People who were diagnosed with cannabis dependence during their teenage and early adult years exhibited declines in IQ scores by their late 30s, according to researchers at Duke University.
The study monitored just over 1,000 New Zealanders, beginning at age 3, who were born in the city of Dunedin in the early 1970s. It found that those who smoked marijuana consistently—at least four times a week in their teens, 20s and in some cases, their 30s—significantly and permanently reduced their IQ scores.
“Participants were frank about their substance abuse habits because they trust our confidentiality guarantee, and 96 percent of the original participants stuck with the study from 1972 to today,” said one of the researchers, professor Terrie Moffitt of King’s College London’s Institute of Psychiatry.
“It is such a special study that I’m fairly confident that cannabis is safe for over-18 brains, but risky for under-18 brains.”
“This is not occasional or recreation use,” said professor Val Curran of the British Association of Psychopharmacology and University College London.
—Posted by Alexander Reed Kelly.
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 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 (P=0.02), Meier and colleagues indicated online in Proceedings of the National Academy of Sciences.
… “Collectively, [the study’s] findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects,” Meier and colleagues wrote.
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