Syrian refugees farm cannabis in Lebanon
The study looked at more than 3,000 people, some cannabis users, some not, to compare the differences Alia Haju/Reuters

Marijuana does not lead to a decline in intelligence in teenage users, the largest longitudinal study on the drug and IQ change has shown. US researchers looked at more than 3,000 individuals, including twins, measuring cognitive decline of cannabis users compared to non-cannabis users.

Researchers from the University of Southern California, Los Angeles noted that marijuana use in adolescence has previously been associated with reduced intelligence, memory, attention and verbal ability. But whether the drug directly causes cognitive decline, or if there are other factors at play, is unclear.

In their study, published in the journal PNAS, the team looked at the relationship between marijuana use and different measures of intelligence among twins. "Marijuana is the most commonly used recreational drug in the United States," they wrote. "Some studies suggest that marijuana use in adolescence is linked to declines in intellectual functioning. Because of the infeasibility of studying this phenomenon experimentally, it is unclear whether the association can be causally attributed to marijuana use itself or is instead the result of confounding factors."

Measures of intelligence were taken at the ages of between nine and 12, before they started using marijuana, and again at between 17 and 20. Marijuana use was self-reported and the scientists adjusted their results for family background and genetic propensities.

Researchers found that in the first sample (not looking at the twins) marijuana users had lower test scores compared to non-users, and that there had been a "significant decline" in crystallised intelligence (vocabulary and information) between preadolescence and late adolescence.

But importantly, researchers showed there was no evidence of a correlation between frequency of use and decline in intelligence and there was no difference in cognitive declines between twins where one sibling used marijuana and the other did not. This, they say, suggests the apparent cognitive decline seen among marijuana users is likely not directly caused by the drug directly.


"Although marijuana users showed greater decline than non-users in areas of [crystallised intelligence], the presence of baseline differences before marijuana involvement, the lack of a dose–response relationship, and an absence of meaningful differences between discordant siblings lead us to conclude that the deficits observed in marijuana users are attributable to confounding factors that influence both substance initiation and IQ rather than a neurotoxic effect of marijuana," the authors wrote.

"Evidence from these two samples suggests that observed declines in measured IQ may not be a direct result of marijuana exposure but rather attributable to familial factors that underlie both marijuana initiation and low intellectual attainment."

The findings echo similar research carried out in the UK on 2,235 teens. Published in the Journal of Psychopharmacology, scientists noted that previous research had shown teenage cannabis users tended to have higher rates of behavioural problems, depressive symptoms, use alcohol and smoke cigarettes.

After adjusting for these factors, they found there was no difference in the IQs and educational performances of teenagers that used marijuana and those who did not. "Cannabis use by the age of 15 did not predict either lower teenage IQ scores or poorer educational performance," they wrote. "These findings therefore suggest that cannabis use at the modest levels used by this sample of teenagers is not by itself causally related to cognitive impairment. Instead, our findings imply that previously reported associations between adolescent cannabis use and poorer intellectual and educational outcomes may be confounded to a significant degree by related factors."