Well, if you read the results what it basically says is that if you started regular use from adolescence there was a clear IQ decline correlated in proportion w

Well, if you read the results what it basically says is that if you started regular use from adolescence there was a clear IQ decline correlated in proportion with "waves" of diagnoses (iow, they measured a number of times several years apart, and IQ loss correlated with how many times they were diagnosed as cannabis-dependent at measurement so presumably length of time using). If you started as an adult, there was no clear correlation, although looks like at least a small trend towards decline was always observed. http://m.pnas.org/content/109/40/E2657/F2.expansion.html http://m.pnas.org/content/109/40/E2657/F2.expansion.html As far as reversal, looks like it stuck pretty hard if you started regular use during adolescence. If you started afterwards, then you bounced back as long as you weren't still using frequently. http://m.pnas.org/content/109/40/E2657/F3.expansion.html http://m.pnas.org/content/109/40/E2657/F3.expansion.html If your friends are adults it's probably either damage done already or nothing irreversible, at least to the extent this study is accurate. One thing that would be interesting would be a breakdown around different standard deviations. We know that drug use is higher in more intelligent people[1,2], and that probably does represent our audience here given subject matter. I'd be curious to know whether the effects are more or less marked once you're a couple of standard deviations out from the norm, given that the population measured seems to be near median-100 (i.e. either a broad cross-section or skewed towards average). [1] https://www.psychologytoday.com/blog/the-scientific-fundamentalist/201010/why-intelligent-people-use-more-drugs https://www.psychologytoday.com/blog/the-scientific-fundamen... [2] http://healthland.time.com/2011/11/15/why-kids-with-high-iq-are-more-likely-to-take-drugs/ http://healthland.time.com/2011/11/15/why-kids-with-high-iq-...