A meta-analysis of 14 studies found that young psychosis patients who currently use cannabis performed worse on IQ, verbal learning, working memory, and motor inhibition tests, contradicting the idea that cannabis-using patients have better cognition.
Read this if you have psychosis and use cannabis and want to understand how it might affect your thinking and memory.
Cannabis-using psychosis patients performed worse on 5 cognitive domains but better on set-shifting
What the researchers found
Previous research had suggested that psychosis patients with lifetime cannabis use appeared to have better cognitive function than non-using patients. This meta-analysis challenged that finding by focusing specifically on current cannabis users and younger patients.
Across 14 studies, young psychosis patients who were currently using cannabis performed worse than non-using patients on several cognitive measures: premorbid IQ, current IQ, verbal learning, verbal working memory, and motor inhibition. This contradicts the narrative of cannabis-using psychosis patients having superior cognition.
One exception was conceptual set-shifting, where cannabis-using patients actually outperformed non-users. This isolated advantage amidst broader deficits suggests different cognitive profiles in cannabis-using versus non-using psychosis patients.
Age was a moderating factor, with older age predicting worse performance in processing speed, sustained attention, and verbal memory among cannabis users, but better performance in verbal learning and verbal fluency.
Why it matters
Understanding the cognitive effects of cannabis in psychosis is important for treatment planning and for understanding the biology of cannabis-associated psychosis. If cannabis-using patients have worse cognition, this challenges the idea that they represent a "healthier" subgroup and suggests cannabis may actively impair cognitive function on top of psychosis-related deficits.
The numbers in context
308 studies identified, 14 met inclusion criteria. Cannabis-using patients performed worse on: premorbid IQ, current IQ, verbal learning, verbal working memory, motor inhibition. Cannabis users outperformed non-users on conceptual set-shifting. Increasing age predicted worse cognitive performance in several domains among cannabis users.
How the study worked
Systematic search of 308 studies identified 14 comparing neurocognition between current cannabis users and non-users with psychotic disorders, with mean age between 15 and 45 years. Effect sizes were extracted from cognitive test performance and analyzed using random effects modeling with moderator analyses.
What this study cannot tell us
The studies used varied cognitive assessments, making direct comparison imperfect. The age range (15-45) is broad. Current cannabis use was defined differently across studies. The meta-analysis cannot determine whether cannabis caused the cognitive deficits or whether patients with worse cognition were more likely to continue using. Sample sizes within individual studies varied.
How to read the evidence
This is a meta-analysis synthesizing 14 studies, providing strong evidence for the cognitive effects of current cannabis use in young psychosis patients.
When this study was published
Published in 2018. Research on cannabis and cognition in psychosis continues to evolve.
The bigger picture
This meta-analysis helps resolve a confusing finding in the literature. The earlier observation that cannabis-using psychosis patients had better cognition likely reflected a comparison of lifetime users (many of whom had stopped) rather than current users. When the analysis focuses on those actively using cannabis, the cognitive picture is worse, suggesting cannabis compounds psychosis-related cognitive impairment.
Questions still open
- Why do cannabis-using psychosis patients perform better on set-shifting but worse on most other cognitive tests? Does the cognitive impairment associated with cannabis use in psychosis reverse with cessation? Could the different cognitive profiles inform different treatment approaches?
Common questions
Does cannabis make psychosis patients think less clearly?
Is there anything cannabis users with psychosis are better at?
Read the original research
Meta-analysis of neurocognition in young psychosis patients with current cannabis use.
Journal of psychiatric research, 99, 22-32
Citation
Bogaty, Sophia E R; Lee, Rico S C; Hickie, Ian B; Hermens, Daniel F. (2018). Meta-analysis of neurocognition in young psychosis patients with current cannabis use.. Journal of psychiatric research, 99, 22-32. https://doi.org/10.1016/j.jpsychires.2018.01.010
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