Fluoride may not be on the periodic table, but that doesn't mean it's not a naturally occurring substance. Fluoride is a mineral that is in the air we breathe, the food we eat, and--yes--occurs quite naturally in the water we drink. That being said, the natural fluoride level in all these things can vary wildly, and thus the debate about adding or subtracting fluoride in water supplies occurs.
Like any chemical or mineral, the dose makes the poison. Thus, if ingested or accumulated at levels too high, over-toxicity will occur. But don't panic quite yet--by this definition, nearly anything could be considered poisonous. For instance, drinking copious amounts of water can be deadly. However, in small amounts, fluoride has been proven to prevent cavities and tooth decay in humans [source: ATSDR].
If you're still not sure how something poisonous can possibly be beneficial, think of the body's relationship with alcohol. In moderation, it's been shown to have health (and psychological) benefits, but if you're taking nine shots of "Tequila Sunrises" within nineteen minutes, you'll find that it's pretty toxic stuff.
So indeed, fluoride is not completely without its risks, which we'll see when we discuss fluorosis. But first, let's explore why it was deemed beneficial in small amounts, and how community water fluoridation amped up the debate about fluoride--and made Colonel Jack D. Ripper in "Dr. Strangelove" so leery of its origins.