Teenage cannabis use and cognitive performance

Does smoking weed as a teenager decrease your IQ?

Writer: Sergio Garcia Busto
Editor: Katie Dale
Artist: Patrick Marenda

Recreational cannabis use is becoming increasingly popular, making it the most commonly consumed illicit drug in many countries and states. Cannabis is also being increasingly prescribed to treat various diseases and has been shown to improve several disease symptoms such as pain and nausea. The principal psychoactive component of cannabis is delta-9-tetrahydrocannabinol (THC), which mainly elicits its effects by binding to the CB1 cannabinoid receptor, partially activating it. It is obvious that cannabis causes acute cognitive changes that last for several hours, but whether it inflicts long-term cognitive impairment resulting in IQ decline is not so clear. One of the main arguments for preventing teenagers from being exposed to it is that it significantly reduces intelligence and cognitive performance, especially if consumed in adolescence. Is there a consensus for this within the scientific community? Does consuming cannabis in your teenage years really decrease your IQ?

It is safe to say that the research does not provide a clear ‘yes or no’ answer to this question; some studies report a strong association between teenage cannabis consumption and IQ decline or poor academic performance, while others observe no significant permanent effect. This confusion in part comes from the fact that cannabis consumption is difficult to isolate, as there are many variables to control for. 

Given that it is unethical to prescribe cannabis to teenagers in order to measure its long-term effects, only observational studies can be done. An important issue here is that subjects involved often report use of other illicit drugs, behavioural problems, poor educational performance and potentially consume tobacco or alcohol. It is not only difficult to determine the extent to which factors like these contribute to cognitive impairment, but also to derive information regarding causality; in other words, what is actually responsible for the observed effect. Therefore, research papers report correlations and associations, but conclusions are neither clear nor unanimous. For example, one of the factors that have been found to considerably alter the association between cannabis consumption and IQ decline is tobacco and specifically cigarettes, sometimes argued to be responsible for poor school performance to a larger extent than cannabis.

Work by Meier et al from 2012, a study that followed participants from a 1972 birth cohort, gave rise to some debate by presenting data indicating that persistent cannabis consumption affects IQ, specifically when consumption starts before the age of 18. The study argued that after controlling for factors including alcohol, tobacco, hard-drug use and pre-existing cognitive impairment, weekly cannabis consumption in adolescence could impair cognitive performance permanently. 

A controversial response to Meier’s work came a few months later—Rogeberg published an article that critiqued Meier’s conclusions and claimed that the methodology used was flawed. He argued that the variables that were controlled for could have a significant effect if taken together, while separately no meaningful effect could be observed. He also explained that the individual IQ trajectory of different exposure groups was not accounted for. It is argued that socioeconomic status (SES) is responsible for long-term IQ changes and that low-SES children could suffer an IQ drop with age. Therefore, Rogeberg says that since low-SES subjects are more likely to consume cannabis as teenagers, Meier’s results were biased. To illustrate this last point, a simulation was created taking the relationship between SES, IQ and cannabis consumption into account, which showed that SES could potentially be entirely responsible for the observed IQ decline. 

A reply to Rogeberg’s response was then published by Moffitt, which showed data indicating that cannabis use in adolescence was distributed across all socioeconomic backgrounds. Moreover, Moffitt’s analysis shows that SES is not predictive of IQ decline, negating the assumption made by Rogeberg in his critique of Meier’s study. Statistical controls for SES did not alter the association between cannabis use in teenagers and IQ fall, arguing against the validity of Rogerberg’s response.

Unfortunately, the question of whether teenage cannabis use reduces IQ looks set to confound us for the foreseeable future. There is evidence of no permanent cognitive impairment in adult consumers, but investigating the effects in adolescents remains experimentally intractable. However, it is somewhat clear that either cannabis consumption or the events and lifestyle choices that can be associated with it may impair cognition, either alone or in combination. Paradoxically, some researchers mention that controlling for additional variables could introduce even more confusion into findings and promote erroneous interpretations and conclusions. Therefore, given the confounding variables that likely influence the association with cognitive performance and IQ, it can only be said for sure that answers to this matter can be subject to doubt. 

If you want to learn more about how cannabis affects human health:

  • Properties of medical marijuana:
  • Depression: 
    • Association of Cannabis Use in Adolescence and Risk of Depression, Anxiety, and Suicidality in Young Adulthood: A Systematic Review and Meta-analysis (JAMA Psychiatry)
  • Is Cannabis Really That Bad? (TheScientist)

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