Americans who take dietary supplements that contain the stimulant ephedrine may put themselves at risk for heart attacks, seizures and other severe problems, the Food and Drug Administration said Wednesday.

Ephedrine alone is a powerful stimulant and the primary ingredient in certain illegal drugs.

But dietary supplements such as the Chinese herb ma huang naturally contain ephedrine also, and are sold with little federal oversight. Proponents claim these supplements boost energy and help weight loss.

Prompted by growing concern that these products contain enough ephedrine to cause serious and sometimes fatal heart and nervous system side effects, the FDA is considering more stringently regulating the supplements.

More than 330 Americans have suffered side effects from ephedrine-containing products, including 12 deaths, says an FDA report presented Wednesday to the agency’s scientific advisers.