Umbrella review of 26 reviews confirms dose-dependent link between cannabis use and psychosis | Research

The takeaway

A systematic review of 26 meta-analyses and systematic reviews confirmed that cannabis use is associated with increased psychosis risk in a dose-dependent manner, earlier psychosis onset, higher relapse rates, and more prominent positive symptoms.

Psychiatrists, public health policy makers, and researchers studying the cannabis-psychosis relationship.

26 reviews confirm dose-dependent cannabis-psychosis association

What the researchers found

Across 26 reviews, the evidence consistently showed: psychosis occurs more frequently in cannabis users than non-users; risk increases with dose; cannabis users develop psychosis earlier; and cannabis use is associated with increased relapse, more hospitalizations, and more pronounced positive symptoms in those with psychotic disorders.

Why it matters

As an umbrella review (review of reviews), this represents the highest level of evidence synthesis available, providing a comprehensive summary of the cannabis-psychosis relationship across the entire published literature.

The numbers in context

26 systematic reviews and meta-analyses included. Dose-dependent risk for psychotic illness. Earlier onset in cannabis users. Higher relapse rates and more hospitalizations in psychotic patients who use cannabis.

How the study worked

Systematic review of meta-analyses and systematic reviews from five databases (2005-2016). 26 publications were included and evaluated for methodological quality, which ranged from high to poor.

What this study cannot tell us

Methodological quality varied across included reviews. The umbrella review inherits the limitations of the underlying studies, which are predominantly observational. Publication bias may favor studies finding positive associations.

How to read the evidence

Strong: umbrella review synthesizing 26 meta-analyses and systematic reviews, the highest level of evidence synthesis.

When this study was published

Published in 2020 in European Archives of Psychiatry and Clinical Neuroscience.

The bigger picture

While the umbrella review confirms the association, the included reviews varied in quality (high to poor), and the fundamental question of whether cannabis causes psychosis or is associated through shared risk factors remains incompletely resolved.

Questions still open

Can the dose-response relationship be used to establish safe use thresholds? What proportion of the association is causal versus due to shared genetic vulnerability? Would reducing cannabis potency reduce psychosis risk at the population level?

Common questions

Does cannabis cause psychosis?

The evidence consistently shows an association that is dose-dependent: the more cannabis a person uses, the higher their risk of psychotic illness. However, proving direct causation is difficult because observational studies cannot fully rule out shared genetic or environmental risk factors.

What about people who already have psychosis?

The review found that cannabis use in people with existing psychotic disorders is associated with higher relapse rates, more hospitalizations, and more prominent positive symptoms (like hallucinations and delusions).

Read the original research

Cannabis use and psychosis: a review of reviews.

European archives of psychiatry and clinical neuroscience, 270(4), 403-412

View on PubMed ↗Publisher’s paper ↗

Citation

Hasan, Alkomiet; von Keller, Rupert; Friemel, Chris Maria; Hall, Wayne; Schneider, Miriam; Koethe, Dagmar; Leweke, F Markus; Strube, Wolfgang; Hoch, Eva. (2020). Cannabis use and psychosis: a review of reviews.. European archives of psychiatry and clinical neuroscience, 270(4), 403-412. https://doi.org/10.1007/s00406-019-01068-z