Key points
- Research shows no overall effect of moderate cannabis use on IQ
- Early age use might affect later IQ
- High usage greater than 4 days per week for greater than 14 years could have a small effect on IQ
- Effects of cannabis on IQ outcomes are confounded by social factors such as smoking, alcohol use, and socioeconomic strata
- The effects of cannabis on IQ do not reach a threshold for robustness as the effect size is within the margins of error for IQ testing reliability and the extremely small effect sizes are not clinically meaningful
While it is commonly suggested that cannabis reduces intelligence quotient (IQ), especially in teenage users of cannabis, the evidence for this is limited, and when confounding influences of socioeconomic factors, cigarettes smoking, and alcohol use are considered, there is typically no effect of cannabis on IQ.
In a 2016 prospective cohort study of 2235 teenagers, once confounding influences had been adjusted for, there was no effect on either IQ or educational performance for those who had smoked cannabis more than 50 times.1
In a study of youth at high risk of psychosis, there was no significant difference in IQ between at-risk youth, healthy controls, or between high, medium, and low-frequency users. However, early-onset users did have a lower IQ, suggesting that early-age use may affect IQ but later does not.2
Early-age use may affect IQ but later does not
In perhaps the most robust cohort studied, researchers looked at data from the famous, large-scale Dunedin Study (n = 1037).3 Change in IQ between the ages of 18 and 38 was related to cannabis use. Never-users gained on average 0.8 of an IQ point, whereas those who used less than 4 times per week lost on average 1.03 IQ points, yet incidentally, still ended up with a marginally higher IQ than non-users. The highest use group, using more than 4 days per week for more than 3 ‘waves’ of the study (each wave corresponds to the time between 18, 21, 26, 32, and 38 years of age), lost around 5 IQ points.
Discussion
The exceedingly small changes in IQ, especially those seen with low and moderate use (i.e. those who use cannabis less than 4 days per week) are not meaningful and would not impact the user’s life. Conversely, there are likely to be benefits for many moderate users of cannabis who may be self-medicating or using the drug for entheogenic purposes. There would also likely be greater harm for people using cannabis for pain relief if they were to substitute this for a prescribed pharmaceutical painkiller which is both more lethal and has long-term health risks.4
exceedingly small changes in IQ are not meaningful and would not impact the user’s life
Furthermore, while IQ tests are considered reliable (with similar repeated test outcomes) there is still some variation in repeated measures. This has been observed in children to as large as 18 IQ points.5 It is also known that socioeconomic and ethnic differences, motivation, and mental state can account for variations in IQ and these variations can account for temporal variability.5
Additionally, ‘never-use’ vs 4 days per week provides an extraordinarily wide range within which true moderate or minimal users have not been accounted for, especially those using once or twice per week or episodic users utilising cannabis for pain relief.
In response to the (Dunedin cohort) study above, Ole Rogeberg suggested that the causality of the findings of cannabis reducing IQ should be disputed because socioeconomic status and temporal factors can affect IQ and secondary analysis simulating these effects resulted in similar findings, suggesting that the causal effects are likely to be overestimated and that the true effect could be zero.6 Also supporting this hypothesis is the fact that those who used the most cannabis and who experience the greatest reduction in IQ also started with the lowest baseline IQ results.
Conclusions
Overall, data suggest that there is little if any effect of cannabis on IQ and any effects seen are so small as to be meaningless, and results are likely to be confounded by social, psychological, and economic factors.
data suggest that there is little if any effect of cannabis on IQ and any effects seen are so small as to be meaningless
References
1. Mokrysz C, Landy R, Gage S, Munafò M, Roiser J, Curran H. Are IQ and educational outcomes in teenagers related to their cannabis use? A prospective cohort study. Journal of Psychopharmacology. 2016;30(2):159-68.
2. Buchy L, Seidman LJ, Cadenhead KS, Cannon TD, Cornblatt BA, McGlashan TH, et al. Evaluating the relationship between cannabis use and IQ in youth and young adults at clinical high risk of psychosis. Psychiatry Research. 2015;230(3):878-84.
3. Meier MH, Caspi A, Ambler A, Harrington H, Houts R, Keefe RSE, et al. Persistent cannabis users show neuropsychological decline from childhood to midlife. Proceedings of the National Academy of Sciences. 2012;109(40):E2657-E64.
4. McCrae JC, Morrison EE, MacIntyre IM, Dear JW, Webb DJ. Long-term adverse effects of paracetamol – a review. Br J Clin Pharmacol. 2018;84(10):2218-30.
5. Neisser U, Boodoo G, Bouchard Jr TJ, Boykin AW, Brody N, Ceci SJ, et al. Intelligence: knowns and unknowns. American psychologist. 1996;51(2):77.
6. Rogeberg O. Correlations between cannabis use and IQ change in the Dunedin cohort are consistent with confounding from socioeconomic status. Proceedings of the National Academy of Sciences. 2013;110(11):4251-4.
