If you're like two-thirds of Americans, fluoride is added to your tap water for the purpose of reducing cavities. But the scientific rationale for putting it there may be outdated. Water fluoridation ... first began in 1945. Those opposed to the process have argued - and a growing number of studies have suggested - that the chemical may present a number of health risks, for example interfering with the endocrine system and increasing the risk of impaired brain function; two studies in the last few months, for example, have linked fluoridation to ADHD and underactive thyroid. Others argue against water fluoridation on ethical grounds, saying the process forces people to consume a substance they may not know is there - or that they'd rather avoid. Despite concerns about safety and ethics, many are content to continue fluoridation because of its purported benefit: that it reduces tooth decay. You might think, then, that fluoridated water's efficacy as a cavity preventer would be proven beyond a reasonable doubt. But new research suggests that assumption is dramatically misguided. Consuming fluoridated water may have no positive impact. The Cochrane Collaboration ... recently set out to find out if fluoridation reduces cavities. They reviewed every study done on fluoridation that they could find. Then they ... published their conclusion in a review earlier this month. "There's really hardly any evidence" the practice works, [said dean of the Hull York Medical School Trevor Sheldon].