Young adult heavy cannabis users showed no hippocampal volume differences compared to non-users at baseline or after an average of 39 months, with both groups showing similar growth trajectories.
Read this if you use cannabis regularly and are concerned about its effects on your brain, particularly memory-related structures.
No hippocampal volume changes in heavy users over 39 months of follow-up
What the researchers found
This longitudinal study tracked hippocampal volumes in 20 heavy cannabis users and 23 matched controls over approximately 39 months using manual brain tracing, considered the gold standard for hippocampal measurement.
At baseline, there were no differences in hippocampal volume between heavy users and non-users. At follow-up, there were still no differences. Both groups showed hippocampal volume increases over time, following similar growth trajectories.
Neither the dose of cannabis used nor the age at which cannabis use began affected hippocampal volumes. Cannabis users in this study began smoking around age 16 and used cannabis approximately five days per week.
The authors noted that their null finding contrasts with prior evidence of hippocampal alterations in older adult cannabis users, suggesting that young adult brains may be more resilient to cannabis effects at these use levels, or that cumulative exposure has not yet reached a threshold for detectable changes.
Why it matters
The hippocampus is central to memory and is rich in cannabinoid receptors, making it a primary concern for cannabis-related brain effects. This longitudinal null finding provides some reassurance for young adult heavy users, though the small sample and age range limit how broadly the finding can be applied.
The numbers in context
20 heavy cannabis users, 23 controls. Mean age 21 (18-24). Follow-up after average 39 months (SD 2.4). Cannabis use: ~5 days/week, onset ~age 16. No significant volume differences at any time point.
How the study worked
Longitudinal neuroimaging study with 20 heavy cannabis users (mean age 21, range 18-24) and 23 matched controls. MRI scans at baseline and follow-up (average 39 months apart). Hippocampal volumes measured using manual tracing on T1-weighted anatomical scans. Cannabis users averaged 5 days/week of use with onset around age 16.
What this study cannot tell us
Small sample size limits statistical power to detect subtle effects. Manual tracing, while considered gold standard, may miss subfield-specific changes. The age range (18-24) captures only early adulthood. Cannabis users averaged 5 days/week, which, while heavy, may not represent the heaviest use patterns.
How to read the evidence
Longitudinal design with gold-standard measurement method. Moderate because the sample size is small despite the strong methodology.
When this study was published
Published in 2017.
The bigger picture
This study contributes to an ongoing debate about whether cannabis changes brain structure. The null finding in young adults, combined with positive findings in older adults from other studies, raises the possibility that cumulative exposure or aging-related factors may be necessary for cannabis to produce detectable hippocampal changes.
Questions still open
- At what age or cumulative exposure level might hippocampal changes become detectable? Would more sensitive measures (hippocampal subfields, functional connectivity) reveal effects that volumetric measures miss? Do these young adults develop hippocampal changes later in life if heavy use continues?
Common questions
Does heavy cannabis use shrink the hippocampus?
Is the brain safe from cannabis at young ages?
Read the original research
Longitudinal study of hippocampal volumes in heavy cannabis users.
Journal of psychopharmacology (Oxford, England), 31(8), 1027-1034
Citation
Koenders, L; Lorenzetti, V; de Haan, L; Suo, C; Vingerhoets, Wam; van den Brink, W; Wiers, R W; Meijer, C J; Machielsen, Mwj; Goudriaan, A E; Veltman, D J; Yücel, M; Cousijn, J. (2017). Longitudinal study of hippocampal volumes in heavy cannabis users.. Journal of psychopharmacology (Oxford, England), 31(8), 1027-1034. https://doi.org/10.1177/0269881117718380
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