As access to marijuana increases—and while acceptance of the drug grows and perception of its harmfulness diminishes—it is important to consider the potential for long-term ill effects.
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[In one study, researchers measured] twins’ intelligence between nine and 12 years of age, before any drug use, and did so again between ages 17 and 20. Exactly as in the [1970] Dunedin study, marijuana users had lower test scores and showed notable reductions in IQ over time. But in [researchers Nicholas] Jackson and [William] Iacono’s analysis, marijuana use and IQ were completely uncorrelated, and IQ measures fell equally in both the users and abstainers.…
How can we explain these discrepancies? First, young marijuana users are many times more likely to also use alcohol and other illicit drugs. And when epidemiologists factor binge drinking, nicotine and other drug use into their models, marijuana’s cognitive effects evaporate. Thus, IQ decline seems more nonspecifically related to general substance use.
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[For now, an eagerly awaited study] is following 11,000 U.S. 10-year-olds in a national epidemiological sample with serial IQ testing and brain imaging to capture the trajectories of normal brain and IQ development prior to any substance use—and to document any longitudinal consequences of such use.
