“It’s a pharmaceutical-grade product which is being directly introduced into the supplement marketplace with absolutely no regulatory oversight,” said Dr. Pieter Cohen, an assistant professor of medicine at Harvard Medical School who has studied dietary supplements.

In the medical literature, DMAA has often been described as a synthetic stimulant similar to amphetamines that can constrict blood vessels, raise blood pressure and heart rate, potentially increasing the risk of heart attacks and strokes. In 2005, supplement makers began to market the substance in workout and weight-loss products, often combining it with caffeine, which may enhance the stimulant’s effects. Products like Jack3d and OxyElite Pro, which USPlabs also markets, became popular among fitness buffs as part of their pre-workout routine.

Although USPlabs now makes a new version of Jack3d — called Jack3d Micro — without DMAA, and the products are no longer sold on military bases, the original Jack3d remains widely available at some stores and Web sites. GNC.com said that Jack3d “produces an intense sensation of drive, focus, energy, motivation and awareness.” Last week, a reporter bought the original version of Jack3d at a GNC outlet in Midtown Manhattan.

But a study commissioned by the United States military after the two soldiers died raised red flags about the safety of DMAA products.

“DMAA in combination with other ingredients may be associated with significant consequences,” a team of military, sports and supplement researchers wrote in case reports about the deaths of the two soldiers that was published last December in Military Medicine, the journal of the Society of Federal Health Professionals. The researchers added, “DMAA continues to be available in dietary supplements despite the lack of evidence that it qualifies as a dietary ingredient.”

The Food and Drug Administration also issued warning letters to 10 marketers of the workout boosters, but critics charge that the agency has been slow to take definitive action against the products. Health regulators in at least seven other countries, including Britain, Sweden, Denmark, Finland, Canada, New Zealand and Australia, have effectively banned supplements containing DMAA.

Last month, a coroner in Britain cited the stimulant in the death of Claire Squires, 30, who took Jack3d before running the London Marathon last year, collapsed late in the race and died. DMAA “on the balance of probabilities, and in combination with extreme physical exertion, caused cardiac failure which resulted in her death,” the coroner wrote in his report.