Teenagers make up most of new cannabis users and also tend to consume it heavily. This is a matter of concern among health professionals and policy makers, mostly due to the possible harms the drug may have on brain development, cognitive faculties, and educational performance.
Research has more or less consistently shown that cannabis causes acute impairments in memory consolidation, working memory, attention, psychomotor control, impulse inhibition, and abstract reasoning. These effects may, in turn, explain why cannabis users often have a poorer educational performance and attainment compared to non-users.
However, the question of whether cannabis related cognitive impairments can become permanent is still hotly debated. The best way to address this question is through longitudinal studies that follow people before they start using drugs, in order to exclude the possibility that symptoms currently seen among users were not already present before the onset of drug consumption.
In a study of this kind, Fried and collaborators revealed that young adult cannabis users have a lower IQ than never-users, but this only held true for those who were still using it and not for those who had stopped. More recently, Meier and collaborators published the results of a study following nearly 900 people born in 1972, showing an IQ decline among those who had started using cannabis weekly in adolescence, but not among those who had only started in adulthood. These results were still significant for those who no longer consumed it.
But these results are still not enough to conclude that cannabis use in adolescence causes permanent cognitive declines. Other variables that are specific to teenager users could account for these effects (and the poorer academic performance), namely antisocial problems, childhood mental illness, parental factors, and use of other substances.
A new study by Mokrysz and collaborators from the U.K., published in the Journal of Psychopharmacology that took into account those confounding variables casts some doubt on the idea that teenage cannabis use is associated with IQ and educational deficits.
Longitudinal data for more than 2000 young adults born in 1991 in Bristol was obtained. In an initial analysis, the authors found that after controlling for IQ at the age of 8, those who had used cannabis more than 50 times had a lower IQ at age of 15, compared to never-users. But when the results took into consideration cigarette use, the differences between the two groups associated with cannabis use disappeared. Strikingly, the same pattern emerged when the authors analyzed educational performance. A third analysis supported the conclusion that both IQ and educational outcomes could be explained by cigarette use alone.
Like any study, this one presents considerable limitations in the conclusions that can be drawn from it. First, the fact that there were no differences in IQ or school performance at such a young age does not preclude the hypothesis that negative effects may emerge later in adulthood or after subsequent exposure. Furthermore, the authors did not consider school dropout, which has been specifically linked to cannabis use by other researchers.
Other limitations include the relatively mild use of cannabis in this population, the adoption of an abbreviated IQ test, and the reliance on self-reports of drug use, which together might have obscured the existence of smaller but still significant effects.
Nevertheless, this study raises important questions about results from previous studies and highlights the importance of using rich longitudinal data to disentangle the effects of other variables.
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