Teenagers lose IQ with chronic marijuana use

Adolescents who smoke marijuana on a regular basis face a decline in intelligence and brain function, even if they quit using the drug as adults. The findings are the first to associate a drop in neuropsychological functioning with cannabis use.

Researchers in New Zealand analyzed 1,037 children in the Dunedin Study, a cohort of children born in 1972 and 1973 and followed until they were aged 38 years, for effects of marijuana use on neuropsychological functioning. The participants were given IQ tests when aged 13 years, before any had begun smoking marijuana, and again when aged 38 years, after those who began smoking marijuana had persistently used the drug. They also were surveyed when aged 18, 21, 26, and 38 years about their drug use.

Approximately 5% of study participants began using marijuana as teenagers. Those who smoked marijuana 4 times a week or more throughout the study period lost on average 8 IQ points. The decline was not associated with factors such as other drug or alcohol use or years of education.

Loss of IQ was also noted in those participants who eventually stopped smoking marijuana. When tested at the completion of the study, their IQs were lower than when tested at the beginning of the study, and continued cessation of smoking marijuana did not reverse cognitive impairment.

The findings suggest that cannabis has a neurotoxic effect on the adolescent brain, causing declines in intellectual functioning, especially in executive function and processing speed. Participants who began smoking marijuana as adults saw a small or no drop in intelligence or functioning.

Researchers hypothesize that cannabis affects the hippocampus and prefrontal cortex, where learning, memory, and planning occur, and that teenagers are more susceptible to marijuana’s effects on the brain than older adults. They say their study highlights the importance of refraining from cannabis use during adolescence and the need for policies that encourage teenagers to stop using the drug altogether.

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