Cannabis Use and Schizophrenia

Research from Denmark has confirmed the findings of previous studies that there is a connection between cannabis use and schizophrenia and has underlined the need for extreme caution when considering legalisation of cannabis use.

Led by Danish researchers, this was a large study of over 60,000 participants diagnosed with cannabis use disorder using data from a population of almost seven million.  Cannabis use disorder occurs when a person’ s use of cannabis cannot be controlled and withdrawal effects are experienced if they stop using it.  This study found that as many as 15% of cases of schizophrenia in young men may be preventable by avoiding cannabis, with young men being more at risk than young women.

The researchers commented that legalisation of cannabis use sends the wrong message, that cannabis is harmless.  Dr Carsten Hjorthøj, the study’s lead, said, “This study adds to our growing understanding that cannabis use is not harmless”.

A global picture is now emerging around the cannabis and schizophrenia connection.  Following Canada’s decision to legalise cannabis use another study, which was published in the Jama Network and reviewed on the Medscape platform by Megan Brooks in February 2025, showed that the change in the laws on cannabis use has led to an increase in the number of diagnoses of schizophrenia in the country.  In addition, there has also been an increase in cases of cannabis use disorder.

This study, carried out in Ottawa, covered a period of 16 years and looked at the period before liberalisation of cannabis use, then a period when cannabis use was liberalised to allow medical use and then the period after full legalisation which included non-medical use.  A total of over 13 million people were studied. It was found that cases of schizophrenia associated with cannabis use disorder almost tripled during this time.

In recent years there has been a growing movement in jurisdictions around the world to legalise the use of cannabis but the evidence of a connection between cannabis use and schizophrenia has been growing.  These changes in laws have led to cannabis becoming one of the most frequently used psychoactive drugs.  Schizophrenia is a cruel, life-changing (and often life-threatening) condition.   Legislators must be made aware that legalisation of cannabis will have real and tangible human costs.  And sufferers of schizophrenia and their carers should know that cannabis use will make their symptoms worse.

There is more about schizophrenia and drug use in the Living with Schizophrenia website at. https://livingwithschizophreniauk.org/information-sheets/schizophrenia-and-street-drugs/

Contact us on email at: info@livingwithschizophreniauk.org

References

Hjorthøj C et al, 2023, Association between cannabis use disorder and schizophrenia stronger in young males than females, Published in Psychological Medicine.

Brooks M, 2023, Young men at highest risk from cannabis abuse. Published on Medscape platform 5/05/2023.

Knapton S., 2023, Almost a third of schizophrenia cases in young men triggered by cannabis use. Daily Telegraph, 4/05/2023.

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