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.
Neuroscience researchers, substance use clinicians, and anyone comparing relative brain effects of alcohol vs. cannabis.
Cannabis showed no cortical thickness effects; alcohol showed both genetic and exposure effects
What the researchers 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.
Why it 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 numbers in context
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 the study worked
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.
What this study cannot 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.
How to read the evidence
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.
When this study was published
2021 study with MRI data from twins aged 24.
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.
Questions still open
- 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?
Common questions
Did this study find cannabis damaged the brain?
Were the alcohol-related brain differences caused by drinking or by pre-existing traits?
Read the original research
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
Citation
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
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