Fluoride & Tooth Decay: Topical VS. Systemic Effects

When waterfluoridation first began in the 1940s, dentists believed that fluoride's main benefit came from ingesting fluoride during the early years of life. This belief held sway for over 40 years.

However, it is now acknowledged by dental researchers to be incorrect. According to the Centres for Disease Control, fluoride's predominant effect is topical (direct contact with teeth) and not systemic (from ingestion).

Hence, there is no need to ingest fluoride to derive its purported benefit for teeth.
As stated by the US Centres for Disease Control:
“[L]aboratory and epidemiologic research suggests that fluoride prevents dental caries predominately after eruption of the tooth into the mouth, and its actions primarily are topical for both adults and children”
(CDC, 1999, MMWR 48: 933-940).

Reference: http://www.fluoridealert.org/health/teeth/caries/topical-systemic.aspx