[koboll]

It actually seems like it is hard:

>Water fluoridation reduces cavities in children, while efficacy in adults is less clear.[9][10] A Cochrane review estimates a reduction in cavities when water fluoridation was used by children who had no access to other sources of fluoride to be 35% in baby teeth and 26% in permanent teeth.[9] However, this was based on older studies which failed to control for numerous variables, such as increasing sugar consumption as well as other dental strategies.[9] Most European countries have experienced substantial declines in tooth decay, though milk and salt fluoridation is widespread in lieu of water fluoridation.[11] Recent studies suggest that water fluoridation, particularly in industrialized nations, may be unnecessary because topical fluorides (such as in toothpaste) are widely used, and caries rates have become low.[3]

https://en.wikipedia.org/wiki/Water_fluoridation