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The Role of Cannabis in Psychosis and Schizophrenia

Learn About Cannabis 5 Health Risks 5 The Role of Cannabis in Psychosis and Schizophrenia

The idea that cannabis can make people insane has been around for a long time. An article in the 1874 Chicago Tribune reported that “a man who had smoked a marihuana cigarette attacked and killed a policeman and badly wounded three others; six policemen were needed to disarm him and march him to the police station where he had to be put into a straitjacket.” It also stated that, “people who smoke marihuana finally lose their mind and never recover it, but their brains dry up and they die, most of times suddenly.”

These wildly exaggerated accounts were further reinforced by the 1936 Refer Madness movie, where high school students do crimes and descend into “insanity” because of their cannabis use. The term “Refer Madness” came to represent an exaggerated and stereotypical portrayal of cannabis use consequences. The reality is that most people who consume cannabis do not experience major health or behavior consequences from their use.  

Unfortunately, the legacy of Refer Madness has become harmful to society’s wellbeing.  As scientific research advances the understanding of the strong connection between cannabis use and the development of psychotic disorders, people tend to dismiss findings as a modern take on Refer Madness propaganda.

But the reality is that there are clear reasons for concern. This article presents what we know about the connection of cannabis use and psychotic disorders, such as schizophrenia. Our hope is that this information will help people better understand the role cannabis plays in the development of psychosis and schizophrenia. We also hope this information will help educate those who are most at risk.

DEFINITIONS

Psychotic disorders are severe mental disorders that cause abnormal thinking and perceptions. People with psychoses lose touch with reality. Two of the main symptoms are delusions and hallucinations.

Substance-induced psychoses are brief episodes of delusions or hallucinations triggered by intoxication or withdrawal, and they are a common cause of first-episode psychosis. A substance-induced psychosis is a psychotic state occurring during intoxication or withdrawal and lasting at least 48 hours.

As cannabis use increases in the U.S., so does the incidence of psychotic disorders.

A U.S. study compared a representative sample of 43,093 people in 2001-2002 with an equally representative sample of 36,309 people in 2012-2013. They found that self-reported psychosis in the last year more than doubled in these ten years, jumping from 0.33% in 2001-2002 to 0.80% in 2012-2013.5  This increase coincides with the increase of past-year marijuana use in the U.S: 4.1% in 2001–2002 and 9.5% in 2012–2013.9

People who used cannabis were more likely to suffer from psychotic disorders in both samples. In 2001-2002, the amount of cannabis use was not related to psychosis. However, in 2012-2013, psychotic disorders were 2.5 times more common among those with frequent or daily nonmedical cannabis use when compared to non-users.

People who use cannabis with high THC concentration daily are 5 times more likely to develop a psychotic disorder than people who do not use cannabis.
A study in 11 cities around the world examined 900 patients suffering from a psychotic disorder such as schizophrenia.1 It compared them with 1200 randomly chosen people with the same characteristics (age, gender, education) who had never experienced any psychotic symptoms. This is known as case-control study. It is a highly effective way to determine whether a particular behavior comes before a disease.

The study divided cannabis use into two categories: low potency (THC levels less than10%) and high potency (THC levels greater than10%). Figure 1 shows the study results: Daily users of any type of cannabis were 2.2 times more at risk of having a psychotic disorder compared with people who never used cannabis. Those who used high potency cannabis daily were about five-times more likely to have a psychotic disorder than those who never used. This finding was true for people living in any of the 11 cities studied.1

People who experience a cannabis-induced psychosis incident are at very high risk of developing schizophrenia years later.
A study in Ontario, Canada2 estimated the risk of developing schizophrenia following an Emergency Department (ED) visit for psychosis induced by any substance use. Data from medical records of all individuals between ages 14-65 who visited an ED in Ontario between 2008 and 2022 (13,784 people total) with substance-induced psychosis were examined according to the drug that caused their symptoms: alcohol, cocaine, opioids, amphetamines, polysubstance use, and cannabis.

Five years later, the medical records of these same people were searched for diagnosis of schizophrenia: 28.4% of people who experienced cannabis induced psychosis had developed schizophrenia, the highest of any other substance (see Figure 2). The results showed an even higher rate for young males (ages 14 to 24) – 40% of them had a diagnosis of schizophrenia 3 years later.2

A similar study was conducted in Denmark.3 The study followed people who experienced substance-induced psychosis for 20 years to find out who later developed schizophrenia or bipolar disorder.3 The study group included 6788 persons who received a diagnosis of substance-induced psychosis between 1994 and 2014 and did not have a previous record of schizophrenia treatment.  They were compared to a random sample in the general population, matched by age and gender.

41% of people who had an incident of cannabis induced psychosis were diagnosed with schizophrenia (see figure 3), much higher than people who experienced psychosis induced by any other substance. Half of the conversion cases happened within 3 years after a cannabis-induced psychosis. Compared to the general population, those who experienced cannabis-induced psychosis were 433 times more likely to develop schizophrenia within the following 20 years. Young males were at a much higher risk.3

However, people who quit using cannabis after experiencing a first episode psychosis incident decreased their chance of developing schizophrenia later.

A 2025 study11 wanted to find out if people who quit using cannabis after experiencing a psychotic episode for the first time would be less likely to be later diagnosed with a psychotic disorder, like schizophrenia, than those who continued to use cannabis. They compared 875 people who had experienced psychosis with a group who had not and found that the chance of being diagnosed with a psychotic disorder became lower the more time that passed since using cannabis. People who had stopped using 1-4 weeks ago, were 7 times as likely to be diagnosed with a psychotic disorder than people who never used cannabis, but after 37 week of no use there was no difference between the two groups.

People who used high potency cannabis daily were most likely to develop a psychotic disorder, and it took longer for their risk to go down than for people who used cannabis less often or used less potent forms of cannabis.

