Alcohol, but not cannabis, was linked to thinner brain cortex in young adults
In a twin study of 436 young adults, alcohol misuse was associated with reduced cortical thickness in brain control networks, while cannabis use showed no such association.
Quick Facts
What This Study Found
Greater alcohol misuse was linked to thinner cortex in prefrontal, temporal, insula, and parietal regions, predominantly right-lateralized. Cannabis use showed no association with cortical thickness. Co-twin analyses revealed that alcohol effects reflected both genetic predisposition to misuse AND direct alcohol exposure effects, particularly in lateral prefrontal and frontal/parietal medial areas.
Key Numbers
436 twins aged 24. Alcohol misuse linked to reduced thickness in prefrontal, temporal, insula, precuneus, and parietal regions. Effects were predominantly right-lateralized. Cannabis showed no associations with cortical thickness in any region.
How They Did This
Co-twin control study of 436 population-based twins aged 24. Dimensional alcohol and cannabis use measures across emerging adulthood were compared with MRI-assessed cortical thickness in cognitive control and salience network regions. The twin design allowed separation of substance exposure effects from shared genetic/environmental factors.
Why This Research Matters
This is one of the few studies that can distinguish between brain differences that existed before substance use (genetic predisposition) and those caused by use itself. The finding that cannabis showed no cortical thickness effects, while alcohol showed both predispositional and exposure effects, has implications for how we rank relative brain risks.
The Bigger Picture
The co-twin design is powerful because it controls for genetics and shared environment. Finding that some alcohol-brain associations reflect pre-existing vulnerability rather than pure damage effects complicates the "alcohol kills brain cells" narrative while also confirming that alcohol does cause some structural changes independently.
What This Study Doesn't Tell Us
Cross-sectional MRI at age 24 with retrospective substance use history. Cannot capture earlier brain changes that may have resolved. Cannabis use levels in this sample may not have been heavy enough to detect effects.
Questions This Raises
- ?Would heavier cannabis use show cortical thickness effects?
- ?Do the alcohol-related brain changes reverse with abstinence?
- ?How do these structural differences relate to functional cognitive outcomes?
Trust & Context
- Key Stat:
- Cannabis showed no cortical thickness effects; alcohol showed both genetic and exposure effects
- Evidence Grade:
- Strong genetically informative design using population-based twin sample. The co-twin control method is among the strongest quasi-experimental approaches for separating causal effects.
- Study Age:
- 2021 study with MRI data from twins aged 24.
- Original Title:
- The Effects of Alcohol and Cannabis Use on the Cortical Thickness of Cognitive Control and Salience Brain Networks in Emerging Adulthood: A Co-twin Control Study.
- Published In:
- Biological psychiatry, 89(10), 1012-1022 (2021)
- Authors:
- Harper, Jeremy, Malone, Stephen M(2), Wilson, Sylia(2), Hunt, Ruskin H, Thomas, Kathleen M, Iacono, William G
- Database ID:
- RTHC-03191
Evidence Hierarchy
Compares people with a condition to similar people without it.
What do these levels mean? →Frequently Asked Questions
Did this study find cannabis damaged the brain?
No. Cannabis use was not associated with cortical thickness changes in any brain region examined. Only alcohol misuse showed associations with thinner cortex.
Were the alcohol-related brain differences caused by drinking or by pre-existing traits?
Both. The twin design revealed that some brain differences reflected genetic predisposition to alcohol misuse, while others (particularly in prefrontal and medial cortex) appeared to be direct effects of alcohol exposure.
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Cite This Study
https://rethinkthc.com/research/RTHC-03191APA
Harper, Jeremy; Malone, Stephen M; Wilson, Sylia; Hunt, Ruskin H; Thomas, Kathleen M; Iacono, William G. (2021). The Effects of Alcohol and Cannabis Use on the Cortical Thickness of Cognitive Control and Salience Brain Networks in Emerging Adulthood: A Co-twin Control Study.. Biological psychiatry, 89(10), 1012-1022. https://doi.org/10.1016/j.biopsych.2021.01.006
MLA
Harper, Jeremy, et al. "The Effects of Alcohol and Cannabis Use on the Cortical Thickness of Cognitive Control and Salience Brain Networks in Emerging Adulthood: A Co-twin Control Study.." Biological psychiatry, 2021. https://doi.org/10.1016/j.biopsych.2021.01.006
RethinkTHC
RethinkTHC Research Database. "The Effects of Alcohol and Cannabis Use on the Cortical Thic..." RTHC-03191. Retrieved from https://rethinkthc.com/research/harper-2021-the-effects-of-alcohol
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.