Government scientists say that despite their confidence in fluoride as a safe and effective cavity-fighter, researchers need to determine how much is enough - and how much may be too much.

A yearlong study by the U.S. Public Health Service, released Tuesday, found no evidence that fluoride causes cancer in humans. It recommended continued addition of the chemical to water supplies."Optimal fluoridation of drinking water does not pose a detectable cancer risk to humans," the report said.