Fluoride in drinking water - credited with dramatically cutting cavities and tooth decay - may now be too much of a good thing. It's causing spots on some kids' teeth.

A reported increase in the spotting problem is one reason the federal government will announce Friday it plans to lower the recommended limit for fluoride in water supplies - the first such change in nearly 50 years.

About 2 out of 5 adolescents have tooth streaking or spottiness because of too much fluoride, a surprising government study found recently. In some extreme cases, teeth can even be pitted by the mineral - though many cases are so mild [meaning some are severe] only dentists notice it.