Heavy Cannabis Use Did Not Shrink the Hippocampus in Young Adults Over 3 Years
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.
Quick Facts
What This Study 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.
Key Numbers
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 They Did This
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.
Why This Research 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 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.
What This Study Doesn't 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.
Questions This Raises
- ?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?
Trust & Context
- Key Stat:
- No hippocampal volume changes in heavy users over 39 months of follow-up
- Evidence Grade:
- Longitudinal design with gold-standard measurement method. Moderate because the sample size is small despite the strong methodology.
- Study Age:
- Published in 2017.
- Original Title:
- Longitudinal study of hippocampal volumes in heavy cannabis users.
- Published In:
- Journal of psychopharmacology (Oxford, England), 31(8), 1027-1034 (2017)
- Authors:
- Koenders, L(2), Lorenzetti, V, de Haan, L(5), 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
- Database ID:
- RTHC-01422
Evidence Hierarchy
Follows a group of people over time to track how outcomes develop.
What do these levels mean? →Frequently Asked Questions
Does heavy cannabis use shrink the hippocampus?
Not in this study of young adults. Heavy cannabis users (about 5 days/week, starting around age 16) showed no hippocampal volume differences compared to non-users over 39 months. However, studies of older adults have found changes, suggesting cumulative exposure may matter.
Is the brain safe from cannabis at young ages?
This study found no structural hippocampal changes in young adults aged 18-24 despite heavy use. However, the hippocampus is just one brain structure, and the small sample size and short follow-up period mean other effects cannot be ruled out.
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Cite This Study
https://rethinkthc.com/research/RTHC-01422APA
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
MLA
Koenders, L, et al. "Longitudinal study of hippocampal volumes in heavy cannabis users.." Journal of psychopharmacology (Oxford, 2017. https://doi.org/10.1177/0269881117718380
RethinkTHC
RethinkTHC Research Database. "Longitudinal study of hippocampal volumes in heavy cannabis ..." RTHC-01422. Retrieved from https://rethinkthc.com/research/koenders-2017-longitudinal-study-of-hippocampal
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.