The U.S. Food and Drug Administration has warned a Web site about selling supplements that claim to prevent the spread of the 2009 H1N1 influenza virus.

The FDA said it joined with the Federal Trade Commission in sending the warning letter -- the first to be issued jointly by the agencies -- advising the owners of the site that they must discontinue the fraudulent marketing of their product or face legal action.

FDA and FTC officials urged consumers to be cautious of promotions or Internet sites offering products that claim to diagnose, prevent, mitigate, treat or cure the 2009 H1N1 influenza virus. Fraudulent H1N1 influenza products come in many varieties, including dietary supplements, as well as products purporting to be drugs, medical devices or vaccines.

"Products that are offered for sale with claims to diagnose, prevent, mitigate, treat or cure the 2009 H1N1 influenza virus must be carefully evaluated," said FDA Commissioner Dr. Margaret Hamburg. "Unless these products are proven to be safe and effective for the claims that are made, it is not known whether they will prevent the transmission of the virus or offer effective remedies against infection. Furthermore, they can make matters worse by providing consumers with a false sense of protection."