July 21, 2021

expert reaction to study looking at cannabis use disorder and schizophrenia in Denmark

A study published in JAMA Psychiatry looks at cannabis use disorder in schizophrenia in Denmark.

Prof Terrie Moffitt, Chair in Social Behaviour & Development, Institute of Psychiatry, Psychology, and Neuroscience (IoPPN), King’s College London, said:

“This study of nationwide medical records adds important evidence that patients with diagnosed cannabis use disorder are more at risk for psychosis now than they used to be.  A study weakness that readers will spot right away is that the study reported on patients who had a clinical diagnosis of cannabis dependence.  However, most cannabis users, even those who are dependent on it, never come in to clinics for treatment.  Also, it is known that people who seek treatment tend to have multiple mental health problems, not solely cannabis problems.  And there are far more recreational cannabis users who manage cannabis well than cannabis-dependent users who cannot manage it.  However, it is not necessary to rely only on medical-record studies.  In Britain and New Zealand, studies tracking large populations of people have interviewed them repeatedly about cannabis use, regardless of whether they ever sought treatment.  In these studies too, cannabis users have elevated risk of psychotic mental disorders.”

Prof David Curtis, Honorary Professor, UCL Genetics Institute, said:

“While it is true that people diagnosed with schizophrenia are also more likely to be diagnosed with cannabis use disorder, the reasons for this association are far from clear and it is extremely difficult to interpret results such as these with confidence.  I am struck by two simple findings.  In Denmark, the incidence in schizophrenia in 2016 was about the same as it had been in 1996 but over the same period of time the prevalence of cannabis use disorder increased nearly five-fold.  If, as the authors suggest, cannabis use disorder can cause schizophrenia then there should have been a quite dramatic increase in schizophrenia incidence and we simply do not see that.  So far as I can see, this study does not really provide support for the hypothesis that cannabis use causes schizophrenia.  In fact, it seems to provide evidence that it does not.”

Dr Amir Englund, a Cannabinoid Psychopharmacology researcher at the Addictions Department of the Institute of Psychiatry, Psychology and Neuroscience, King’s College London, said:

“This study is an impressive analysis of over 7 million people in Denmark to explore the potential causal relationship between cannabis use and schizophrenia, specifically cannabis use disorder (CUD) or as it is more commonly known, cannabis addiction.  The study is one of many of its kind, an observational study exploring if cannabis addiction increases the risk of schizophrenia.  The study also explored what proportion of schizophrenia cases could be prevented if people were not addicted to cannabis (or PARF), and how this changed over time.  The study found that being addicted to cannabis is associated with an increased risk of schizophrenia 4-fold, which is similar to what previous studies have found.  The PARF increased from 2% in 1972 to 8% by 2010, but is still lower than what has been observed in the UK (30%).

“In terms of what this study adds to our current understanding of the relationship between cannabis use and schizophrenia – it strengthens an already well-established association between the two.  However, it is unable to shed additional light on whether cannabis causes schizophrenia or not, due to the observational nature of the study.  In my opinion, the current scientific view of cannabis use as a ‘component cause’, which interacts with other risk factors to cause schizophrenia, but is neither necessary nor sufficient to do so on its own – still stands.”

Prof Sir Robin Murray, Professor of Psychiatric Research, Institute of Psychiatry, Psychology, and Neuroscience (IoPPN), King’s College London; and Dr Marta Di Forti, MRC Senior Clinical Fellow, Institute of Psychiatry, Psychology, and Neuroscience (IoPPN), King’s College London, said:

“This is an outstanding carefully conducted study showing that over four decades the proportion of schizophrenia attributable to cannabis use in Denmark rose from 2 to 8%.  As the accompanying editorial points out this is likely to be a considerable underestimate.  This study adds to the now overwhelming evidence that cannabis use is an important and growing cause of psychosis.  Public education to draw attention to the risks of heavy cannabis use is needed urgently.”

‘Development Over Time of the Population-Attributable Risk Fraction for Cannabis Use Disorder in Schizophrenia in Denmark’ by Carsten Hjorthøj et al. was published in JAMA Psychiatry at 16:00 UK time on Wednesday 21 July 2021.

DOI: 10.1001/jamapsychiatry.2021.1471

Declared interests

Prof Terrie Moffitt: “I have no conflict of interest.  I am PI of a grant from the US National Institute on Aging to study effects of cannabis use on the mental health of the baby boomer generation, who used cannabis in their youth and continue using as they age.”

Prof David Curtis: “I have no conflict of interest.”

Dr Amir Englund: “No conflicts to report relating to this.”

None others received.