The CDC says that the level at which it is added to the water (1 ppm) is safe and effective. Kerry Maguire, of the Forsyth Institute, an independent (not-for-profit) research institute in the United States specializing on oral health and its connection to overall wellness, agrees with the CDC. 

"For the children I treat, fluoridated drinking water translates into fewer cavities requiring a trip to the dentist and more time in school," she said. "As a dentist and scientist – as well as a mother and grandmother – I welcome the CDC’s affirmation of the safety and effectiveness of community water fluoridation.”

http://www.livescience.com/37123-fluoridation.html