Cannabis and Psychosis

Alasdair M. Barr, PhD, Heidi N. Boyda, BSc and Ric M. Procyshyn, PharmD, PhD

Reprinted from "Cannabis" issue of Visions Journal, 200, 5 (4), p. 8

What is cannabis and how does it affect the brain?

Cannabis is a flowering herb, originally from Asia. It has been used for thousands of years for a variety of purposes, including drug use. Cannabis is now the world’s most popular recreational drug. In BC, more people have used cannabis than anywhere else in Canada.1

There are many different chemicals in cannabis that can affect the brain. The most powerful of these is a compound called delta-9-tetrahydrocannabinol (THC). Once THC has entered the bloodstream through smoking cannabis or ingesting it (e.g., eating brownies), it can rapidly enter the brain. This is where it produces its main effects.

Like many other drug factors that are derived from plants, the THC molecule is able to interact directly with nerve cells in the brain. By chance, the THC molecule closely resembles several naturally occurring chemicals in the brain (endocannabinoids) that allow nerve cells to communicate with each other. The THC is therefore able to bind to a part of these nerve cells known as the CB1 receptor, similar to the way a key fits into a lock. These receptors are present on many different nerve cells, spread widely throughout the brain.2 THC is much more effective at binding to the CB1 receptors and affecting the nerve cells than the natural endocannabinoids are. This is why you feel “high” or “stoned” after consuming cannabis.

What are the harmful effects of cannabis?

Cannabis is generally known as a “soft” drug. This means that it is considered less harmful to the general population than “hard” drugs such as cocaine or heroin. Cannabis is less addictive than caffeine, and it doesn’t have any serious withdrawal effects compared to a drug like alcohol.

A large number of scientific studies have used techniques like magnetic resonance imaging (MRI) to study the brain in living people who have previously used large amounts of cannabis. Those studies have found very little evidence for major brain damage in people who use cannabis on a regular basis.3 For many, the major health hazards of cannabis use are those related to smoking the drug—such as lung damage—rather than the effects of THC on the brain.4

So why is there so much concern about cannabis and mental illness?

A major concern is that ongoing use of cannabis by people who have developed psychosis (i.e., a loss of contact with reality, commonly associated with hallucinations and delusions) results in a poorer future outcome. Relapses and hospitalization are more common. This is now well-established.5

At the BC Mental Health and Addictions Research Institute, we’ve recently completed a study of first-episode psychosis patients in the south region of the Fraser Health Authority. (Lead investigator is Dr. G.W. MacEwan; results are currently being prepared for submission to a scientific journal.) We examined the links between cannabis use and psychosis. Cannabis use in this group of patients was notably more common than in the general population. Importantly, we observed that the patients who used cannabis had an earlier onset of psychosis than the patients who didn’t use cannabis.

Based on this study and other recent studies, researchers have started to question whether cannabis use itself can actually cause psychosis.6

What is the link between cannabis and psychosis?

It is commonly known that smoking cannabis can produce a number of effects in the average person that resemble some of the symptoms of psychosis. These include anxiety, paranoia and delusional beliefs. Generally, these effects are fairly short-lasting, and there are no remaining problems after the drugs have worn off.

Researchers are now concerned that there may be a small proportion of the general population who are much more vulnerable to the effects of cannabis. With enough use of the drug, these people may develop long-lasting or even permanent psychosis.

There have been several major international studies that followed a large number of healthy young people, mostly in their teens, over a period of 10 or more years.7-8 These studies found that people who were heavy cannabis users were more likely to develop psychotic disorders, such as schizophrenia, as they got older than were people who didn’t use the drug. However, this may be a fairly subtle effect: a review of the literature found that using cannabis increased the risk of developing schizophrenia two-fold.9 While this may sound dramatic, remember that only 1% of the general population ever develops schizophrenia.10

What researchers have learned from these studies is that cannabis can potentially cause psychosis, but only in a select group of people who are naturally vulnerable.

What makes you vulnerable to the effects of cannabis?

We’re not sure yet what makes people vulnerable to the effects of cannabis. As with most forms of illness, there is a complex mix of genetic factors and effects of the environment you are surrounded by.

We have some pretty good ideas, though. What we know is that people who have a stronger predisposition to psychosis are at greater risk. They may have a family history of psychosis or may have had previous brief psychotic experiences. Age is an important factor: teenagers are more at risk than adults. Scientists have even identified a gene, known as catechol-O-methyltransferase, which may make a person more vulnerable to the effects of cannabis. That is, they may have an up to five-fold greater risk after cannabis use in adolescence of exhibiting psychosis and developing schizophreniform disorder (like schizophrenia, but lasting no longer than six months).11


Cannabis is a common recreational drug. The majority of people who use cannabis are unlikely to develop any lasting mental illness as a result of its use. However, continued cannabis use may result in a poorer long-term outcome in those with an existing psychotic disorder. Cannabis use may increase the chances of developing psychosis. Its use may also cause psychosis to begin at an earlier age in those who are at a greater risk for psychosis than the general population.

About the authors

Alasdair is an Assistant Professor in the Department of Anesthesiology, Pharmacology & Therapeutics at the University of British Columbia. He is also a Senior Scientist with the BC Mental Health and Addictions Research Institute.

Ric is a Clinical Associate Professor in Psychiatry and an Adjunct Professor in Pharmaceutical Sciences at UBC. He is also a senior scientist with the BC Mental Health and Addictions Research Institute.

Heidi is a doctoral graduate student in Alasdair and Ric’s research laboratory.

Their research interests include understanding the causes of mental illness and addiction, from the molecular level through to clinical trials with patients.

  1.  UVic Communications, University of Victoria. (2006, Oct 4). Cannabis use highest in BC [media release].

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  9. Henquet, C., Krabbendam, L., Spauwen, J. et al. (2005). Prospective cohort study of cannabis use, predisposition for psychosis, and psychotic symptoms in young people. BMJ, 330(7481):11.

  10. Health Canada. (2002). A report on mental illness in Canada.

  11. Caspi, A., Moffitt, T.E., Cannon, M. et al. (2005). Moderation of the effect of adolescent-onset cannabis use on adult psychosis by a functional polymorphism in the catechol-O-methyltransferase gene: Longitudinal evidence of a gene X environment interaction. Biological Psychiatry, 57(10), 1117-1127.