Cannabis-using first-episode schizophrenia patients developed psychosis earlier but had better cognitive function at onset (due to higher premorbid IQ), and three-quarters quit cannabis spontaneously over time.
Read this if you want to understand the relationship between cannabis use, psychosis onset, and recovery.
75% of cannabis-using schizophrenia patients quit spontaneously
What the researchers found
Ninety-nine first-episode schizophrenia patients were divided into lifetime cannabis users and never-users. Cannabis users developed psychosis at a younger age, with a strong linear relationship between age of first cannabis use and age of psychosis onset, including prodromal symptoms.
Paradoxically, cannabis users showed better cognitive function at psychosis onset, but this was explained by their higher premorbid IQ, not by a protective effect of cannabis. Social functioning was also better in users.
Remarkably, three-quarters of cannabis users quit spontaneously over the follow-up period. Those who started cannabis later in life quit sooner, and earlier cessation was linked to fewer positive psychotic symptoms and fewer hospital days.
Why it matters
This study resolved an apparent paradox: cannabis users had earlier psychosis (bad prognosis indicator) but better cognition (good indicator). The explanation was that cannabis brought forward psychosis in people who were otherwise cognitively healthier, meaning early onset was caused by cannabis rather than reflecting intrinsically worse illness.
The numbers in context
99 patients, 15-month follow-up. Strong linear relationship between age of first cannabis use and psychosis onset. 75% of cannabis users quit spontaneously. Later cannabis initiation predicted earlier cessation and fewer symptoms.
How the study worked
Prospective cohort study of 99 first-episode schizophrenia patients without other substance abuse. Compared lifetime cannabis users to never-users on premorbid function, cognition, clinical outcomes, and cannabis use trajectory over 15 months.
What this study cannot tell us
Excluded patients with other substance abuse, limiting generalizability. Cannabis use was self-reported. The linear relationship between cannabis onset and psychosis onset does not prove causation (though the dose-response pattern strengthens the case). 15-month follow-up is relatively short.
How to read the evidence
Prospective cohort with careful subgroup analysis. Excludes other substance abuse for cleaner cannabis signal. 15-month follow-up is moderate.
When this study was published
Published in 2012. The relationship between cannabis and psychosis onset timing has been confirmed in larger studies since.
The bigger picture
The finding that most cannabis users with schizophrenia quit on their own challenges the perception that cannabis use is intractable in psychotic patients. The relationship between cannabis cessation and better outcomes provides motivation for treatment programs.
Questions still open
- Would active cannabis cessation intervention improve outcomes further than spontaneous quitting? Is there a critical cannabis exposure threshold that triggers psychosis? Can premorbid IQ be used to identify cannabis users at highest psychosis risk?
Common questions
Does cannabis cause earlier psychosis?
Do people with schizophrenia keep using cannabis?
Read the original research
The effect of cannabis use and cognitive reserve on age at onset and psychosis outcomes in first-episode schizophrenia.
Schizophrenia bulletin, 38(4), 873-80
Citation
Leeson, Verity C; Harrison, Isobel; Ron, Maria A; Barnes, Thomas R E; Joyce, Eileen M. (2012). The effect of cannabis use and cognitive reserve on age at onset and psychosis outcomes in first-episode schizophrenia.. Schizophrenia bulletin, 38(4), 873-80. https://doi.org/10.1093/schbul/sbq153
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