The U.S. Food and Drug Administration warned five companies on Thursday to stop selling dietary supplements containing an unapproved stimulant known as beta-methylphenylethylamine, or BMPEA.

BMPEA is an amphetamine-like substance that has been shown to raise blood pressure and heart rate in animals and is classified as a doping agent by the World Anti-Doping Agency.

An FDA investigation found nine out of 21 supplements containing Acacia rigidula also contained BMPEA even though the plant itself does not contain the substance. The agency published its findings in 2013.

Earlier this month, a study in the journal Drug Testing and Analysis showed BMPEA-containing products were still on the market more than a year after the FDA published its findings. Asked why, the agency said it had not identified a safety problem.

In its warning letters the FDA cited product misbranding. It said BMPEA does not meet the definition of a dietary ingredient and Acacia rigidula does not contain BMPEA.

Dr. Pieter Cohen, an assistant professor at Harvard Medical School and lead author on the Drug Testing and Analysis study, said he was "delighted" the FDA had confirmed that BMPEA does not belong in dietary supplements but said it "begs the question as to what took them so long."

A spokeswoman for the agency, Jennifer Corbett Dooren, said the FDA prioritizes its enforcement actions based on available resources and the level of safety concern.

"The agency faces the challenge of having limited resources to monitor the marketplace for potentially harmful dietary supplements," she said.

In January 2014 Michael Landa, then-director of the agency's Center for Food Safety and Applied Nutrition, listed the BMPEA research among the division's notable accomplishments in 2013.

In an open letter published on the agency's website he praised the researchers for identifying "Acacia-containing dietary supplements adulterated with an amine compound not found in the Acacia plant."

Cohen said the warnings do not go far enough since they cover only those products that explicitly list BMPEA on the label. Many products use Acacia rigidula as code and do not mention BMPEA, he said.

Corbett Dooren declined to say why the agency had only targeted a subset of products. She said the agency "can't comment on additional enforcement actions."

The warning letters were sent to Hi-Tech Pharmaceuticals, Tribravus Enterprises, Train Naked Labs, Better Body Sports and Human Evolution Supplements. Affected products include Fastin-XR, Lipodrene, Sudden Impact, Core Burner and Phoenix Extreme.