters.

Cohen's study was published in the April 7 issue of Drug Testing and Analysis.