A prospective neuroimaging study found that brain activation differences and poorer executive function existed at age 12 - before any cannabis use - in teens who would go on to initiate cannabis by age 15, supporting a risk model over a cannabis-caused damage model.
Neuroscience researchers, adolescent health professionals, cannabis policy makers.
Brain activation differences and poorer executive function were already present at age 12 in teens who would initiate cannabis by 15 - before they ever used.
What the researchers found
At age 12, before any cannabis use, teens who would initiate cannabis by 15 already showed activation differences in frontoparietal (increased) and visual association (decreased) regions, and poorer executive planning scores. At follow-up, a cannabis dose-response relationship was found in cuneus activation, but this was not associated with cognitive performance. Limited support for a direct cannabis exposure effect.
Why it matters
This is one of the strongest designs for disentangling cause and effect in cannabis neuroscience. By measuring brains before anyone used cannabis, researchers could show that many "cannabis effects" actually preceded cannabis use, fundamentally challenging the assumption that cannabis causes these brain differences.
The numbers in context
85 participants, 22 initiated cannabis by age 15. Baseline (age 12) differences found in frontoparietal and visual association activation and Stockings of Cambridge executive planning scores before any cannabis use.
How the study worked
85 participants completed working memory fMRI and cognitive assessments at age 12 (before cannabis use) and age 15. By follow-up, 22 had initiated cannabis. Baseline neurocognitive measures were compared between future initiators and non-initiators.
What this study cannot tell us
Relatively small cannabis-initiating group (n=22). Only 3 years between baseline and follow-up. More prolonged exposure may be needed to observe true cannabis effects. Cannot fully rule out other substance use effects.
How to read the evidence
Strong - prospective design with pre-cannabis baseline measurements is the gold standard for this research question.
When this study was published
Published in 2018.
The bigger picture
Many retrospective studies have attributed brain differences in cannabis users to drug effects. This prospective study shows the same differences exist before use begins, suggesting they reflect pre-existing vulnerability to substance use rather than drug-induced damage. This has major implications for how we interpret cannabis brain research.
Questions still open
- Do the pre-existing brain differences represent a general substance use vulnerability or something specific to cannabis? Would longer follow-up reveal true cannabis exposure effects? Could these baseline measures be used to identify at-risk youth before they begin using?
Common questions
Does cannabis cause brain changes in teens?
Are brain differences in cannabis users caused by cannabis?
Read the original research
Early Cannabis Use and Neurocognitive Risk: A Prospective Functional Neuroimaging Study.
Biological psychiatry. Cognitive neuroscience and neuroimaging, 3(8), 713-725
Citation
Tervo-Clemmens, Brenden; Simmonds, Daniel; Calabro, Finnegan J; Montez, David F; Lekht, Julia A; Day, Nancy L; Richardson, Gale A; Luna, Beatriz. (2018). Early Cannabis Use and Neurocognitive Risk: A Prospective Functional Neuroimaging Study.. Biological psychiatry. Cognitive neuroscience and neuroimaging, 3(8), 713-725. https://doi.org/10.1016/j.bpsc.2018.05.004
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