NZ Herald 28 Aug 2012
The persistent use of cannabis before age 18 has been linked to lasting harm to intelligence, according to a large study.
Analysis of more than 1000 New Zealanders found those who took up cannabis in adolescence and used it for years afterwards experienced an average decline in IQ of eight points when measured at age 13 and 38.
People who did not begin using cannabis until they were adults, with fully formed brains, did not show the same declines.
Experts here and abroad say the findings are significant and could offer some explanation for the “teenage stoner” stereotype.
Lead researcher Madeline Meier of Duke University in Durham, North Carolina, said quitting cannabis later in life did not appear to reverse the loss of intelligence.
Higher IQ correlated with higher education and income and better health, she said.
“Somebody who loses eight IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come.”
The study, to be published in the Proceedings of the National Academy of Sciences, examined individuals in the Dunedin Cohort study, which has followed 1037 people born in 1972-73 in Dunedin from birth.