The study in the linked article found that lead reduced the IQ of exposed Americans 3-6 IQ points.

A New Zealand study found that persistent cannabis users had a 6-8 point decline, even if they had given up cannabis: https://www.pnas.org/doi/abs/10.1073/pnas.1206820109

Here's a meta review that found a 2 IQ point drop across a wider range of usage types: https://www.cambridge.org/core/journals/psychological-medici...

There's a lot less known about cannabis at this point, but what we know so far isn't very positive.

That's not actually what the study found.

  Further, cessation of cannabis use did not fully restore neuropsychological functioning *among adolescent-onset cannabis users*.

They did not find a 6-8 point decline for folks who gave up cannabis if they started later in life. In fact they found no decline in IQ whatsoever for folks who started later in life - both when using it and after stopping.

   In contrast, within-person IQ decline was not apparent among adult-onset persistent cannabis users who used cannabis infrequently (median use = 6 d) or frequently (median use = 365 d) in the year before testing.

> There's a lot less known about cannabis at this point, but what we know so far isn't very positive.

... for adolescents. Both your studies only show impact in youth.

I don't think my summary was unfair. I was only asked if there was evidence for persistent effects. There clearly is, even if just for adolescents.

It's also important to point out that for the adult-onset group they only looked at 1 year back. This doesn't preclude the possibility that longer periods as an adult find different results.

Sorry, no, your claims and summary were definitively and unquestionably false.

There is a large, large difference between 'Americans' and 'adolescents'. Vast.

Your summary missed a critical piece, which is that it is just for adolescents.

I don't think, even within the marijuana community, that there is any debate that marijuana use in adolescents is harmful. I regularly see /r/trees comments telling teens to stop until they're at least 18 and their brain finishes developing.

Sorry, I took a look at the study and I couldn't tell - what is an adolescent? Like, 10-18? 14-25? I know what the word means, but for a scientific study I assume there's a specific range they're referring to.

I hadn't seen the meta-analysis, thanks. 2=3 IQ points drop is pretty significant and enough to be worth consideration but it's not too worrisome.