A Danish study involving more than 6 million participants found that cannabis use disorders significantly increased the risk of developing depression and bipolar disorder. Despite the association, the researchers stressed that the study did not conclusively prove that cannabis use causes these mental health conditions, and cautioned against use, and argued for legalization. recommended to consider.
New research suggests that cannabis use disorders lead to an increased risk of developing bipolar disorder and depression.
Cannabis is one of the most commonly used illegal drugs in the world. A new Danish study suggests that cannabis use disorders are more strongly associated with the development of psychiatric disorders than previously thought.
The study included registry data from over 6 million Danes, and the results show that cannabis use disorders increase the risk of psychotic and non-psychotic depression and bipolar disorder.
“Considering differences such as gender, age, socioeconomics and family history, we find that cannabis use disorders are almost twice as likely to develop depression and two to three times more likely to develop depression. Both men and women have bipolar disorder,” says Oscar Hougaard-Jeffsen, a doctoral student in the University’s Department of Clinical Medicine.[{” attribute=””>Aarhus University. He is the lead author of the study, which has just been published in the scientific journal JAMA Psychiatry.
“As far as we know, we’ve conducted the world’s largest study of the link between cannabis use disorder and affective disorders,” says Oskar Hougaard Jefsen, a PhD student from the Department of Clinical Medicine at Aarhus University. He is the first author of the study, which includes register data from more than six million Danes. Credit: Simon Byrial Fischel
According to the Danish Health Authority, one in three Danes under the age of 25 has smoked cannabis. However, the new study only focuses on people with a significant consumption of cannabis such that they have been registered with a substance use disorder – e.g. because they have been in contact with the substance abuse treatment system or other parts of the healthcare system.
More countries are legalizing cannabis
Several studies have supported the hypothesis that extensive cannabis use is not harmless to human mental health. For example, previous studies suggest that cannabis use disorder can increase the risk of developing schizophrenia. But until now, the risk of other mental disorders has been sparsely studied.
In this study, researchers from Aarhus University and the University of Copenhagen have analyzed data from Danish nationwide registers such as the National Patient Register, the Danish Psychiatric Central Research Register, and the Danish Register of Pharmaceutical Sales.
“The study is the largest of its kind in the world, and our findings suggest that cannabis use disorder is also associated with an increased risk of developing depression and bipolar disorder. The results recommend caution when it comes to using cannabis. This applies to people with an increased risk of developing mental illness, and to politicians and other decision-makers who are discussing the possibilities of legalizing cannabis,” says Oskar Hougaard Jefsen.
An increasing number of countries are legalizing the production and sale of cannabis for medicinal and recreational use. Since 2018, general practitioners in Denmark have been able to write prescriptions for drugs based on cannabis for patients as part of a trial scheme that also gives companies and individuals the opportunity to produce cannabis for medicinal or industrial use.
Oskar Hougaard Jefsen believes that the results of the study should be considered when it comes to legalizing and controlling cannabis use.
“We should conduct more research into whether there are people for whom cannabis is particularly harmful. This could strengthen preventative measures,” he says, adding that there is a particular need for more knowledge about the dose-dependent effects of cannabis use on the brain, cognition, and behavior, and for identifying risk factors for the transition from cannabis use disorder to psychiatric disorders.
No conclusive evidence
Oskar Hougaard Jefsen points out that, despite the indications in the study, it does not provide conclusive evidence that cannabis causes these mental disorders.
For example, he cannot rule out that undiagnosed depression or bipolar disorder has led some of the people in the register-based study to develop cannabis use disorder – i.e. the disease resulted in the abuse and not the other way around.
“But when we see an increased disease risk – even ten years after the cannabis use disorder has been registered – I don’t think that self-medication can be the only explanation. It seems unlikely that so many people would go undiagnosed for so long,” he says.
“Danish register data really gives us a unique opportunity to take into account many of the crucial factors that could affect the results. However, conclusive evidence would require a randomized controlled trial in which a group of people would have to smoke large amounts of cannabis to see if this increased their risk of developing mental illness in the long term, and such a study would, of course, be unethical,” he says.
The research results – more information
- The study is a register-based epidemiological cohort study of 6,651,765 people born in Denmark before 2006, and who lived in Denmark between 1995 and 2021. An equal number of men and women appear in the study.
- The partners are Associate Professor Carsten Hjorthøj, Senior Researcher Annette Erlangsen and Clinical Professor Merete Nordentoft – all three from the University of Copenhagen.
Reference: “Cannabis Use Disorder and Subsequent Risk of Psychotic and Nonpsychotic Unipolar Depression and Bipolar Disorder” by Oskar Hougaard Jefsen, MD; Annette Erlangsen, PhD; Merete Nordentoft, DMSc and Carsten Hjorthøj, PhD, 24 May 2023, JAMA Psychiatry.
DOI: 10.1001/jamapsychiatry.2023.1256
