Cetylpyridinium chloride is the active ingredient in Crest Pro Health mouthwash. Proctor and Gamble product research says the rinse is safe and that their single blind study showed, "Side effects were minimal, with no between-group differences in hygienist-rated calculus or stain accumulation..." However, many have gathered at the Amazon page for the product to complain that it stains teeth brown. The only mention of this in the media that I can find is an entertaining op-ed rant from earlier this year. One anectdote from a dental professional claims it takes an hour of cleaning to get the stains off. Did Crest rush its product to market, missing a critical side effect? Or was it meant to boost sales of another product line?