An analysis of sports supplements that claim to burn fat or improve your workout may contain a stimulant drug banned by the World Anti-Doping Agency. Researchers from Harvard Medical School say the stimulant may pose health risks.

The stimulant drug is oxilofrine, also known as methylsynephrine or p-hydroxyephedrine. Researcher Pieter Cohen told NPR that oxilofrine can stimulate the heart and has some medical uses, such as boosting heart function in patients under anesthesia.

Oxilofrine is not supposed to be in dietary supplements, which by law are supposed to contain only “dietary ingredients,” the article notes. The Food and Drug Administration (FDA) has stated oxilofrine does not meet the definition of a dietary ingredient.

Cohen started analyzing supplements containing oxilofrine after several well-known athletes tested positive for the drug. He tested 27 supplement brands with methylsynephrine on the label, and found 14 contained oxilofrine. Six contained high amounts, including one that had more than twice the usual adult dose, and three times the usual adolescent dose.

Studies of lower doses of oxilofrine suggest it may cause adverse effects such as heart palpitations, irregular heart rhythms and increased blood pressure, the researchers wrote in the journal Drug Testing and Analysis.

Earlier this month, the FDA sent warning letters to seven manufacturers, stating oxilofrine should not be in dietary supplements. The FDA said it has received 47 reports of adverse events associated with supplements containing oxilofrine. The agency said it cannot say whether the supplements caused harm. The FDA does not review supplements for safety and effectiveness before they come to market.