If Oprah's name is on it, it's a near guarantee it will sell.
A book, magazine or diet with her golden seal of approval gives a consumer a sense of trust. After all, if Oprah Winfrey uses it, it has to be good, right?
So last year, when she and Columbia University heart surgeon Dr. Mehmet Oz talked about the acai berry on "The Oprah Winfrey Show," products featuring the now-famous fruit exploded onto the market, into television pitches, Internet popup ads and e-mails clogging your inbox.
These wild claims may seem more plausible when Winfrey and Oz's names are used as endorsements.
"Companies used the fact that Oprah and Memhet Oz talked about the acai berries on their shows to create the impression that Oprah and Oz were selling these products -- and endorsing them," said David Schardt, from the Center for Science and Public Interest.
The Center for Science and the Public Interest says there is no evidence acai actually helps you lose weight.
Now, Winfrey and Oz have filed suit against more than 40 companies, some selling acai, others pushing other products, with their names right on them.
"Defendants are fabricating quotes or falsely purporting to speak in Dr. Oz's and/or Ms. Winfrey's voice about specific brands and products that neither of them has endorsed," the complaint reads.
Oz spoke exclusively to "Good Morning America" against the claims that marketers have allegedly made with his name, calling them "hurtful."
"Many Americans have seen images of me, and Oprah and others supporting, it would appear, products that actually don't work in the ways that are described," Oz said. "And more importantly, when consumers trusting us try to buy these products over the Web, what they end up getting are fake products, pills that don't really have what's promised in them. They're often duped into paying more than they should. If my picture is next to a product, endorsing it and supporting your purchase of it, I did not give them permission."
Illinois Attorney General Lisa Madigan is also filing suit on behalf of consumers who were allegedly tricked.
"For thousands of dieters, the quest for a miracle product has become a nightmare," Madigan said in a press release. "Far too often, consumers end up losing their money -- not weight -- in these deals."
"Hopefully these companies will think twice about exploiting celebrities and cheating people by making them think that celebrities really endorsed these products," Schardt said.
The FTC is considering new rules insisting that celebrities must actually use any product they endorse.
The acai berry was virtually unknown outside the United States until 2001, when two brothers, Ryan and Jeremy Black, began to sell acai through Sambazon Inc., promoting its antioxidant properties.
Harvested as a deep purple pulp from 60-foot palm trees, acai (pronounced "ah-sigh-ee") is exported as a thick pulp and sold in a capsule, powder or juice form at health food stores and online.
Today billion-dollar beverage giants, including Coca-Cola, PepsiCo and Anheuser-Busch, are adding the fruit to their beverage lineups, according to The Wall Street Journal. It's also found in products from Stonyfield Farm and Haagen-Dazs. Procter & Gamble recently infused acai into its Herbal Essence shampoos and conditioners.
Oz, who appears on "The Oprah Winfrey Show" as a medical expert, told ABCNews.com earlier this year that he had never officially endorsed acai.
"Acai is a powerful antioxidant," said Oz. "Colorful, dark foods like red wine, pomegranates, concord grapes, blueberries -- they call them brain berries -- are full of nutrients."
As far as those weight loss properties, Oz said, "I'd be surprised if by itself acai could help."
Acai products are distributed through such stores as Whole Foods, Wild Oats and Jamba Juice, as well as many conventional grocery chains and the Web.