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CAMH Stories Centre for Addiction
and Mental Health

CAMH study unpacks complex links between schizophrenia and cannabis

People with schizophrenia are much more likely to smoke pot -- an estimated 25 to 60 per cent of them may meet criteria for a cannabis use disorder, compared to about 5 per cent of the general population.

But why is use so high, given that cannabis is linked to increasing the risk of psychosis, and can contribute to other serious health harms?

A new CAMH pilot study is investigating the complicated relationship between schizophrenia and cannabis.

“By understanding the underlying mechanisms, we hope to make it easier for a person with schizophrenia to abstain from cannabis,” says Dr. Mera Barr, an Independent Scientist with CAMH’s Schizophrenia Division, and an Assistant Professor of Psychiatry at the University of Toronto.

Dr. Mera Barr
Dr. Mera Barr

On the one hand, there are many potential harms related to cannabis use in this vulnerable population, she notes. Cannabis may increase the risk of psychotic symptoms in young people who are predisposed to develop schizophrenia, for example. For those already living with schizophrenia, “harms may include lung and cardiac disease, increased schizophrenia symptoms, longer hospital stays and legal problems. There is also the high cost of the drug for patients who are often on very low incomes.”

On the other hand, the drug has been shown to have some positive effects on cognitive symptoms – such as short-term memory problems -- related to schizophrenia, she notes.

In a pilot study, Dr. Barr and her team are studying 18 patients with schizophrenia who are also cannabis-dependent. The patients were asked to go cannabis-free for 28 days in return for a financial incentive. Abstinence was also confirmed by biological tests.

Ten of the 18 participants were successfully abstinent for the four-week study period.  Results were compared to a control group of non-psychiatric patients who did not use cannabis.

Preliminary results showed that successful abstinence in the study group actually impaired patients’ working memory slightly (this is defined as very short-term memory required to complete tasks). Abstinence also slightly impaired cortical inhibition, a neuron-based cornerstone of many cognitive functions. The decreases in function were 14 per cent and 19 per cent respectively in participants of this small pilot group sample assessed before and after abstinence.  (In the control group of non-psychiatric cannabis-dependent participants, there was no change in working memory, but there was a decrease of 20% in cortical inhibition with abstinence.)

In addition, certain measures taken at the start of the study could predict abstinence at 28 days. For example, those with better cortical inhibition function at baseline were more likely to be abstinent after 28 days.

“An obvious question is whether we can use another means to improve cortical inhibition and working memory, while supporting a patient’s abstinence from cannabis,” says Dr. Barr.

“And we do have a tool we can test,” she believes.

Enter rTMS – or Repetitive Transcranial Magnetic Stimulation. This therapy stimulates nerve cells in the brain through a series of short magnetic pulses limited to a small area in the frontal lobe. The client stays awake during the 30-minute procedure and can resume normal activities right away. The therapy is being conducted at CAMH’s Temerty Centre for Therapeutic Brain Intervention.

Approved by Health Canada for depression, the treatment has already shown promise for improving executive functioning in people with schizophrenia and youth with depression.  A recent study conducted by Dr. Barr and colleagues showed that a program of intense stimulation – rTMS five days a week for four weeks – led to a significant improvement in working memory for people with schizophrenia.  (Dr. Barr notes that she reassures all research participants that the therapy is safe – in fact she has received rTMS herself).

Dr. Barr wants to take that a step further – to see if rTMS “will offset the cognitive disruption we are seeing from cannabis abstinence.” She is applying to fund a broader study on this question through an Ontario Mental Health Foundation research grant.

Schizophrenia and cannabis use combined can represent a tremendous burden for patients and their families. (Prevalence of cannabis use disorder in people with schizophrenia is analyzed in a study by Koskinen et al, 2010). And as cannabis moves towards decriminalization, access to the drug may increase.

“By gaining a better understanding of the mechanisms in the relationship between schizophrenia and cannabis, we hope to develop better treatments and to support abstinence for our patients,” says Dr. Barr.


 Related research


CAMH Chief of Schizophrenia Dr. Tony George, CAMH Scientist Dr. Jeff Daskalakis, Dr. Barr and colleagues are investigating the promise of rTMS for tobacco cessation in patients with schizophrenia. As with cannabis, tobacco rates and related health harms are very high in this vulnerable population. Read more.

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