Marijuana does not cause decline in intelligence among teens - US Researchers

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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.