Cannabis and psychosis – new study finds no significant association

A new study finds ‘no significant association’ between cannabis use and the onset of psychosis, contrasting with previous data and longstanding claims to the contrary. 

Even as cannabis becomes more widely accepted and increasingly legalised across the globe, the link between its consumption and the risk of developing psychosis remains a controversial topic. 

While some studies have reported associations between cannabis use and psychosis, particularly when consumed by young people and adolescents, whether it plays a causal role in the onset of psychotic symptoms remains unclear. 

The authors of a new study say their findings ‘contrast with epidemiological data’ that suggests cannabis use increases the risk of developing psychotic disorder. 

Researchers examined the association between cannabis use and the incidence of psychotic disorders in people at clinical high risk of psychosis. 

Current and previous cannabis use were assessed using in over 300 individuals deemed to be at a ‘clinical high risk of psychosis’ and 67 healthy participants. 

All participants were followed up for two years from the start of the study, during which they were assessed using the Global Assessment of Functioning disability scale. Transition to psychosis and persistence of psychotic symptoms were assessed using the Comprehensive Assessment of At-Risk Mental States criteria. 

According to the findings, during the follow up 16.2% of the clinical high-risk sample developed psychosis. Of those who did not become psychotic, 51.4% had persistent symptoms and 48.6% were in remission. 

The authors say: “There was no significant association between any measure of cannabis use at baseline and either transition to psychosis, the persistence of symptoms, or functional outcome.

“These findings contrast with epidemiological data that suggest that cannabis use increases the risk of psychotic disorder.”

Previous research on cannabis and psychosis 

 Factors such as THC potency, frequency of use, and the age and genetics of the consumer may all contribute to the risk of developing psychosis. Experts generally advise caution around cannabis use for anyone who has a family history of psychosis or may be predisposed to developing symptoms.

However, despite the lack of conclusive evidence, concerns around the risks of psychosis are often used by critics as an argument against policy reform.

As more jurisdictions legalise cannabis for adult-use, new studies are helping to paint a clearer picture.

In a paper published earlier this year, researchers from the Hospital of the University of Pennsylvania in Philadelphia examined commercial and Medicare Advantage claims data from more than 63 million individuals between 2003 to 2017. 

They found no ‘statistically significant difference’ in the rates of psychosis-related diagnoses, or prescribed antipsychotics, in states with medical or recreational cannabis policies compared to those where the plant is still prohibited. 

Elsewhere, a study from 2021 looked at the relationship between adolescent cannabis use and adult-onset psychosis in a longitudinal co-twin control analysis.

They identified no dose-response relationship in models that compared the one twin with greater cannabis use to the other using less, in relation to psychosis-proneness in adulthood.

The researchers also reported that there were no potentially differing effects on the twin’s levels of cannabis exposure and risk of developing schizophrenia later in life.

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