The Food and Drug Administration warned makers and distributors of dietary supplements Wednesday that it is coming after hundreds of products that contain drugs, or compounds acting as drugs.

The illegal supplements generally fall into three categories - weight-loss agents, bodybuilding products and "sexual enhancers." Many contain known pharmaceuticals or similar chemicals whose precise effects in human beings have not yet been studied.

In a media briefing, the FDA's principal deputy commissioner, Joshua M. Sharfstein, said that since 2007 the agency has identified about 300 pharmaceutical products "masquerading as dietary supplements." It has also investigated reports of "adverse events," including deaths, linked to products in all three categories.

Although FDA actions have included recall of supplements and referral of companies for criminal prosecution, "we do not think that the problem is solved," Sharfstein said. The latest strategy is a letter dated Dec. 15 warning makers of dietary supplements to police themselves, and their suppliers and distributors, to ensure that nobody is slipping drugs into their products. The agency enrolled five trade organizations to do the same.

"We share the concern of the FDA over the proliferation of these adulterated products," Scott M. Melville, president of the Consumer Healthcare Products Association, told reporters.

"The spiking of supplements with drugs is a crime," said John F. Gay, head of the Natural Products Association. "It endangers the public and undermines our efforts."

"We want to drive these pirates out of our industry," said Loren Israelsen, head of the United Natural Products Alliance.

The letter, signed by FDA Commissioner Margaret A. Hamburg, said that "responsible individuals and companies should be aware that the government may initiate criminal investigations" when drug-laced supplements are found.

Dietary supplements are regulated much less rigorously than drugs, which must undergo extensive testing before they are allowed to go on the market.

Federal law requires that supplement makers be "responsible for marketing a safe product," but they don't have to actually prove the safety or effectiveness of their products. There are limits, however, to claims they can make about a supplement's physiological action.

Most of the "sexual enhancers" contain sildenafil, the compound in Viagra, or other members of that chemical family. They are sold under names such as Mr. Male Magic Enhancer and Ejaculoid XXTreme. Many of the weight-loss supplements (Solo Slim, Joyful Slim Herb Supplement) contain sibutramine, an FDA-approved drug. The bodybuilding supplements (Clomed, Off Cycle II Hardcore) contain anabolic steroids or a class of compounds called "aromatase inhibitors" that increase testosterone production.

Steroid hormones have extremely complex structures, and chemists can synthesize compounds that differ only slightly from natural ones. Many "analogues" retain the original effects, or in some cases magnify them.