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.
Acai berry was marketed as the answer to weight loss, sexual dysfunction, even longer life.
VIDEO: The talk show host and her on-air physician sue dozens of companies.Play
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."