Proponents and opponents have been both criticized for overstating the benefits or overstating the risks, and understating the other, respectively.[21][22] Systematic reviews have cited the lack of high quality research for the benefits and risks of water fluoridation and questions that are still unsettled.[14][22][23] Researchers who oppose the practice state this as well.[24] According to a 2013 Congressional Research Service report on fluoride in drinking water, these gaps in the fluoridation scientific literature fuel the controversy.[15]