NEW evidence suggests that both cannabis and tobacco are associated with changes in brain volume, with a large meta-analysis identifying reduced amygdala volume among users, raising important questions about the long-term neurological effects of cannabis exposure.
Cannabis and Brain Structure Background
Cannabis is the most widely used illicit drug globally and is frequently co-used with tobacco, yet their distinct effects on brain structure remain unclear. Understanding how cannabis exposure relates to brain volume is critical for evaluating potential neurological risks and informing public health strategies.
Methods and Results on Cannabis Brain Volume
This systematic review and meta-analysis included 103 studies identified from SCOPUS, PubMed, and PsycINFO up to September 5th 2024, following PRISMA guidelines and preregistered on PROSPERO. Of these, 77 studies were included in meta-analysis with 72798 participants, including 44 cross-sectional cannabis studies with 18247 participants, 30 tobacco cross-sectional studies with 51194 participants, and four tobacco longitudinal studies with 3357 participants.
In cross-sectional analyses, cannabis use was associated with smaller amygdala volume with k = 17, g = 0.13, 95% CI 0.03 to 0.23. Tobacco use was also associated with smaller amygdala volumes (k = 5, g = 0.17, 95% CI 0.04 to 0.31), as well as reduced insula, pallidum, and total grey matter volume. Longitudinal tobacco data showed greater total grey matter volume decline (k = 5, g = 0.05, 95% CI 0.01 to 0.10).
Clinical Implications for Cannabis Use
These findings indicate that cannabis use may be associated with region specific brain volume differences, particularly affecting the amygdala, although evidence remains limited and heterogeneous. Clinicians should consider these potential associations when discussing cannabis use with patients, particularly in populations at risk of neurological or psychiatric conditions. The stronger and more consistent associations observed with tobacco highlight differing neurobiological impacts between substances. Future research will be essential to determine the clinical relevance of these structural findings and to guide evidence-based policy and patient counselling regarding cannabis and tobacco exposure.
Reference
Sawyer K et al. Associations of cannabis use, tobacco use and co-use with brain volume: a systematic review and meta-analysis. Addiction. 2026;DOI:10.1111/add.70361.
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