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April 8, 2020

Will Legalization and Commercialization of Cannabis Use Increase the Incidence and Prevalence of Psychosis?

Author Affiliations
  • 1Institute of Psychiatry, Psychology, and Neuroscience, Department of Psychosis Studies, King’s College, London, United Kingdom
  • 2Centre for Youth Substance Abuse Research, The University of Queensland, Brisbane, Australia
JAMA Psychiatry. 2020;77(8):777-778. doi:10.1001/jamapsychiatry.2020.0339

There is a worldwide trend toward liberalizing cannabis policy and commercializing its sale. Uruguay legalized recreational cannabis in 2013, as did Canada in 2018, as well as 10 US states.1 Other countries have decriminalized the drug. In Holland, cannabis can be bought in designated cafes, and in Portugal, the police refer those who regularly use cannabis for counseling. Psychiatrists have played a prominent role in the debate over the health consequences of legalization in many countries, especially in the UK, but the public debate in the US has been notable for the absence of input from psychiatrists.2

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4 Comments for this article
Public Health Issue
Catherine Ratliff, JD, NCC | Disability Lawyer
Who should take responsibility for educating individual users and users at large, and the public, about the dose that induces a safe high and the dose that over time induces addiction? Who should take responsibility for regulating the THC levels and labeling, prescription instructions and warnings? It's a public health issue. Failure to bring THC distribution into the public health system is a failure of government responsibility.
Correlation Still Does Not Imply Causation, But Incarceration and Racism are Bad for Your Health
Carl Hart, Ph.D. | Columbia University
We read with great interest the Viewpoint by Murray and Hall suggesting cannabis legalization will increase psychosis in the U.S. This is an important issue, not least because eleven states have already legalized the drug for adult use. Unfortunately, our initial excitement gave way to disillusionment and skepticism, because the manuscript contains less than appropriate interpretations of relevant data and fails to acknowledge that cannabis prohibition exacts an unjust toll on targeted groups in racialized environments.

It’s critically importance for readers to understand that Murray and Hall drew their conclusion—legalization will increase psychotic disorders—from correlational data. Each
of the cited studies caution that causation has not been demonstrated; because psychosis has been shown to be correlated with numerous variables, including social disadvantage and ethnic minority status. In our own review, we found that anxiety, bipolar, and mood disorders all have been correlated with cannabis use, and that psychosis has been correlated with heavy tobacco smoking, heavy alcohol use, and stimulant misuse. There was no clear evidence for a causal relation between cannabis and psychosis. The lack of evidence for causality reduces the argument against legalization for psychiatric reasons to the level of speculation.

The consequences of continued criminalization are, in contrast, clear. More than six hundred thousand Americans are arrested annually for cannabis possession. Blacks and Latinos are arrested at considerably higher rates than whites, even though all use the drug at similar rates. Considering concerns about the rapid spread of coronavirus (COVID-19) in jails and prisons and extraordinarily high rates of COVID-19-related deaths among black people, it’s particularly disconcerting that the authors neglected a discussion on harmful consequences of cannabis prohibition. Even without the threat of COVID-19, incarceration and racism have detrimental health repercussions that are too often ignored by psychiatrists.

Co-authored with Charles Ksir, Ph.D.
Cannabis Ise is a contributory cause of psychosis
Robin Murray, MD | Institute of Psychiatry, Psychology and Neuroscience
Hart and Ksir doubt that cannabis use is a contributory cause of psychosis because they say much of the evidence is correlational. We have answered such arguments in detail elsewhere 1. In brief, Bradford Hill proposed criteria that should be fulfilled before an association could be regarded as causal 2 . We consider that these are fulfilled in regard to cannabis use and psychosis. There is a consistent relationship between cannabis use and psychosis – 10 out of 13 prospective studies show a significantly increased risk in users. Cannabis use precedes psychosis in the vast majority of patients (temporality); the frequency of cannabis use and cannabis potency show a dose-response with the risk increased up to ninefold with heavy use of high potency cannabis (strong relationship). The risk is specific to psychosis and not to psychiatric illness in general. There are plausible and coherent neurochemical theories linking the two. By analogy, other psychoactive drugs can also induce psychosis e.g. methamphetamine and synthetic cannabinoid agonists. Experimental studies show that administration of THC to volunteers can induce a brief psychosis while administration to juvenile animals induces behaviours thought to model psychosis proneness. Bradford Hill proposed his criteria to convince sceptics that tobacco smoking was a cause of lung cancer. Unfortunately, it took nearly three more decades before there were public health measures to counteract the effects of tobacco. Hopefully we won’t have to wait three decades before there are health education messages about the risk of psychosis with heavy cannabis use. Finally, while we agree with Hart and Ksir that there is a need to reform the way that the US justice system deals with cannabis use by ethnic minorities, we also believe that people of all ethnicities should be informed about the risks of daily use of high potency cannabis.
1. Murray RM,Englund A, Abi-Dargham A, et al. Cannabis-associated psychosis: Neural substrate and clinical impact, Neuropsychopharmacology, 2017, 124. 89-104
2. Bradford Hill A. The Environment and Disease: Association or Causation? Journal of the Royal Society of Medicine, 1965, 58, 5, 295-300

written with Wayne Hall
As Use Rises, Problems Occur
Reed Allyn, MB, MSc. | University of British Columbia
Physicians in Canada have had the opportunity to study the effects of cannabis in medical settings upwards of 20 years now, due to federal legalization of medicinal cannabis in 2001. Some research into adolescent schizophrenia and triggering mechanisms with use of cannabis have been undertaken. I believe among Canadian physicians, that there is broad understanding that cannabis can play a role in the potential emergence of psychiatric illness in adolescents. However it is not widely understood the exact mechanisms, nor if there is an unknown disposition for potential psychosis in those patients who become ill after use with cannabis. It is also not clear if just one dose of cannabis can trigger psychosis, or what level of use has most risk. Like anything else, the greater the use of anything, the more potential for problems and issues to appear. This is no different than increasing antibiotic use, the greater spread of utilization will undoubtedly result in more antibiotic resistance, for example.
Consequently, if there is a significant uptake in recreational use of cannabis, there is sure to be additional complications. However what is proving to be interesting, in that over the year that cannabis has been federally legal (Oct 2019 to present) for those of age to use, there appears to be a flat-line in terms of an increase in the development of psychiatric disease in particular, the adolescent and higher risk populations. I believe as many of my colleagues do, that Canada's lax regulations for numerous years, has contributed to a very small jump in usage of cannabis in higher risk groups and younger populations, as fear of punishment for possession and use, has not been a deterring factor for habitual users.