Intelligence quotient decline following frequent or dependent cannabis use in youth: a systematic review and meta-analysis of longitudinal studies
Open accessAbstract
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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IQ decreased by 2 points each year after the start of frequent or dependent cannabis use.
“IQ, which decreased by 2 points each year after the start of frequent/dependent use [30].”
From Cannabis, cannabinoids and health: a review of evidence on risks and medical benefits (2025) · cites this paper as [30]
The Results (Full-scale IQ decline; page unavailable) report a 1.98-point IQ decline associated with frequent or dependent cannabis use. The Abstract describes approximately 2 points following exposure, not 2 points each year. The annual rate in the citing sentence is unsupported. (LLM, conf=0.91)
Experimental — accuracy not yet measuredThere is mixed evidence on the effects of the age of first cannabis use and duration of use on cognitive performance.
“There is mixed evidence on the effects of the age of first cannabis use and duration of use on cognitive performance”
From Cannabis, cannabinoids and health: a review of evidence on risks and medical benefits (2025) · cites this paper as [30]
The Introduction (page unavailable) reports inconsistent findings on cannabis use and cognitive performance generally. The Discussion (page unavailable) says evidence on longer durations of use is sparse and that there are no longitudinal IQ data for onset between ages 18 and 25. It does not specifically report mixed findings about how age at first use and duration of use affect cognitive performance. (LLM, conf=0.82)
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Metan: fixed- and random-effects meta-analysis
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“…evidence from prospective longitudinal studies suggests that adolescent (mostly frequent) PWUC have lower or declining IQs than non-using peers, but the possible role of confounders is unclear ( Gonzalez, Pacheco-Colon, Duperrouzel, & Hawes, 2017 ; Lorenzetti et al., 2020 ; Power et al., 2021 ).”
BackgroundCannabis, cannabinoids and health: a review of evidence on risks and medical benefits
Hoch, E., Volkow, N. D., Friemel, C. M., Lorenzetti, V., Freeman, T. P., & Hall, W. (2025) · European Archives of Psychiatry and Clinical Neuroscience · doi:10.1007/s00406-024-01880-2 · Cited by 139 · see citing works
“There is mixed evidence on the effects of the age of first cannabis use and duration of use on cognitive performance, except for IQ, which decreased by 2 points each year after the start of frequent/dependent use [30].”
Adolescent Substance Abuse, Transgenerational Consequences and Epigenetics
Salmanzadeh, H., Ahmadi-Soleimani, S. M., Azadi, M., Halliwell, R. F., & Azizi, H. (2021) · Current Neuropharmacology · doi:10.2174/1570159x19666210303121519 · Cited by 23 · see citing works
“Despite the public notion that cannabis consumption is low risk, there is recognition of the detrimental effects of this drug on cognition, increased risk of anxiety, depression and psychosis [18-20].”
BackgroundPrenatal cannabis exposure, the brain, and psychopathology during early adolescence
Baranger, D., Miller, A. P., Gorelik, A. J., Paul, S., Hatoum, A., Johnson, E., Colbert, S., Smyser, C. D., Rogers, C., Bijsterbosch, J., et al. (2024) · Nature Mental Health · doi:10.1038/s44220-024-00281-7 · Cited by 21 · see citing works
“Concerningly, there is now growing evidence that PCE in humans is associated with related behavioral outcomes during childhood, adolescence, and early adulthood, including increased psychopathology and reduced cognition 7–10 (see also refs.”
The Neurocognitive Effects of Cannabis Across the Lifespan
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Cannabis and Mental Health: A Review.
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“These data raise concern because human longitudinal studies have linked habitual cannabis use in adolescence to persistent deficits in executive functioning, impulse control, and cognition [2,3] which might be attributable to impaired prefrontal cortical development [4-6].”
BackgroundBaral, A., Hanna, F., Chimoriya, R., & Rana, K. (2024) · Epidemiologia · doi:10.3390/epidemiologia5010007 · Cited by 13 · see citing works
“A meta-analysis with more than 650 study subjects and 5600 controls found that cannabis use was associated with the development of cognitive decline, as measured by a decrease in IQ by 2 points compared to non-users in the United States [40].”
