In a warning last month, the Food and Drug Administration (FDA) urged consumers to avoid supplements containing dimethylamylamine, or DMAA, because the drug could trigger heart problems.
GOP lawmakers are now asking why USPlabs continues to use DMAA in Jack3d, and why GNC is still selling the supplement.
Reps. Tim Murphy (R-Pa.), Joe Barton (R-Texas) and Michael Burgess (R-Texas) also asked the FDA to detail its efforts to remove DMAA products from the market.
"The agency said that 'FDA can also take a seizure action to remove the products from the market or obtain an injunction against a company to prevent it from manufacturing and distributing illegal products,'" the lawmakers wrote.
"That being said, as of May 7, 2013, USPlabs continues to market Jack3d and General Nutrition Centers, Inc. continues to sell it."
Lawyers for USPlabs have said DMAA is legal, and that the company should be allowed to use it.