From what I have read, fluoride’s action on teeth is purely topical. Which is why it is in toothpaste (which is not swallowed). The “minuscule” amount in drinking water is reported as not enough to be toxic, yet somehow enough to strengthen teeth through internal blood circulation. Any fluoride you ingest, even a few atoms, is considered a toxin by the body and removed. So while the minuscule amounts added to water may not harm you, they are still adding to the “workload” your body has in dealing with all the minuscule amounts of other toxins you acquire daily.