But many experts now question the scientific basis for the intervention. In June 2015, the Cochrane Collaboration—a global independent network of researchers and health care professionals known for rigorous scientific reviews of public health policies—published an analysis of 20 key studies on water fluoridation. They found that while water fluoridation is effective at reducing tooth decay among children, “no studies that aimed to determine the effectiveness of water fluoridation for preventing caries [cavities] in adults met the review’s inclusion criteria.” *

The Cochrane report also concluded that early scientific investigations on water fluoridation (most were conducted before 1975) were deeply flawed. “We had concerns about the methods used, or the reporting of the results, in … 97 percent of the studies,” the authors noted. One problem: The early studies didn’t take into account the subsequent widespread use of fluoride-containing toothpastes and other dental fluoride supplements, which also prevent cavities. This may explain why countries that do not fluoridate their water have also seen big drops in cavity rates (see chart).