Does Cannabis Use Increase Risk of Schizophrenia? New Canadian Research

The link between cannabis use and the risk of schizophrenia has been controversial for years. Many cannabis advocates argue that it is a relatively safe substance, with minimal risk of serious health consequences. Opponents, on the other hand, point to research suggesting that long-term and heavy use of cannabis, especially with high THC content , may increase the risk of psychotic disorders, including schizophrenia.

In recent years, Canada has become a particularly interesting case – one of the first countries in the world to fully legalize cannabis for recreational use. Legalization was intended to limit the black market, control the quality of products, and reduce the number of cannabis-related crimes. However, it also paved the way for easier access to cannabis, increased consumption, and the emergence of products with exceptionally high THC content. Has easier access to cannabis caused a sudden increase in schizophrenia diagnoses?

In this article, we present the results of a study conducted in Ontario that examined the association between cannabis legalization and the increase in schizophrenia associated with cannabis use disorder (CUD). This study included data from 2006 to 2022, allowing for a broad time perspective and an assessment of the long-term effects of the legislative changes.

Rise in cases of schizophrenia linked to cannabis addiction

Since legalization, the percentage of Canadian adults reporting cannabis use has increased from about 14% in 2018 to more than 25% in 2022. Particularly high increases in use have been observed in the age groups 19–24 and 14–18, raising concerns about youth mental health. Cannabis use among youth is considered particularly risky due to the sensitivity of the developing brain to the effects of THC.

A study conducted in Ontario among a population of over 13.5 million people found a significant increase in the proportion of schizophrenia cases attributable to cannabis use disorder (CUD). The primary measure used in the analysis was the population attributable fraction (PARF) , which measures the percentage of schizophrenia cases that could be attributed to prior heavy cannabis use.

Increased PARF for CUD-related schizophrenia

Before legalization (2006–2015) : 3.7% of schizophrenia cases were attributable to CUD.
After legalization (2018–2022) : this value increased to 10.3%, which is almost a threefold increase.

The highest increase was recorded in the group of young men (19–24 years old):

In this group, as many as 18.9% of schizophrenia cases were associated with cannabis addiction
Males aged 14–18 also had a high PARF rate of 17.8%
By comparison, in women aged 45–65 the PARF rate was only 1.8%

Increase in NOS (unspecified) psychosis diagnoses

83.7% more cases of NOS psychosis after legalization (increase from 30 to 55.1 cases per 100,000 people).

NOS psychoses are often diagnosed when the patient’s symptoms do not meet all the criteria for schizophrenia, which may mean that some patients may develop the disorder later.

The largest increase in NOS psychosis cases occurred after 2015 , i.e. during the liberalization of medical cannabis regulations.

The overall impact of legalization on mental health

Despite the increase in PARF for CUD, the overall number of cases of schizophrenia in the population did not change significantly . Increases in younger age groups were offset by decreases in schizophrenia diagnoses in older adults (45–65 years).

This may suggest that the increase in schizophrenia cases among heavy cannabis users does not affect the overall epidemiology of the disease, but changes the profile of patients – shifting the incidence to younger age groups.

Increased incidence of unspecified psychosis following liberalization of medical cannabis

After 2015, there was a sharp increase in cases of NOS psychosis, suggesting that some patients may develop full-blown schizophrenia at a later stage.
These changes have coincided with the increased availability of marijuana on the medical and grey market.

Possible explanations:

Increased awareness and diagnosis of psychosis cases in the context of increasing cannabis consumption.

The actual increase in morbidity is due to the increase in cannabis consumption , especially that with high THC content.

Since 2020, products containing high concentrations of THC (vaporizers, extracts, oils, edibles) have appeared on the legal market. Previous studies suggest that high-THC products can double or even triple the risk of a psychotic episode .

Discussion and interpretation of results

Results from the Ontario study indicate a significant increase in the population-based etiologic fraction (PARF) for cannabis use disorder (CUD) associated with schizophrenia. There are several possible explanations for this phenomenon:

Greater availability of cannabis and increased use after liberalization

The legalization of cannabis has significantly increased its availability , which has led to an increase in the number of users, including in groups particularly susceptible to developing mental disorders. After legalization, the percentage of adult Canadians reporting cannabis use increased from about 14% in 2018 to over 25% in 2022.

The increase in use was particularly prevalent among young adults (19–24 years old) , the group at highest risk of developing schizophrenia. Increased exposure to cannabis, especially at a young age, may have influenced the development of psychosis in genetically predisposed individuals.

Increased THC content in products available on the market

With legalization came products with significantly higher THC content than those previously available on the black market. Strains exceeding 20% ​​THC are now sold in legal stores , and concentrates and oils can contain as much as 80–90% THC . Studies indicate that higher THC content is directly related to a greater risk of psychosis.

Regular users of high-THC cannabis are 2–5 times more likely to develop schizophrenia compared to users of weaker strains of cannabis. The increase in the consumption of stronger products may partially explain the increase in cases of schizophrenia among cannabis users.

Better diagnosis of CUD and schizophrenia (possible overrepresentation of cases)

The increase in cases may be due in part to improved diagnostics and greater awareness of the problem. Since the legalization of cannabis, physicians and psychiatrists have been more likely to identify patients with CUD and monitor their mental state, which may have led to an earlier diagnosis of schizophrenia.

It is possible that in the past, some cases of schizophrenia were not classified as CUD-related, but are now better identified, leading to an increase in PARF. It is also worth noting that the mere fact of a correlation does not imply causation . It is possible that people with schizophrenia are more likely to use cannabis as a form of self-medication (e.g., to reduce anxiety), which could explain the increase in CUD cases in this population.

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(Featured image by geralt via Pixabay)

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First published in Fakty Konopne. A third-party contributor translated and adapted the article from the original. In case of discrepancy, the original will prevail.

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