Neither cannabis smoke nor THC exposure during adolescence produced robust behavioral changes in adult rats after abstinence, suggesting that adverse effects of adolescent cannabis use in humans may be driven by non-cannabinoid factors.
Neuroscientists studying adolescent cannabis effects, harm reduction advocates, and researchers interpreting human observational studies.
No lasting behavioral effects
What the researchers found
Despite testing both cannabis smoke and THC at multiple doses during the adolescent period (P29-49 or P35-45), adult rats showed no significant effects on anxiety (open field, elevated plus maze), depression (sucrose preference, forced swim), or cognition (novel object recognition) after abstinence until P70. Some subtle sex differences were slightly attenuated.
Why it matters
This null finding challenges the assumption that adolescent cannabis exposure directly causes lasting psychological harm. If the behavioral deficits seen in human studies are not reproduced in controlled animal experiments, non-cannabinoid factors (social environment, other substance use, pre-existing vulnerabilities) may drive the human associations.
The numbers in context
Cannabis smoke: PD 29-49. THC ascending doses: PD 35-45. Adult testing: PD 70. No significant effects on open field, elevated plus maze, sucrose preference, forced swim, or novel object recognition. Some sex-dependent measures slightly attenuated.
How the study worked
Two experiments in Long-Evans rats. Cannabis smoke exposure PD 29-49 or ascending THC doses PD 35-45. Adult behavioral testing at P70 for anxiety, depression, and cognition. Both sexes tested.
What this study cannot tell us
Rat behavior may not fully model human psychiatric conditions. The abstinence period (to P70) may not be long enough. Cannabis smoke exposure methods may not replicate human consumption patterns. Only behavioral outcomes were measured, not neurochemical or structural changes.
How to read the evidence
Rated preliminary because this is an animal study, though the systematic negative finding across multiple behavioral domains is notable.
When this study was published
Published in 2019.
The bigger picture
Null findings in well-designed animal studies are important because they control for the confounds that plague human observational research. If controlled cannabinoid exposure does not produce lasting behavioral changes, the human literature may be capturing correlation rather than causation.
Questions still open
- Would longer follow-up or different behavioral tests reveal effects? Are human associations with adolescent cannabis use driven by confounds? Would higher potency exposure show different results?
Common questions
Does adolescent cannabis use cause lasting mental health problems?
Why would animal results differ from human studies?
Read the original research
Effects in rats of adolescent exposure to cannabis smoke or THC on emotional behavior and cognitive function in adulthood.
Psychopharmacology, 236(9), 2773-2784
Citation
Bruijnzeel, Adriaan W; Knight, Parker; Panunzio, Stefany; Xue, Song; Bruner, Matthew M; Wall, Shannon C; Pompilus, Marjory; Febo, Marcelo; Setlow, Barry. (2019). Effects in rats of adolescent exposure to cannabis smoke or THC on emotional behavior and cognitive function in adulthood.. Psychopharmacology, 236(9), 2773-2784. https://doi.org/10.1007/s00213-019-05255-7
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