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.
Millions of people in England receive fluoridated water. This means fluoride has been added to bring it up to around 1mg of fluoride per litre of water, which is a level found to reduce tooth decay levels.
Now I know there is a running joke around the English having bad teeth. But I imagine if millions of people were having issues we’d know about it.
I always thought we started fluoridation of water so the Soviets couldn't figure out how much uranium (and later plutonium) we were producing. UF6 is a bi-product of uranium enrichment and it's fairly straight-forward to estimate how much we're producing by taking water samples at the mouths of various rivers. But if everyone is brushing their teeth with fluoride, then it's a lot harder to accurately measure how much is due to enrichment.
This is very clearly a conspiracy theory, but water fluoridation is one of those topics that seems to attract them.