Heavy Cannabis Use Tied to IQ Decline

The participants were members of the Dunedin (New Zealand) Study, a prospective study of a birth cohort of 1,037 individuals followed from birth in 1972/1973 to age 38. Cannabis use was ascertained in interviews at ages 18, 21, 26, 32, and 38. Neuropsychological testing was conducted at age 13, before initiation of cannabis use, and again at age 38, after a pattern of persistent cannabis use had developed.

The results showed that 242 people never reported cannabis use, nor were they ever diagnosed with dependence; 479 indicated some use but never received a diagnosis; and 80, 35, and 38 had received one, two, or three or more dependence diagnoses in the adult evaluations. In the small group of participants who became cannabis dependent before age 18--a total of 23 cohort members-- the decline translated to an average decline of about 8 IQ points, whereas 14 participants who also showed heavy cannabis use but only beginning in adulthood showed only a very small drop in full-scale scores. The study was published on July 30, 2012, in the Proceedings of the National Academy of Sciences of the USA (PNAS).

“Collectively, findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects,” concluded lead author Madeline Meier, PhD, of Duke, and colleagues. “Prevention and policy efforts should focus on delivering to the public the message that cannabis use during adolescence can have harmful effects on neuropsychological functioning, delaying the onset of cannabis use at least until adulthood, and encouraging cessation of cannabis use particularly for those who began using cannabis in adolescence.”

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