The link between cannabis use and severe mental disorders
- Branko Ristić
- 1 hour ago
- 4 min read
A researcher’s perspective
In recent years, cannabis has become a hot topic, from recreational legalisation debates to discussions about medical applications. But beyond its social and legislative implications, there's a growing body of evidence suggesting that cannabis use might be linked to severe mental health disorders, including psychosis, schizophrenia, bipolar disorder, and depression. Understanding these associations is crucial as cannabis consumption rises worldwide, especially among youth and vulnerable populations. This article explores the scientific evidence of how cannabis use correlates with severe mental illnesses and discusses the potential public health implications.
I am a PhD student in Psychiatry at the University of Verona, and my research focuses on risk factors associated with mental disorders, particularly psychosis. Together with my supervisor, Prof. Sarah Tosato, I recently co-authored a book chapter on the evidence for the risk of severe mental disorders in the context of cannabis use. In this piece, I present this work.
The scope of cannabis use worldwide
Cannabis remains the most widely used illicit drug globally. According to the United Nations' 2018 estimate, approximately 192 million adults, or about 4% of the adult population, had used cannabis in the previous year. Usage patterns vary significantly across regions; for instance, about 12% of people in North America reported using cannabis in the past year, with similar levels found in West and Central Africa and in Oceania. Conversely, Asia reports much lower figures at around 1.8%, though trends are shifting with increasing acceptance and legalisation in some countries.
People usually start using cannabis in their late adolescence, around 18-19 years, with the age often earlier in high-income countries. Notably, in the United States, about one in five teenagers said they had used cannabis in the past month, and similar rates were observed among Canadian adolescents. Cannabis use is especially common among young adults, which is worrying because this age group is more vulnerable to mental health problems.
Cannabis and psychosis: A clear connection
A large number of studies show that using cannabis, especially often or in very strong forms, is linked to psychosis and similar mental health problems. Research suggests that cannabis is one of the risk factors for psychosis that people can actually change. Using strong cannabis products every day, such as those high in THC (Δ-9-Tetrahydrocannabinol, the main active chemical in cannabis that affects the brain), can make the chances of developing a psychotic disorder much higher than in non-users.
Studies show a strong link between cannabis use and psychosis. People who use cannabis every day are up to four times more likely to develop psychosis than non-users, especially if they use stronger forms of the product, that is, with higher concentrations of THC. The risk increases proportionally to how much and how often someone uses it, and starting at a younger age, particularly before 15, makes the chances even higher.
Studies that follow people over time indicate that using cannabis during adolescence can double or even sextuple the risk of schizophrenia later in life. For instance, a large-scale study involving over 50,000 males found that those who smoked cannabis by age 18 had twice the risk of schizophrenia diagnosis, with chronic users having a six times higher risk. Moreover, individuals with Cannabis Use Disorder (CUD) are more likely to experience other psychiatric conditions, including anxiety and affective disorders.
Genetics can also influence how people respond to cannabis. Some individuals have specific genetic differences that make them more likely to experience psychosis when using cannabis. This shows how our genes and environment can work together - if someone is genetically predisposed, cannabis use may increase their risk of developing psychosis or other severe mental disorders.
Severe mental disorders beyond psychosis
Patients with major depressive disorder or bipolar disorder exhibit higher rates of cannabis dependence and CUD compared to the general population. For example, CUD is present in more than 20% of bipolar disorder patients, with higher severity correlating with more rapid mood cycling and poorer outcomes.
In depression, cannabis use appears to both increase the risk of developing depressive symptoms and exacerbate existing depression. Several studies show that individuals with CUD are 2 to 5 times more likely to suffer from major depressive disorder, and heavy cannabis use in adolescence can lead to earlier depression onset. However, the causal nature of this relationship remains complex, with some evidence suggesting a bidirectional link: depression may predispose individuals to cannabis use as a form of self-medication, while cannabis itself may worsen mood symptoms.
Public health implications and prevention strategies
Given the accumulating evidence, reducing cannabis consumption, particularly high-potency forms, could significantly lower the incidence of severe mental disorders. Models estimate that if daily use of cannabis and high-potency strains were eliminated, up to 20-30% of new psychosis cases might be preventable.
Public health policies should emphasise education about the mental health risks associated with cannabis, especially for youth. As cannabis becomes more accessible and socially acceptable, awareness campaigns and early intervention programs are vital. Furthermore, recognising genetic susceptibility could inform screening and prevention efforts.
Navigating a complex relationship
In conclusion, the epidemiological evidence points to a strong association between cannabis use and severe mental disorders, especially psychosis and schizophrenia. While causality is difficult to establish definitively, the consistent findings across studies underscore the potential harm of cannabis, particularly in vulnerable groups and at early ages.
As cannabis policies evolve, integrating scientific insights into public health strategies is essential. Encouraging responsible use, restricting high-potency products, and promoting education can help mitigate the mental health burden linked to cannabis. Ultimately, understanding and respecting this complex relationship will be key to safeguarding mental health in an era of changing cannabis legality and perception.