A recent article in Newsweek magazine (June 29, 2015) references a study that questions the validity of using fluoride in tap water to prevent cavities. In my practice, it is empirically evident that patients who have fluoride in their water, or those who take fluoride supplements, have far fewer cavities than those who do not have fluoride. Of course, the consumption of soda and other sugar/white flour products will also affect this outcome. This brings us to the Newsweek article, which seems to cast a negative shadow on the use of fluoride, but their conclusions are based on data collected prior to 1975, when fluoride introduction into toothpaste began. All of their data is relative to children, not adults who use fluoride toothpaste. Their date actually supports reductions in cavities in children using fluoridated water (15% reduction for baby teeth, 14% for permanent teeth.)
It may be well that with fluoridated toothpaste, adding fluoride into the water is less important , but we cannot tell from this research. They have no date for or against using fluoridated water in adults.