Smoking marijuana during one’s teenage years may not have a direct impact on cognitive development, although those who do use cannabis may be more at risk of stunted intelligence due to a range of other environmental factors. This is according to a pair of studies investigating how the IQ of adolescent marijuana users compares to that of their abstinent peers, while accounting for other factors that may contribute to any differences.

The first of these appeared in the Journal of Psychopharmacology last week, and found that in a sample of 2,235 British teenagers, IQ levels were lower among those who had smoked marijuana at least 50 times by the age of 15. However, their results also revealed higher rates of childhood behavioral problems, depressive symptoms, and other substance use – particularly alcohol and cigarettes – among cannabis smokers, raising the possibility that these factors may have been responsible for their reduced IQ.

Through statistical analysis, the researchers report that cannabis use could not be used to predict reduced teenage IQ scores once these other variables had been accounted for. Interestingly, however, they discovered that even after excluding those who had never tried cannabis, cigarette smokers exhibited lower educational performance, dropping by up to two grades in any given academic subject. Since cannabis users were 17 times more likely to also smoke cigarettes, the study authors posit that cigarette use may be a more reliable predictor of reduced IQ in this group.

By way of explanation, they propose that those who smoke both tobacco and cannabis may do so due to “social adversity during adolescence,” possibly stemming from behavioral problems or an unstable family environment. This, in turn, could have a negative effect on IQ and academic attainment. While this idea is backed up by previous research, currently it is just speculation as they have no supportive data.

Building on this research, a second study appeared in the Proceedings of the National Academy of Sciences, in which researchers examined the effects of marijuana on the intelligence of twins, where one twin was a regular user while the other was not.

To conduct the study, the authors measured the IQ of around 2,000 adolescent twins living in Los Angeles and Minnesota, over a period of roughly a decade. As with the previous study, they found that those who began smoking marijuana during this period tended to develop lower-than-average intelligence levels. However, they discovered that the non-using twins of marijuana smokers also obtained lower scores, indicating that this outcome may have been determined by a range of background factors such as family environment or genetic predisposition, rather than cannabis.

Research has indicated that marijuana-using twins do not suffer from a lower IQ than their non-using twin. Stokkete/Shutterstock

In addition, they note that participants who became marijuana users during the course of this study also attained lower baseline IQ scores, which were recorded before their teenage years, at a time when they had not yet been introduced to cannabis. Subsequently, the researchers claim that reduced baseline IQ may be an indicator of a child’s likelihood to begin using cannabis, and may well be a product of genetic or family-wide confounding factors.

Finally, the authors of this study are keen to point out that their research differs from most previous work on the subject since it is of a longitudinal rather than a cross-sectional nature. In other words, it tracks subjects’ development over a long period of time, rather than merely observing a group of participants at a single point. This approach provides more reliable data since it allows researchers to observe how a range of factors may impact a child’s development, thereby enabling them to pinpoint the various causes of their lowered IQ as opposed to placing it all down to cannabis.