Study: Cannabis legalisation not linked to increased psychosis rates in short-term

The legalisation of adult-use cannabis in Canada is not linked to increased rates of psychosis in the short-term, new research shows, but longer studies are needed. 

A new study has found that following the legalisation of adult-use cannabis in Canada, there was no short-term increase in health service use or frequency of psychotic disorders. 

Researchers examined changes in health service use and incident cases of psychotic disorder following the introduction of the legislation, from January 2014 to March 2020. During this time, there were tight restrictions on retail stores and available product types.

The team identified psychosis-related outpatient visits, emergency department visits, hospitalisations, and inpatient length of stay, as well as incident cases of psychotic disorders, among people aged 14 to 60 years.

They ‘did not find evidence’ of an increase in health service use or incident cases of psychotic disorders over a short-term period (17 months). 

However, they did identify ‘clear increasing trends’ in health service use and incident cases of substance-induced psychotic disorders over the entire study period.

This has led to calls for more longer-term studies to be carried out, including those looking at the effects of the expansion of the commercial cannabis market.

The authors conclude: “Our findings suggest that the initial period of tight market restriction following legalization of non-medical cannabis was not associated with an increase in health service use or frequency of psychotic disorders. A longer post-legalization observation period, which includes expansion of the commercial cannabis market, is needed to fully understand the population-level impacts of non-medical cannabis legalization; thus, it would be premature to conclude that the legalization of non-medical cannabis did not lead to increases in health service use and incident cases of psychotic disorder.”

READ MORE: Getting the balance right: a conversation about cannabis and psychosis

Previous findings on cannabis policy and psychosis

There is much debate about the relationship between cannabis and psychosis, with some concern that the introduction of more liberal legislation would lead to an increase in psychotic disorders among the population. 

It is not yet clear whether this is the case, with some early research appearing to refute this.

In one study scientists 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 and 2017. 

Researchers 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 its use is still prohibited. 

A small number of previous studies have reported increased rates of psychosis in association with local cannabis policies, although most have focused on either medical or recreational policies in isolation.