What's also interesting is that you would expect that such extreme polarization would show equally extreme negative effects, if they exist, since it means you have a large population of users exposed to high doses where negative effects should be most obvious.

(Imagine that it takes 10 times per month to put someone at risk of going crazy, robbing or killing people, torching houses, etc. If the distribution of use was a straight line down to just a few % of the heaviest users reaching 10 times per month, you wouldn't necessarily see much of an increase in population numbers. But with a crazy distribution like that, it's >20% of users who would be exhibiting side effects!)