Skip to main content

Verified by Psychology Today


New Research Highlights the Cannabis Connection to Psychosis

U.S. resembles Europe in study of cannabis-related hospital visits.

Key points

  • A 2019 European study found hospitalizations for psychosis are related to the average THC concentration in locally available cannabis.
  • A new U.S. study found hospitalizations for psychosis are higher in geographical areas with the most liberal cannabis legalization laws.
  • Quebec has responded to research confirming the connection between cannabis and psychosis by restricting THC concentration to 30%.

The relationship between cannabis and psychosis remains a topic of intense scientific investigation. For example, “Marta Di Forti surveyed 11 sites across Europe and correlated variations between … patients aged 18-64 experiencing first-episode psychosis with data on the expected concentration of THC in the different types of cannabis available at each local site.(1) Daily use of greater than 10% THC increased the odds of a psychotic episode nearly five times the rate of psychotic episode found in those who never used cannabis. Di Forti concluded that differences in frequency of daily cannabis use and in use of high potency cannabis contributed to the striking variation in the incidence of psychotic disorder across the 11 studied sites.”(2)

Similar data has recently been reported in the U.S. by Lauren Moran et al (Psychiatry Res, Jan 4, 2022). This Harvard research team divided geographical areas of the U.S. into those that had no legal use of cannabis, those that had legal medicinal cannabis, and those that had both legal medicinal and recreational cannabis. They assumed that increased legal availability of cannabis is associated with increased and/or wider use. Especially states with legalized recreational cannabis, with sales of concentrated products such as vaping cartridges with 90% THC, would offer greater opportunity for consuming larger quantities of the component in cannabis primarily responsible for producing psychotic symptoms. They compared hospitalizations for psychosis associated with cannabis among these different geographical areas and found a correlation between legal status and hospitalization for cannabis-induced psychosis. In addition, Pacific regions with the most liberal cannabis laws in the U.S. also had higher rates of hospitalization.

In truth, the rate of schizophrenia-like psychosis associated with cannabis is only a few multiples of the baseline 1% rate of schizophrenia naturally occurring in the general public (see my post "Cannabis and Psychosis: New Research"), and this is not high enough to raise much concern among most cannabis users. The vast majority have neither known anyone with cannabis-induced psychosis nor experienced any significant symptoms of psychosis. The perception of risk for schizophrenia remains low.

On the other hand, parents with a child who has fallen into serious psychosis after heavy cannabis use may view the connection between the two very differently. I vividly remember a father back in the 1980s angrily berating me for not being able to provide scientific proof for his conviction that pot had robbed him of his beloved son. But, at the time, the research was much less clear. “The anguish of parents who have watched their child descend into profoundly disabling delusions and hallucinations after using cannabis is heartrending. To them, the cause-and-effect relationship between cannabis and psychosis is clear and certain. Anyone denying that cannabis can cause schizophrenia is seen as a dangerous enemy.”(4)

When a Toronto survey of 3,720 adolescents between 13-17 demonstrated a clear association of cannabis use frequency with increased psychotic symptoms, and not vice versa(5), psychiatrists in Quebec called for restrictions on the THC concentration permitted in legal cannabis products. Today, the maximum THC content allowed in Quebec is 30%, and the maximum amount of cannabis flower that can be purchased per day is 30 grams. These restrictions counter the race-to-the-top THC concentrations pursued by some entrepreneurs. In addition to high THC vaping devices sold throughout the U.S. in states with legal cannabis, both medicinal and nonmedicinal, California has recently announced a state fair prize for the highest content cannabis variety, precisely the component of cannabis responsible for psychosis.

I repeat that cannabis can be used and enjoyed safely by most well-informed adults (see my post, "5 Signs of Using Cannabis Too Frequently"). But states have done little to inform the public of the risks to be avoided. Why have states not followed the Quebec model? What has happened to public health protections promised by advocates of legalization? My next post will explore how this promise has been broken in California.


1. M. Di Forti, et al. The contribution of Cannabis Use to Variation in the Incidence of Psychotic Disorder Across Europe (EU-GEI): A Multicenter Case-Control Study. Lancet. 2019 March 19. DOI:

2. Excerpt from T Cermak, From Bud to Brain, Cambridge University Press, 2020.

3. L Moran, et al, Geographical variation in hospitalization for psychosis associated with cannabis use and cannabis legalization in the United States, Psychiatry Res, 2022 Jan 4;308:114387.

4. Excerpt from T Cermak, Marijuana on My Mind: The Science and Mystique of Cannabis, Cambridge University Press, release date 4-20-22

5. J. Bourque, et al. Association of Cannabis Use with Adolescent Psychotic Symptoms. JAMA Psychiatry, 2018; 75(8): 864–6.1.

More from Timmen L. Cermak MD
More from Psychology Today