(WXIA) -- The Federal Trade Commission wants to give consumers the "skinny" on several major companies accused of making false promises.

Ads for a product called Sensa claim all you have to do is sprinkle a powder on your food and watch the weight fall off.

Americans have spent $364 million on Sensa, but the FTC says there's no proof it actually works.

"Our message for consumers is, be very wary of pills, patches, powders that claim rapid and substantial weight loss without diet or exercise," said Jessica Rich with the FTC.

Sensa is one of four companies charged with deceptive advertising. The FTC is demanding the companies pay $34 million in settlements and return customers' money. Sensa has already agreed to give back more than $26.5 million in refunds.

Missouri Sen. Claire McCaskill helped drive the investigation into the diet companies, but she warns that going after them is like playing a game of Whac-A-Mole.

"There are many companies that are doing this, and with every one that is found, another one will pop up," McCaskill said.

The government has also targeted ads for LeanSpa, HCG Diet Direct and slimming creams from L'Occitane.

Several accused companies have agreed to settle without admitting any wrongdoing.

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