Both self-reported cannabis use and hair THC levels at age 20 predicted increased psychotic-like experiences, internalizing symptoms, aggression, and poorer well-being by age 24.
Young adult mental health researchers, substance use prevention specialists, and policymakers.
Cannabis use at age 20 predicted worse outcomes at 24 regardless of whether measured by self-report or hair THC
What the researchers found
In a community sample of 863 young adults, cannabis use at age 20 (measured by both self-report and hair THC) predicted increases in psychotic-like experiences, internalizing symptoms, aggression, problematic substance use, and decreased general well-being from ages 20 to 24. Cannabis users also had higher odds of not being in employment, education, or training. Effects were consistent across six different operationalizations of cannabis exposure.
Why it matters
Using objective hair analysis alongside self-report strengthens confidence that the associations are real rather than artifacts of self-report bias. The consistency across measurement methods is particularly compelling.
The numbers in context
863 young adults. Weekly-to-daily self-reported users: n=150. Hair THC detected: n=110 at age 20. Both measures predicted worse outcomes at 24 with small effect sizes. Composite scores combining self-report and hair data were not more informative than either alone.
How the study worked
Longitudinal community study with N=863 young adults assessed at ages 20 and 24. Cannabis exposure measured by self-reported frequency and hair THC/CBN concentrations via LC-MS/MS. Multiple linear and logistic regression models adjusted for sex, sociodemographics, and baseline outcomes at age 20.
What this study cannot tell us
Observational design cannot establish causation. Small effect sizes. Cannot rule out reverse causation (pre-existing mental health driving cannabis use). Hair analysis detects cannabis over months, not acute use patterns. Community sample may not capture highest-risk users.
How to read the evidence
Longitudinal community sample with objective biomarker validation of self-report. Adjusted for baseline outcomes. Consistent results across multiple operationalizations.
When this study was published
2025 publication.
The bigger picture
This study demonstrates that the negative outcomes associated with cannabis use in young adulthood are not merely reporting artifacts. Whether these effects represent causation or shared vulnerability factors remains the key unanswered question.
Questions still open
- Would these associations hold after controlling for genetic liability for psychosis?
- At what frequency does cannabis use begin to predict negative outcomes?
Read the original research
Cannabis use is associated with changes in psychological and functional well-being during young adulthood: evidence from self-reports and hair analyses.
Psychological medicine, 55, e246
Citation
Johnson-Ferguson, Lydia; Loher, Michelle; Bechtiger, Laura; Janousch, Clarissa; Baumgartner, Markus R; Binz, Tina M; Ribeaud, Denis; Eisner, Manuel; Quednow, Boris B; Shanahan, Lilly. (2025). Cannabis use is associated with changes in psychological and functional well-being during young adulthood: evidence from self-reports and hair analyses.. Psychological medicine, 55, e246. https://doi.org/10.1017/S003329172510144X
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