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Research using data from the "Dunedin Study" has shown heavy use of marijuana before the age of 18 causes lasting harm to intelligence, attention and memory.
The study, by an international research team, showed those dependent on cannabis in adolescence suffered an average decline of eight IQ points from when they were tested at age 13 and age 38. Those who did not take up pot until they were adults "with fully-formed brains" did not show similar mental declines.
The research used data from the University of Otago's ongoing Dunedin Multidisciplinary Health and Development Study, also known as the "Dunedin Study", which has followed a group of 1037 children born in 1972-73 in Dunedin from birth.
About 5% of the study group were considered marijuana-dependent, or were using more than once a week before they were aged 18.
Lead researcher Madeline Meier, a post-doctoral researcher at Duke University in Durham, North Carolina, said the study showed giving up the habit later in life did not appear to reverse the loss in IQ caused by adolescent dependency.
Otago University National Addiction Centre senior lecturer Dr Simon Adamson said the results should help inform policy-makers.
"The results ... suggest adult onset regular cannabis use does not lead to cognitive decline ... it was the individuals ... who were already using heavily by age 18 who experienced impairment.
"Clearly, we must focus energy on reducing the prevalence of cannabis use in adolescence."
The study's authors said its findings were unique because it was among the first to be able to take into account deficits in its subjects' IQ before they were dependent on cannabis.
It was also able to show cannabis-associated cognitive decline did not occur solely because cannabis users completed fewer years of education.