Young adult cannabis users showed decreased cortical thickness in multiple frontal brain regions rich in cannabinoid receptors, while earlier onset of use in those with ADHD was associated with a different pattern of cortical changes.
Read this if you have ADHD and use cannabis, or are interested in how cannabis affects brain development in young adults.
Cannabis users had thinner cortex in 5 frontal regions rich in cannabinoid receptors
What the researchers found
Researchers compared brain structure in young adults with and without childhood ADHD who did or did not use cannabis regularly. After controlling for age, gender, brain size, nicotine, and binge drinking, cannabis use was associated with decreased cortical thickness in several brain regions.
Specifically, cannabis users (regardless of ADHD status) had thinner cortex in the right superior frontal sulcus, anterior cingulate, isthmus of the cingulate gyrus, and left superior frontal and precentral regions. These areas are rich in cannabinoid receptors and involved in cognitive control.
Interestingly, ADHD diagnosis alone did not predict brain structure, but persistent ADHD (symptoms continuing into adulthood) was associated with thinner cortex in regions controlling movement and inhibition. For cannabis users with ADHD specifically, earlier onset of regular cannabis use predicted a different pattern of cortical changes.
Why it matters
ADHD is a major risk factor for cannabis use, and many young people with ADHD use cannabis. This study shows that cannabis affects brain structure in regions important for the very cognitive functions (attention, inhibition) that are already compromised in ADHD, suggesting a potential compounding effect.
The numbers in context
Four groups: ADHD+cannabis (n=37), ADHD-only (n=44), cannabis-only (n=18), controls (n=21). Cannabis users: decreased cortical thickness in 5 frontal/cingulate regions. Persistent ADHD: decreased left precentral/postcentral thickness. All effects controlled for nicotine and binge drinking.
How the study worked
Cross-sectional MRI study comparing four groups: ADHD with cannabis use (n=37), ADHD without use (n=44), non-ADHD cannabis users (n=18), and non-ADHD non-users (n=21). Multiple regression and MANCOVA controlled for age, gender, total brain volume, nicotine, and binge drinking.
What this study cannot tell us
Cross-sectional design cannot determine causality. Groups were unequal in size. Cannabis use was self-reported. Could not control for all potential confounders (e.g., socioeconomic status, other substance use). Earlier cannabis onset in ADHD may reflect ADHD severity rather than a cannabis-specific effect.
How to read the evidence
MRI study with multiple groups and extensive confound control, but cross-sectional design and modest sample sizes limit causal conclusions.
When this study was published
Published in 2016. Neuroimaging research on cannabis and ADHD continues to evolve with larger longitudinal studies.
The bigger picture
The convergence of cannabis effects and ADHD vulnerability in frontal brain regions suggests that cannabis use in ADHD populations may be particularly concerning for cognitive development. However, longitudinal studies are needed to determine if cannabis causes thinning or if thinner cortex predisposes to cannabis use.
Questions still open
- Does cortical thinning from cannabis use worsen ADHD symptoms? Would the brain changes reverse with abstinence? Is the interaction between ADHD and cannabis onset age clinically significant for treatment decisions?
Common questions
Does cannabis thin the brain?
Is cannabis worse for people with ADHD?
Read the original research
The impact of ADHD persistence, recent cannabis use, and age of regular cannabis use onset on subcortical volume and cortical thickness in young adults.
Drug and alcohol dependence, 161, 135-46
Citation
Lisdahl, Krista M; Tamm, Leanne; Epstein, Jeffery N; Jernigan, Terry; Molina, Brooke S G; Hinshaw, Stephen P; Swanson, James M; Newman, Erik; Kelly, Clare; Bjork, James M. (2016). The impact of ADHD persistence, recent cannabis use, and age of regular cannabis use onset on subcortical volume and cortical thickness in young adults.. Drug and alcohol dependence, 161, 135-46. https://doi.org/10.1016/j.drugalcdep.2016.01.032
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