Centralized fluoridation is a safe, simple, and effective way to improve the dental health of the population at large (especially children in the candy cavity-prone years). It is also extremely cost-effective: one study estimated that for every $1 a city invested in fluoridation it saved the average citizen (with a mean dental health cost) $38 on dental care. The Centers for Disease Control and Prevention considers water fluoridation to be one of the top 10 public health advances of the 20th century. The American Dental Association "unreservedly endorses the fluoridation of community water supplies as safe, effective and necessary in preventing tooth decay." The World Health Organization states that "fluoridation of water supplies, where possible, is the most effective public health measure for the prevention of dental decay.".
Having been around for 70-odd years and having been the focus of much research, water fluoridation has literally been studied to death (