Persistent cannabis users show neuropsychological decline from childhood to midlife

Dunedin Multidisciplinary Health and Development Study, n=1,037 followed age 13 to 38. Persistent adolescent-onset cannabis users showed mean IQ decline of 8 points by age 38 vs. controls; effect persisted after accounting for confounders. Adult-onset users did not show this decline. Single most-cited study for "adolescent brain vulnerability to cannabis". Caveats: replications mixed (Mokrysz 2016 disputed the magnitude after controlling for SES), but the Dunedin signal is robust to multiple sensitivity analyses.