That's very, very debatable. Just google "flouride" and you will see thousands upon thousands of sites on the subject. I have friends in the medical field who told me of the harm it causes. (Just don't demand to know what harm, since you will need to find it yourself.)
I did look up.
Here's the real problem with their argument, they're saying there were harms as a result of fluoridation but more people were harmed by having dental caries and losing teeth. There's a strong link between heart disease and advancing gum disease.
Stained teeth as a result of too much fluoride is not considered harmful even though it's unsightly. Not much is known about long term systemic effects but considering that it started in 1940's and our lifespan increasing, I doubt it has any effect.
Also, remember that fluoride is a NATURAL element and is found in many water sources. In areas where there's less than optimal amount, we simply add it. The effects are apparent - rate of dental caries dropped.
So, let's imagine this... if there's an unique disease called fluoriditis - 1 out of every 100,000 gets that disease and it's caused by fluoride in water and they die. On the other hand, 1 out of 10 gets dental caries which is known to cause serious systemic health problems and can cause premature death. About 1 out of 200 die prematurely from it. If we add fluoride, we may save a few more lives.
Now, do you think we should ban fluoride just because one out of 100,000 people got fluroiditis? That would be irrational.
It's like saying that seat belts is wrong because it's an infringement of your rights - well, that's where it has to stop. If seat belts save more lives than without it, then it should be for the public's safety. You can die from wearing seat belts (trauma from seat belt) but you're more likely to die from not wearing it!
I know the lithium water studies are pretty sparse compared to fluoride water but I am intrigued and I have a public interest in it - I want to see fewer mentally ill people.