The July 2000 issue of the Journal of the American Dental Association reported that “fluoride incorporated during tooth development is insufficient to play a significant role in caries protection.”

Researchers have now overwhelmingly rejected the notion that swallowing fluoride is either necessary or effective for preventing decay. Instead, the current consensus is that fluoride’s benefit (whatever it may be) comes from topical contact with teeth after the teeth have erupted into the mouth. As the Centers for Disease Control (CDC) stated in 1999: “fluoride prevents dental caries predominately after eruption of the tooth into the mouth, and its actions primarily are topical for both adults and children.”

The CDC repeated this position in 2001, affirming that “fluoride’s predominant effect is posteruptive and topical.”

Note the word "topical".