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To examine the relationship between adolescent- or adult-onset persistent cannabis use and cognitive changes, investigators analyzed prospective data on the 1037 participants in the longitudinal Dunedin Study. The data included interviews at ages 18, 21, 26, 32, and 38; IQ testing in childhood (biannually at ages 7–13) and at age 38; and neuropsychological tests at age 38. Only seven participants reported any cannabis use by age 13.
Compared with adult-onset persistent users, adolescent-onset persistent users with at least three cannabis dependence diagnoses had an average 8-point decline in IQ by age 38. Regardless of dependence diagnoses, adolescent-onset users showed statistically significant impairments across multiple domains of cogniti…