Fluoride in drinking water is credited with dramatically reducing cavities, but has also been linked to a reported increase in spots on some children's teeth, which is one reason the U.S. government announced Friday it plans to lower the recommended levels of the mineral.

The first such change in nearly 50 years comes after a surprising government study recently found that about two out of five adolescents have tooth streaking or spottiness because of too much fluoride. In some extreme cases, teeth can even be pitted by the mineral — though many cases are so mild only dentists notice it.

Health officials note that most communities have fluoride in their water supplies, and toothpaste has it, too. Some kids are even given fluoride supplements.

The U.S. Department of Health and Human Services proposed a recommended fluoride level of 0.7 milligrams per litre of water to replace the current recommended range of 0.7 to 1.2 milligrams per litre.

The U.S. Environmental Protection Agency will review whether the maximum allowable concentration it has set for the mineral — 4.0 milligrams per litre — is too high.

In 2008, a panel of experts convened by Health Canada recommended lowering fluoride levels in drinking water to 0.7 mg/L from the maximum acceptable concentration of 1.5 mg/L.

About 13.5 million Canadians, or about 43 per cent of the population, live in communities with fluoridated tap water, but almost no fluoridation is done in British Columbia or Quebec, according to Health Canada.

The U.S. Centers for Disease Control and Prevention reports that the splotchy tooth condition, fluorosis, is unexpectedly common in children age 12 through 15. And it appears to have grown much more common since the 1980s.