I think it's worth reminding people of Occam's Razor, and the point of it is: a simple explanation is often better than a complicated one. The US has had water fluoridation for 65 years, affecting 346 million people. That's a pretty big god damn sample size and long amount of time in which to observe effects. If we still have no proof of significant negative health effects, it's probably not bad for you . That said:…
The problem is there will have been a lot of confounders.
E.g. despite huge sample sizes, isolating the cause of the obesity crisis is too hard because so many different things changed at once.