The earlier people start using high THC cannabis daily, the earlier their psychotic symptoms begin.

A study was conducted in a United Kingdom psychiatric hospital with 410 people already diagnosed with psychotic disorders to find out if those who started using cannabis early in life also had an earlier start of their psychosis compared to those not using cannabis. The study found that those who started to use cannabis before the age of 15 and consumed “skunk” (a stronger cannabis common in Europe) daily had their first episode of psychosis 6 years earlier than non-cannabis users.4

People with a psychotic disorder who use cannabis have higher rates of hospitalization and poorer prognosis than people who have a psychotic disorder but do not use cannabis.

In a review of 24 studies, researchers found that continued cannabis users had a greater increase in relapse of psychosis than did both non-users and discontinued users, as well as longer hospital admissions than non-users. This was independent of the stage of illness.8

Studies also documented that ongoing cannabis use among those with psychosis is also associated with lower psychosocial functioning and recovery, less regular use of antipsychotic medications, reduced effectiveness of anti-psychotic medications, and increased rates of mental health treatment drop out.9

When people who developed psychosis quit cannabis, their psychotic symptoms get better, and they have better functioning.
This study followed adults with psychosis for 8 years following entry into an early psychosis treatment program. Researchers compared three groups that differed in regard to cannabis use. The green line in the graph represents people who used cannabis prior to treatment but stopped their cannabis use when they entered the program. The black dotted line represents people who never used cannabis. The red dotted line represents people who continued to use cannabis after starting treatment for psychosis. The scores on the left side of the graph represent overall functioning. The higher the score, the better the person’s functioning.6

As can be seen in the graph, people who either stopped using upon entering treatment or never used cannabis showed better functioning over time than those who continued to use (i.e. less episodes of delusions and hallucinations).

We continue to learn more about how cannabis affects mental health. However, it has become increasingly clear that cannabis use contributes to increasing rates of psychotic disorders, earlier symptoms, and poorer treatment outcomes.

References

  1. Di Forti M, Quattrone D, Freeman TP, et al. The contribution of cannabis use to variation in the incidence of psychotic disorder across Europe (EU-GEI): a multicentre case-control study. Lancet Psychiatry. 2019;6(5):427-436. doi:10.1016/S2215-0366(19)30048-3
  2. Myran DT, Harrison LD, Pugliese M, et al. Transition to Schizophrenia Spectrum Disorder Following Emergency Department Visits Due to Substance Use With and Without Psychosis. JAMA Psychiatry. 2023;80(11):1169-1174. doi:10.1001/jamapsychiatry.2023.3582
  3. Starzer, MSK, Nordentoft, M, & Hjorthøj, C (2018). Rates and Predictors of Conversion to Schizophrenia or Bipolar Disorder Following Substance-Induced Psychosis. The American journal of psychiatry, 175(4), 343–350. https://doi.org/10.1176/appi.ajp.2017.17020223
  4. Di Forti M, Sallis H, Allegri F, et al. Daily use, especially of high-potency cannabis, drives the earlier onset of psychosis in cannabis users. Schizophr Bull. 2014;40(6):1509-1517. doi:10.1093/schbul/sbt181
  5. Livne O, Shmulewitz D, Sarvet AL, Wall MM, Hasin DS. Association of Cannabis Use-Related Predictor Variables and Self-Reported Psychotic Disorders: U.S. Adults, 2001-2002 and 2012-2013. Am J Psychiatry. 2022;179(1):36-45. doi:10.1176/appi.ajp.2021.21010073
  6. González-Pinto, A, Alberich, S., Barbeito, S, Gutierrez, M, Vega, P, Ibáñez, B, Haidar, MK, Vieta, E, Arango, C. 2011. Cannabis and First-Episode Psychosis: Different Long-term Outcomes Depending on Continued or Discontinued Use, Schizophrenia Bulletin, 37(3): 631–639, https://doi.org/10.1093/schbul/sbp126
  7.  Gafoor R. (2022). Is Cannabis Abstinence Related to Subsequent Reduced Risk of Psychosis? a Nested Retrospective Case Control Hierarchical Survival Analysis. BJPsych Open, 8(Suppl 1), S51. https://doi.org/10.1192/bjo.2022.192
  8. Schoeler T, Monk A, Sami MB, et al. Continued versus discontinued cannabis use in patients with psychosis: a systematic review and meta-analysis. Lancet Psychiatry. 2016;3(3):215-225. doi:10.1016/S2215-0366(15)00363-6
  9. Marino, L., Scodes, J., Richkin, T., Alves-Bradford, J. M., Nossel, I., Wall, M., & Dixon, L. (2020). Persistent cannabis use among young adults with early psychosis receiving coordinated specialty care in the United States. Schizophrenia research, 222, 274–282. https://doi.org/10.1016/j.schres.2020.05.035
  10. Hasin, D. S., Saha, T. D., Kerridge, B. T., Goldstein, R. B., Chou, S. P., Zhang, H., Jung, J., Pickering, R. P., Ruan, W. J., Smith, S. M., Huang, B., & Grant, B. F. (2015). Prevalence of Marijuana Use Disorders in the United States Between 2001-2002 and 2012-2013. JAMA psychiatry, 72(12), 1235–1242. https://doi.org/10.1001/jamapsychiatry.2015.1858
  11. Bond, B.W., Duric, B., Spinazzola, E., Trotta, G., Chesney, E. et al. (2025) Cannabis use cessation and the risk of psychotic disorders: A case-control analysis from the First-Episode Case-Control EU-GEI WP2 Study. The Canadian Journal of Psychiatry, 70(3):182-193. Doi: https://doi.org/10.1177/07067437241290187

Last updated August 2024.