The e-mail circulating in Washington last month said a former government lawyer knew a guy whose dog had to be put to sleep because he walked on a floor cleaned with Procter & Gamble's Swiffer WetJet, licked his paws and developed liver disease. Two of the cats owned by the man's maid, who also used the product at home, were said to have died, too.

A quick Google search found that the rumor — which started last year — had been totally refuted by Snopes.com and BreakTheChain.org, two websites that are dedicated to online-myth-busting. But it has been neither quick nor easy for P&G to squelch the story, which was also proved false last year by toxicologists at the American Society for the Prevention of Cruelty to Animals.

Thanks to the Internet, hoaxes, legends and scurrilous rumors about companies and their products are living longer than ever. Companies from Target to Starbucks have had to battle a seemingly ceaseless online rumor mill, which is sometimes fueled by myths perpetuated by competitors.

Some targeted companies fear that disputing the rumors can actually make matters worse.

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