Study after study: recommended levels of fluoride; no cancer risk, no adverse effect on IQ. You can get adverse effects from too much fluoride, sure. So, here's a recommendation: don't get too much fluoride. You can get adverse effects from too much anything. Food. Water. Even oxygen.
In fact, IQs in the US have increased 3 pts per decade since 1949. The practice of fluoridation became widespread in this country in the 30s. There are lots of adverse health risks from caries -- tooth decay -- that are mitigated by fluoridation. Oddly, as politically fanned opposition to public health practices have increased, there has been both an increase in cancers, a decrease in lifespan, and for the first time in 70 years, a decrease in IQ. Coincidence? Opposition to public health leads to adverse health results? Sounds reasonable to me. But, hey, you can't reason paranoia out of people.