Any opponent of liberalizing cannabis laws who cites the Dunedin results showing persistent IQ declines in people who start to use cannabis heavily as adolescents and keep using heavily for the next 20 years and doesn’t:
(a) point out that what’s being studied is long-term drug abuse, not the mere use of cannabis and
(b) acknowledge another analysis, published in the same journal, showing that the results of the Dunedin study may not reflect a causal link from cannabis abuse to IQ decline
is trying to persuade, not to inform.
To be clear: It’s quite possible that very heavy and very prolonged cannabis use has measurable and lasting cognitive impacts. It’s also possible that some of those impacts might stem from actual cell- and tissue-level brain changes; you’d want measurements after more than a month of abstinence to figure that out. But there’s been entirely too much “It’s a scientific fact that pot-smoking makes you stupid” around. Note that the original paper found no measurable IQ change in those who didn’t start heavy cannabis use in adolescence, even if they smoked like chimneys as adults.