Persistent Cannabis Use and Cognitive Decline
The landmark Meier et al. PNAS 2012 study followed 1,037 New Zealanders from birth, measuring cognitive function at age 13 (before cannabis use) and again at age 38. People who became persistent heavy cannabis users lost an average of 8 IQ points over that 25-year span. People who started before age 18 (during brain development) showed the largest declines.
The most important finding from a quitting perspective: stopping cannabis use in adulthood did not fully restore cognitive function in those who had started in adolescence. The earlier and heavier the use, the more durable the deficit.
This page is a draft. The full writeup will cover the test battery used, the dose-response, the adult-onset vs adolescent-onset distinction, and subsequent replications and critiques.