The government is lowering the recommended level of fluoride in drinking water for the first time in more than 50 years.
Health officials say some people are getting too much fluoride now because it is also added to toothpaste, mouthwash and other products.
Too much fluoride has become a common cause of splotches on teeth. One study found about 2 out of 5 adolescents have tooth streaking or spottiness from too much fluoride.
The government first began urging cities to add fluoride to drinking water in 1962.
The change announced Monday finalizes a proposal first made four years ago.