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A review of 40 years' worth of studies suggests cannabis may not be effective in treating mental health disorders, but experts say that might have more to do with the lack of high-quality research than the drug itself.

The review, published in Lancet Psychiatry this week, looked at 83 studies dating back to 1980 on cannabis and constituent cannabinoids as a treatment for depression, anxiety, attention-deficit hyperactivity disorder, Tourette syndrome, post-traumatic stress disorder and psychosis.

The study concluded there was "scarce evidence" to suggest cannabis, including active ingredients such as cannabinoids tetrahydrocannabinol (THC) and cannabidiol (CBD), improves the symptoms of any of these conditions based on 3,513 participants.

There was also "very low-quality evidence" that it leads to a "small improvement" in anxiety symptoms for individuals, but only in those with other medical conditions like chronic pain and multiple sclerosis.

"There remains insufficient evidence to provide guidance on the use of cannabinoids for treating mental disorders," Prof. Michael Farrell, co-author of the report and director of the National Drug and Alcohol Research Centre in New South Wales, Australia, said in an email.

'Risk of error' in research

While experts say the review itself is credible, the decades-old research could be flawed due to a number of challenges — including the fact that cannabis is still illegal in much of the world, which has made securing funding for research challenging.

"The research in these conditions, in general, have been hampered by, obviously, the illegality of these compounds and these products," said Dr. Peter Selby, chief of medicine in the psychiatry division of the Centre for Addiction and Mental Health in Toronto.

"So the quality of the research, where you find positive findings, have lots of risk of error."

Dr. Hance Clarke, director of pain services at Toronto General Hospital, prescribes cannabis as a treatment for patients with a variety of chronic pain issues. He says the data from the studies examined in the review isn't necessarily up-to-date.

"You're mining old data where we know there's not much evidence for this. So I don't know that this should be so surprising of a finding … If you're dealing with low-quality evidence, you're going to get that answer," he said.

"The biggest missing link, I think, of this paper, is there's never discussion of dose. There's never a discussion in terms of what people are actually consuming. It's simply mining the results of various data sets."

Dr. Hance Clarke, director of pain services at Toronto General Hospital, says one shortcoming of the review is it doesn't discuss cannabis dosage. (CBC)

Clarke said most of the data was from pain and multiple sclerosis studies and other areas of research, while very few of the studies featured randomized controlled trials related to cannabis and psychiatric conditions.

Fardous Hosseiny, interim CEO of the Canadian Mental Health Association, said there is a gap in the research on the medical benefits of cannabis, and until the gap is filled, patients' individual experiences can't be ignored.

"When you talk to individuals who tell you that they have certain mental illnesses, or they even have certain physical ailments, and they tell you the reason that they can get out of bed is because of their access to cannabis. We can't dismiss that," he said.

"But it's also ensuring that if that's the case, then we're doing it with the safest approach that we can."

Cannabis as a treatment option

Selby said cannabis shouldn't be ruled out as a treatment option for mental health disorders, but it shouldn't necessarily be the first line of defence either.

"It wouldn't be your first choice," he said. "Anecdotes are not how one should make health-care decisions, because that is fraught with problems."

Dr. Deepak D'Souza, a psychiatry professor at Yale University School of Medicine who wrote a commentary on the study, said that for many psychiatric disorders there are medications that have undergone rigorous testing and approval processes, while there are still many unanswered questions with cannabis.

"How often should a person take it? How much should they take? How long should they take it for? We don't have that basic information," he said. "So we've clearly gotten ahead of ourselves."

Selby said it wouldn't be prudent for a physician to recommend cannabis for mental health disorders without first trying things like therapy and other proven medications.

Dr. Deepak D'Souza, a psychiatry professor at Yale University School of Medicine, says there's evidence exposure to cannabis can make certain psychiatric disorders, such as schizophrenia, worse over time. (Thomas Samson/AFP/Getty Images)

"Physicians have an obligation to certainly be up on the science and the literature and know how to curate information and present it to a patient so that they can jointly make the appropriate decision," he said.

"'I took it and I felt better,' fails to account for the natural recovery, fails to account for belief, fails to account for some of the intoxicating effects of the drug."

Cannabis can make certain psychiatric disorders worse

D'Souza said he's concerned that the "cart is before the horse," because clinicians are prescribing cannabis off-label as a treatment option without the evidence to back it up.

"If we want to have marijuana or its constituent cannabinoids available as a treatment, we need to first do the studies," he said. "But this seems somewhat backwards, what's happening right now with cannabis."

D'Souza says there is compelling evidence of risks of exposure to cannabis for patients with certain psychiatric disorders, such as schizophrenia, which can make their condition worse over time.

Hosseiny, the CEO of the CMHA, points to Canada's lower-risk cannabis use guidelines as a way of helping patients understand the potential health risks.

"The key here is to ensure that you're protecting those individuals that might be at risk, but also not stigmatizing individuals that are using it and actually thriving because of it," he said.

"It's really a balanced approach and I think legalizing it is has been the right step."

Future research could hold more answers

The review concludes that further high-quality studies that directly examine the effect of cannabis on treating mental disorders are needed.

Hosseiny says Canada is in a unique position to do the research — especially after legalization.

"We are in uncharted territories where we have this opportunity to be leading on the research globally," he said.

Clarke, the head of pain services at Toronto General Hospital, said the legalization of cannabis has allowed for more research funding from organizations in Canada, such as the Canadian Institutes of Health Research and the Arthritis Society, and he's hopeful more research will soon be released.

"We're heading in that direction," he said. "That's when we're going to have the opportunity to answer these questions definitively."

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