While I can't source the IQ claim, the evidence "showing" that fluoridation is safe to drink (as opposed to measuring by tooth health) is fairly scarce.

OTOH I've heard teeth are especially poor around Portland Oregon where they don't fluoride the water. I'd simply say the jury is still out.

I tend to think of iq claims specifically with a particularly large grain of salt: it's an index of society that people follow closely and is related to many, many factors.

The practice of fluoride in drinking water does seem to be getting some push back of late. I did a quick google scholar search for "fluoride drinking water" and found some articles on both sides.

So does the practice of vaccinations, and without strong evidence that it's not safe, I'm inclined to take the lack of such evidence as significant, in light of the abundance of evidence that it seems to be helpful.