Frequent Cannabis Use by Young People Linked to Decline in IQ

Summary: Frequent cannabis use was associated with a two point decline in IQ. The decline was primarily related to verbal IQ.

Source: RCSI

A study has found that adolescents who frequently use cannabis may experience a decline in Intelligence Quotient (IQ) over time. The findings of the research provide further insight into the harmful neurological and cognitive effects of frequent cannabis use on young people.

The paper, led by researchers at RCSI University of Medicine and Health Sciences, is published in Psychological Medicine.

The results revealed that there were declines of approximately 2 IQ points over time in those who use cannabis frequently compared to those who didn’t use cannabis. Further analysis suggested that this decline in IQ points was primarily related to reduction in verbal IQ.

The research involved systematic review and statistical analysis on seven longitudinal studies involving 808 young people who used cannabis at least weekly for a minimum of 6 months and 5308 young people who did not use cannabis. In order to be included in the analysis each study had to have a baseline IQ score prior to starting cannabis use and another IQ score at follow-up. The young people were followed up until age 18 on average although one study followed the young people until age 38.

“Previous research tells us that young people who use cannabis frequently have worse outcomes in life than their peers and are at increased risk for serious mental illnesses like schizophrenia. Loss of IQ points early in life could have significant effects on performance in school and college and later employment prospects,” commented senior author on the paper Professor Mary Cannon, Professor of Psychiatric Epidemiology and Youth Mental Health, RCSI.

This shows a head made up of cannabis leaves
Further analysis suggested that this decline in IQ points was primarily related to reduction in verbal IQ. Image is in the public domain

“Cannabis use during youth is of great concern as the developing brain may be particularly susceptible to harm during this period. The findings of this study help us to further understand this important public health issue,” said Dr Emmet Power, Clinical Research Fellow at RCSI and first author on the study.

The study was carried out by researchers from the Department of Psychiatry, RCSI and Beaumont Hospital, Dublin (Prof Mary Cannon, Dr Emmet Power, Sophie Sabherwal, Dr Colm Healy, Dr Aisling O’Neill and Professor David Cotter).

The research was funded by a YouLead Collaborative Doctoral Award from the Health Research Board (Ireland) and a European Research Council Consolidator Award.

About this neuroscience research news

Source: RCSI
Contact: Jane Butler – RCSI
Image: The image is in the public domain

Original Research: Open access.
Intelligence quotient decline following frequent or dependent cannabis use in youth: a systematic review and meta-analysis of longitudinal studies” by Mary Cannon, et al. Psychological Medicine


Intelligence quotient decline following frequent or dependent cannabis use in youth: a systematic review and meta-analysis of longitudinal studies

Previous systematic reviews and meta-analyses of cross-sectional data assessing the effect of cannabis on cognitive functioning and intelligence show inconsistent results. We hypothesized that frequent and dependent cannabis use in youth would be associated with Intelligence Quotient (IQ) decline. This study is a systematic review and meta-analysis. We searched Embase, PubMed and PsychInfo from inception to 24 January 2020. We included studies with non-treatment seeking samples and pre- and post-exposure measures of IQ. We requested data from authors if summary data was not available from published work. We preregistered our review with PROSPERO (ID no. CRD42019125624). We found seven cohort studies including 808 cases and 5308 controls. We found a significant effect for the association between frequent or dependent cannabis use in youth and IQ change, Cohen’s d = −0.132 (95% CI −0.198 to −0.066) p < 0.001. Statistical heterogeneity between studies was also low at I2= 0.2%. Study quality was moderate to high. This translates to an average decline of approximately 2 IQ points following exposure to cannabis in youth. Future studies should have longer periods of follow up to assess the magnitude of developmental impact.

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  1. As someone who’s used THC for both medical and recreational purposes, I only used it in adulthood but it clearly impacted my functioning on a chronic basis with development of tolerance. When used with nutrition, supplements and other things on a cycling basis or on average (high) doses no more than twice a week on isolated days, I experienced significant reversal of brain symptoms which combined with intervention fixed half my life. It’s how you use it

  2. I would suspect that cannabis doesn’t lower IQ, but rather that frequent cannabis consumption is indicative of various general problems possibly the easiest of which to point out is the difference in homework and tutelage between the users and non-users. If students are attentive during class, and then fully complete their workload including miscellaneous “studying” tasks so as to ensure they are as equally versed as ALL of their peers, then I would expect the effects of cannabis consumption to be much more negligible. The fact that kids use substances in the manners that they do suggests there is a much more concerning problem with their lives. Kids should be kept busy and healthy, not stressed or disparaged about their lot in life.

    In short I would suspect the noted decrease in IQ scores to be equally attributable to the lack of time/effort put forth by the students in their studies. The kids who consume cannabis are likely dodging their assignments. Whether or not they are actively using cannabis INSTEAD of completing assignments is not particularly relevant to the fact that they did not do the work.

    Let’s focus on providing for our nation’s children while they are at risk. Not whether or not marijuana may be slightly negative towards the development of adolescents.

    Nobody should be given drugs, unless absolutely necessary, until about age 25. At the very least kids won’t develop tolerance/addiction to chemicals as a part of their inherent physiology.

    1. Well said sir. On the one hand I agree that there could easily be a negative impact if someone uses too much in their developing years. But if taken at its word this article would indicate the vilification of a fantastic plant with innumerable medical benefits.

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