I think calling it a conspiracy theory is not entirely fair. It's a genuine scientific debate, hindered by the lack of proper evidence and studies that apply to the US.

Read https://www.statnews.com/2025/01/06/fluoride-iq-jama-pediatrics-critiques-meta-analysis/ for example, it highlights a recent meta-study that found a small, but non-negligible effect on neurodevelopment if people were ingesting fluoride. But a lot of studies it relies on have some asterisks attached. Those are properly accounted for in the meta-study, but ultimately the answer is "we don't really know".

Many western countries don't add fluoride to the drinking water; many used to do so in the past but stopped. There were the concerns about neurotoxicity (albeit minor) but also some ethical concerns regarding mass-medicating the population without any realistic opt-out. But the other major reason is that those countries have the population exercise good dental practices like brushing twice a day with fluoridated toothpaste, which is spat out instead of swallowed. This avoids concerns of neurotoxicity but maintains the dental benefits: a best of both worlds basically (also endorsed by most scientists).

The US has systemic poverty issues, and a large part of the population do not observe these good dental practices, not necessarily through ill-will but rather because they lack the money to buy toothpaste. Because of that, removing fluoride will likely increase cavities in the US, unlike in other western countries.

Ideally the US keeps the fluoride around until these systemic poverty issues are largely resolved. But knowing the current shitstains in government...