Across 19 studies with 5,191 participants, cannabis users showed significantly greater formal thought disorder severity than non-users, regardless of whether they were healthy, had first-episode psychosis, or had schizophrenia.
Psychiatrists, psychosis researchers, and clinical psychologists.
Cannabis worsened thought disorder across healthy, FEP, and schizophrenia groups
What the researchers found
Cannabis users had higher FTD severity overall (SMD 0.21, 95% CI 0.12-0.29, p=0.00009). This held across subgroups: healthy individuals (SMD 0.19, p=0.02), first-episode psychosis (SMD 0.21, p=0.04), and schizophrenia (SMD 0.25, p=0.005). Between-group differences were not significant.
Why it matters
Formal thought disorder (disorganized thinking and speech) is a core psychosis feature. This meta-analysis shows cannabis worsens it across the entire spectrum, from healthy individuals to those with established schizophrenia.
The numbers in context
Studies: 19. Cannabis users: 1,840. Non-users: 3,351. Overall SMD: 0.21 (p=0.00009). Healthy: 0.19 (p=0.02). FEP: 0.21 (p=0.04). Schizophrenia: 0.25 (p=0.005).
How the study worked
Systematic review and meta-analysis searching six databases through July 2022. Included 19 studies with 1,840 cannabis users and 3,351 non-users. Random-effects model. Subgroup analyses by clinical population.
What this study cannot tell us
Mostly cross-sectional studies. Cannot determine causation. FTD measurement varied across studies. Could not assess confounding by other substance use. Moderate heterogeneity.
How to read the evidence
Comprehensive meta-analysis with consistent results across subgroups, though primarily based on cross-sectional data.
When this study was published
Published in 2022 with searches through July 2022.
The bigger picture
The consistent effect across healthy, FEP, and schizophrenia populations suggests cannabis impacts thought organization through a fundamental mechanism rather than simply worsening pre-existing psychosis.
Questions still open
- Which dimensions of FTD (poverty of speech, loose associations, tangentiality) are most affected by cannabis? Does the effect reverse with cannabis cessation?
Common questions
Does cannabis affect thinking organization?
Is the effect only in people with psychosis?
Read the original research
Association between formal thought disorder and cannabis use: a systematic review and meta-analysis.
Schizophrenia (Heidelberg, Germany), 8(1), 78
Citation
Argote, Mathilde; Sescousse, Guillaume; Brunelin, Jérôme; Fakra, Eric; Nourredine, Mikail; Rolland, Benjamin. (2022). Association between formal thought disorder and cannabis use: a systematic review and meta-analysis.. Schizophrenia (Heidelberg, Germany), 8(1), 78. https://doi.org/10.1038/s41537-022-00286-0
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