A study published in JAMA Pediatrics has given new life to a long-running debate: whether adding fluoride to drinking water is a prudent way to prevent tooth decay, or a potentially toxic mistake. The research, which focused on mother-child pairs from six Canadian cities, found that high fluoride exposure during pregnancy was correlated with lower IQ scores among young children, especially boys. "Based on the current evidence, it is a reasonable recommendation to tell women to reduce their fluoride intake during pregnancy," says study co-author Christine Till. The U.S. began adding fluoride to some public water supplies in the 1940s, in an effort to strengthen teeth and prevent decay, and research on it has been accumulating in the subsequent decades. At high doses, fluoride can actually damage people's teeth, according to the World Health Organization - and some research, much of it in animals, suggests it's also tied to more serious side effects, including bone cancer and cognitive impairments. In part due to that controversy, more than 300 communities in North America have voted to end fluoridation programs over the past 20 years, according to the anti-fluoride activist group Fluoride Action Network. Today, about 66% of Americans and 39% of Canadians had access to fluoridated water. Drinking an extra milligram of fluoride per day - about a liter of properly fluoridated water, plus a mug of tea (which is itself a source of fluoride) - during pregnancy translated to an average 3.66-point IQ drop for boys, the [new] study shows.