THE stimulant DMAA has been banned by the World Anti-Doping Agency, the National Collegiate Athletic Association and Major League Baseball. But it is still found in Jack3d and other supplements sold at stores like GNC and the Vitamin Shoppe.

Dimitar Kutrovsky found that out the hard way.

Mr. Kutrovsky came to the United States from Bulgaria in 2006. He had a full athletic scholarship to the University of Texas at Austin, where he was the tennis team captain and an all-American singles and doubles player. He turned pro after graduation, and in August 2011, exhausted from his travel schedule and the sweltering Texas heat, he visited a GNC store in Austin.

Looking for an energy lift, Mr. Kutrovsky asked the sales clerk for advice, according to an arbitrator’s ruling in an anti-doping proceeding brought against him by the International Tennis Federation. He told the clerk that he was a professional tennis player subject to drug testing. The clerk recommended Jack3d and told him that it was used by a lot of athletes. It was “like a Red Bull but stronger,” he told Mr. Kutrovsky, according to the ruling.

At home, Mr. Kutrovsky compared Jack3d’s ingredients with those banned in Olympic sports. Finding no matches, he began using Jack3d. The arbitrator found that Mr. Kutrovsky did not realize that the stimulant listed on the label — dimethylamylamine, or DMAA — had been banned by many sports associations under another name, methylhexaneamine.