Does the word Tuskegee mean anything to you? Because that's basically what you are asking for.

Nor is there any need for such a study as we have a natural one: some areas have more fluorine in the water than others. We started putting fluorine in the water because we noticed that the places with higher natural levels had better teeth. There comes a point where it's too much and downsides appear, but, again, we already knew that. Note that this is a completely normal thing--there is nothing which is not toxic in sufficient quantity. Including *everything* that is necessary for human life. What would be strange is if there wasn't some maximum safe level. Some things the body easily eliminates and the range between minimum and maximum is quite wide. Things which are not so easily eliminated have narrower ranges. Thus we have the situation where overdose of water-soluble vitamins is basically unheard-of, but overdose of fat-soluble vitamins very definitely happens.