Cannabis and psychosis: Is there evidence of causal association?

It has been well known since historic times that cannabis may cause a variety of psychiatric symptoms. In fact, the desire to take cannabis or marijuana is primarily to obtain mental effects, and the line may be thin even in an occasional user between experiencing a pleasant and exciting psychoactive effect and a real psychotic episode. “Cannabis psychosis” is a term widely used for psychotic episodes resulting from cannabis use. These occur during or shortly after intake and may last days or weeks, but subside after discontinuation of the drug. They may require hospitalization and medication. Comprehensive summaries of mental health effects of cannabis have been published by Murray and Hall & Degenhardt.

It has often been debated whether use of cannabis can cause long-term psychotic states, and in particular schizophrenia and other chronic psychoses. Seeing patients with a combination of heavy cannabis use and schizophrenia, I was intrigued to assess the causal direction of the association. It was in the 1980s when I found out there was a survey on drug use in a national cohort of 50,000 Swedish 18-19 year old male conscripts (one year of military service was compulsory in Sweden until 2010) that we could link to data on occurrence of schizophrenia later in life. We found that those who reported use of cannabis in adolescence had a doubled risk of schizophrenia compared to those who did not use cannabis. With data on social background, psychological characteristics, and psychiatric condition assessed at conscription, we could control for such factors that might influence the association.

We have continued to follow this cohort and the men are now over 50 years old. The contribution of cannabis to new cases of schizophrenia has declined in occasional users but those who reported heavy use of cannabis in adolescence still have a twofold increased risk of schizophrenia, even at older ages. We do not know whether this is due to continued use of cannabis, or whether heavy early use could indeed have had very long lasting effects.

In recent years, several other studies have also found an association between cannabis use and later onset of chronic psychosis. A review was published in 2007 concluding that there is now “sufficient evidence to warn young people that using cannabis could increase the risk of developing a psychotic illness later in life.” The paper was accompanied by an editorial in which the prestigious journal the Lancet admitted that they had previously underestimated the risk of harmful effects of cannabis.

We recently studied the pattern of care of the patients with schizophrenia in our cohort of male conscripts, and it turns out that those patients with a history of cannabis use had double the number of total days in hospital and around double the number of hospitalizations that were twice as long in duration of those who did not have a history of cannabis use.

Thus, there is now evidence that cannabis is indeed a contributory cause of chronic psychoses, including schizophrenia. Certainly, cannabis is not the only cause of chronic psychosis. There generally needs to be other factors, such as genetic factors, personality characteristics, etc. to cause schizophrenia or other long-standing psychoses. It has been shown that the risk of psychosis in cannabis users is especially strong in psychologically vulnerable persons. Thus young people, and especially persons with mental health illness, should be warned about the risk of chronic psychotic disorders as an effect of cannabis use. Not only because of the risk of chronic psychosis, but also a number of other negative physical and mental side effects.

*Please note that the material presented here does not necessarily imply endorsement or agreement by individuals at the Centre for Addictions Research of BC


Peter Allebeck, Professor of Social Medicine, Karolinska Institute, Stockholm, Sweden



Should the minimum legal drinking age of 19 years old in British Columbia be reduced?

Alcohol use is the largest contributor to injury, disability, and death among adolescents and young adults aged 10-24 years old. As a result, Canada and other countries worldwide have implemented minimum legal drinking age (MLDA) legislation for many decades now, and drinking-age laws are considered a cornerstone of alcohol-control policies designed to reduce harms among young people. In Canada, the minimum legal drinking age is 18 years of age in Alberta, Manitoba, Québec, and 19 years of age in the rest of the country.

No other alcohol policy has generated more research or debate than minimum legal drinking age legislation. Usually the debates about the most appropriate drinking age involve competing views about how society should assign importance to the harms versus the benefits of such legislation. Some people put more emphasis on the importance of individual choice in alcohol consumption, while other people place more value on restricting alcohol consumption in some age groups so as to safeguard young adults and improve overall public health. And so, there is a healthy tension between individual rights and the greater public good underlying debates about the minimum legal drinking age, and where individuals sit on this continuum will often affect how they interpret the scientific findings.

The large majority of the scientific evidence on this topic, however, is consistent and clear: raising the MLDA is associated with reductions in alcohol-related harms (such as motor vehicle collisions, injuries, assaults, and deaths), and lowering the MLDA is associated with increases in alcohol-related harms.

Advocates for lowering the MLDA might argue that most MLDA studies rely on United States’ data from the 1970s and 1980s, and usually have fatal/nonfatal motor vehicle collisions as their most harmful outcomes. And, critics of MLDA legislation might continue, the large MLDA impacts on motor vehicle collisions observed in the 1970s and 1980s might be substantially diminished in the contemporary British Columbia context due, in large part, to advances in both traffic safety and other alcohol policies (e.g., improvements in road safety/motor vehicle safety; introduction of provincial graduated driver licensing legislation; increases in the severity of penalties for drinking and driving).

Could it be that the MLDA has lost much of its effectiveness in reducing alcohol-related harms among young people in British Columbia?

In four recent studies, my colleagues and I have demonstrated that drinking-age legislation continues to have a powerful impact on alcohol-related harms among young people in Canada. Relative to youth slightly younger than the minimum legal drinking age, young adults just older than the MLDA incur immediate and significant increases in a range of serious alcohol-related harms, including: motor vehicle collisions; inpatient/Emergency Department admissions for alcohol-use disorders, attempted suicides, injuries, assaults; and death. An example of our work was published recently in the American Journal of Public Health.

Along with the prior MLDA literature, our Canadian studies provide up-to-date and persuasive evidence that lowering the MLDA will likely result in significant increases in serious alcohol-related harms among young people in British Columbia. Proponents of a lower MLDA in BC will need to make an even more convincing argument that such substantial damages to youth in British Columbia will be outweighed by the benefits of lowering the drinking age in our province.


Dr. Russ Callaghan, PhD (; Jodi Gatley, BSc (


Jodi Gatley