The Federal Trade Commission on Thursday fined the marketers of four weight loss pills millions of dollars for making false advertising claims ranging from rapid weight loss to reducing the risk of cancer.

FTC Commissioner Deborah Platt Majoras said the products would remain on store shelves, but that the companies would have to stop making the false claims.

"What we challenge is the marketing of the claims," she said. "The marketers are required to back up the claims with the science and if they can't do that they can't make the claim. But we don't ban the products from the shelves."

The FTC investigated a variety of claims made — including rapid weight loss and reduction in the risk of osteoporosis, Alzheimer's and even cancer, Majoras noted.

Fines were levied against marketers of Xenadrine EFX, One A Day Weight Smart, Cortaslim and TrimSpa. Majoras did not specify how much each marketer was being fined and did not identify the marketers. The FTC scheduled a new conference later Thursday to announce details.

Efforts to reach the product manufacturers were not immediately successful.

Majoras said that some of the money paid as civil fines would be returned to consumers. "We always try to get money back when consumers have been deceived," she said. "In this instance I'm pleased to say that I believe we're going to get millions back from some of these products to be able to return it to consumers."

Majoras, speaking on NBC's "Today" show, said the FTC investigation found that the marketers of Xenadrine had a study that said those who took a placebo actually lost more weight than those taking the pill.

"They not only didn't have studies to support the claim, they actually had a study that went the other way," she said.

Some of the products marketed their claims through infomercials or celebrity endorsements. Anna Nicole Smith, for example, has endorsed TrimSpa.

"Testimonials from individuals are not a substitute for science," Majoras said. "And that's what Americans need to understand."