To be clear: drinking water sources often contain flouride and swallowing water is generally not optional. The source can be natural or artificial, but the outcome will be the same if you get an unusually excessive amount during developmental years. As with all nutrients, too much will inevitably become bad for you (thankfully, in the case of flouride, the therapeutic index is quite generous)
With all of this being said, once you're an adult there's no longer any particularly viable pathways from the bloodstream to your outer teeth, so flouride in the body becomes mostly disconnected from flouride in the teeth (and vice versa). Flouride's effects on the body are less well understood, albeit only because we struggle to measure such apparently small effects on a general population.
My mom was obsessed with flouride in water so I actually asked my dentist about it. According to an actual health professional it is quite good for the teeth and excessive amounts only affect the appearance not structure.
> The severity of the condition is dependent on the dose, duration, and age of the individual during the exposure.
> Severe : 5 : All enamel surfaces are affected and hypoplasia is so marked that the general form of the tooth may be affected. The major diagnostic sign of this classification is discrete or confluent pitting. Brown stains are widespread and teeth often present a corroded-like appearance.
Fluoride catalyzes remineralization of dissolved calcium and phosphate atoms back to hydroxyapatite; this also happens without fluoride, but less efficiently. In addition fluorine ions can substitute the hydroxide ions in hydroxyapatite, forming fluorapatite which is more resistant against acid.
With all of this being said, once you're an adult there's no longer any particularly viable pathways from the bloodstream to your outer teeth, so flouride in the body becomes mostly disconnected from flouride in the teeth (and vice versa). Flouride's effects on the body are less well understood, albeit only because we struggle to measure such apparently small effects on a general population.