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“Thus, several studies demonstrated cannabis exposure being associated with lower academic performance [8, 9], intelligence quotient decline [10, 11], and lower cognitive performance in adolescents [8, 10].”
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“93 When taken in pregnancy, cannabis use increases the risk of premature birth, decreased birthweight, and cognitive de fi cits in the newborn.”
Cannabis use in youth is associated with chronic inflammation
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“Cannabis use in youth has been linked to a range of deleterious neurodevelopmental outcomes including accelerated cortical thinning, declines in verbal intelligence and the development of psychotic disorders (Albaugh et al., 2023; Power et al., 2021, 2023).”
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“Yet, a recent systematic review and meta-analysis found that cannabis exposure in youth led to a decline in IQ [77].”
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Adolescent polysubstance use: Time for a new public health approach
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“Substance use in youth is strongly linked to long-term cognitive and social consequences (Brennan et al., 2024; Power et al., 2021).”
Self-citationCognitive performance in young adults who endorse a cannabis use disorder.
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“…adults is associated with the development of psychosis [29, 30], ADHD [29–31], depression [29, 32], and cognitive decline later in adulthood [29, 33, 34] One study focusing on LGBTQA + university students found cannabis use resulted in diminished motivation and emotional regulation and elevated…”
Fleury, R., Staines, L., Smyth, B., & Cannon, M. (2026) · Psychological Medicine · doi:10.1017/s0033291726104504
“Academic decline, social disconnection, criminalization, and mental health deterioration are all part of the polysubstance use landscape (Brennan et al., 2024, 2025; Power et al., 2021).”
Self-citationSanctuary, M., Hudacheck, C. L., Jones, A., Murphy, B., Welsh, N., Klawitter, J., Hoffenberg, E., & Collins, C. B. (2024) · bioRxiv · doi:10.1101/2024.07.08.602495
“This concern is further amplified in adolescent IBD patients, where any symptomatic relief provided by cannabis use must outweigh well-established negative implications of cannabis on cognitive function within the still developing brain 5 .”
Background
- Authors
- Power, E., Sabherwal, S., Healy, C., O’ Neill, A., Cotter, D., & Cannon, M.
- Affiliations
- Royal College of Surgeons in Ireland (IE)
- Corresponding author
- Power, Emmet
- Venue
- Psychological Medicine
- Publisher
- Cambridge University Press (CUP)
- Published
- 2021-01
- Volume
- 51
- Issue
- 2
- Pages
- 194-200
- ISSN
- 0033-2917 (ISSN-L)
- 1469-8978
- PMC
- PMC7893511
- OpenAlex
- W3122807857
- Cited by
- 34
- References
- 0
- Citations by year
- 2026: 6 · 2025: 7 · 2024: 7 · 2023: 4 · 2022: 6 · 2021: 4
- FWCI
- 0.66
- Type
- journal article
- Language
- en
- Open access
- hybrid
- doi.org
- Locations
- Psychological Medicine (publishedVersion, cc-by)
- PubMed Central (publishedVersion, cc-by)
- PubMed (publishedVersion)
- Figshare (submittedVersion, cc-by)
- Funders
- Health Research Board
- H2020 European Research Council (724809)
- Topics
- Health Sciences › Medicine › Pharmacology › Cannabis and Cannabinoid Research
- Health Sciences › Medicine › Epidemiology › Substance Abuse Treatment and Outcomes
- Health Sciences › Medicine › Pediatrics, Perinatology and Child Health › Prenatal Substance Exposure Effects
- Keywords
- cannabis use, cognitive function, youth, cannabis use disorder, adolescent development, longitudinal cohort
- MeSH terms
- Marijuana Use, Adolescent, Cognition, Humans, Intelligence, Intelligence Tests, Longitudinal Studies, Cohort Studies, Young Adult
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