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Unpacking the links between schizophrenia and cannabis use

CAMH Discovers: News from CAMH Research and the Campbell Family Mental Health Research Institute
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​Unpacking the links between schizophrenia and cannabis use

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?

Dr. Mera BarrA 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, Scientist in the Biobehavioural Addictions and Concurrent Disorders Laboratory in CAMH’s Schizophrenia Division and in the Temerty Centre for Therapeutic Brain Intervention at CAMH.

Negative – and positive – effects

On the one hand, there are many potential harms related to cannabis use in this vulnerable population, notes Dr. Barr. For example, cannabis may increase the risk of psychotic symptoms in young people who are predisposed to develop schizophrenia. 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,” she explains.

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.

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 participants who are cannabis-dependent and have not been diagnosed with a psychiatric illness.

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 participants who are cannabis-dependent and have not been diagnosed with a psychiatric illness, there was no change in working memory with abstinence, but there was a decrease of 20% in cortical inhibition.)

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.

Testing a new approach

“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 says.

Enter rTMS – or repetitive transcranial magnetic stimulation. This non-invasive 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 as a treatment for depression, rTMS 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. As the next area of her research, Dr. Barr wants to take that a step further – to see if rTMS “will offset the cognitive disruption we are seeing from cannabis abstinence.”

Schizophrenia and cannabis use combined can represent a tremendous burden for patients and their families. And as cannabis moves towards legalization, 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.


​Smoking and schizophrenia

CAMH researchers Dr. Tony George, Dr. Jeff Daskalakis, Dr. Mera Barr and colleagues are investigating the promise of repetitive transcranial magnetic stimulation (rTMS) for tobacco cessation in patients with schizophrenia. An earlier preliminary CAMH study showed that rTMS successfully reduced cigarette cravings among smokers with schizophrenia. As with cannabis, tobacco rates and related health harms are very high in this vulnerable population.

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