Some products marketed as dietary supplements contain an “alarming variety” of active ingredients that can pose “considerable dangers” to consumers but are not listed on their labels, the Food and Drug Administration says.

In a letter sent to dietary supplement makers Wednesday, the FDA cited particular concern about weight loss, body building and sexual enhancement supplements. The agency urged consumers to avoid products that claim they are alternatives to anabolic steroids or to FDA-approved drugs, and warned manufacturers that they could face criminal prosecution if they sell products laced with active ingredients not on their labels.

The letter, from FDA Commissioner Margaret Hamburg, said “undeclared ingredients” endanger consumers who are unaware “that the ingredients are present, that the ingredients may be associated with serious side effects, or that they may interact in dangerous ways with other products consumers may be taking.”

“These tainted products can cause serious adverse effects, including strokes, organ failure, and death,” Dr. Hamburg said. “The manufacturers selling these tainted products are operating outside the law.”

The FDA previously has worked with the dietary supplement industry to recall more than 70 products marketed for sexual enhancement, more than 40 sold for weight loss and more than 80 for bodybuilding.

Unlike drugs, dietary supplements do not have to be approved by the FDA before they hit store shelves.