I'd honestly thought the fluoride in your water is a poison went away with the idea that at one point it was a Communist plot, and then this meme found its way to my timeline (I honestly don't know how this Facebook page keeps finding its way onto my timeline, but I have other memes from the same group).
So I Googled "dangers of fluoride in drinking water."
Of the many sites that popped up, most were far from scientific. There were actually two concerns that did show up. Over-fluoridation can cause fluorosis or slight tooth and other bone discoloration. A problem that I suffer from having started life on less than well controlled cistern water. I had a dentist that not only identified it, but could tell me what part of Colorado I was raised in. The other concern is one that has been going on for about 50 years. Fluoride was also once used to treat a hyperthyroid. The opponents to the use in water are concerned it may suppress the thyroid gland. Most studies have found no link to the problem. A hand full have said otherwise. And therein lies the problem. Point only to the studies over the last 50 years that support this meme or point to the preponderance of science that says you are living in fear and should be wearing a tin foil hat.
Don't worry, we will all be killed by GMO's or red dye #12 long before the fluoride discolors our bones and kills us.