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Medical marijuana is shown in Toronto on Nov. 5, 2017.Graeme Roy/The Globe and Mail

There is little to no research to support the supposed benefits of medical cannabis, and what evidence exists suggests that using marijuana as medicine may do more harm than good, family doctors' associations across Canada are telling their members.

A trio of advisories prepared by the Alberta College of Family Physicians has been distributed to more than 32,000 clinicians, summarizing the scientific literature, or lack thereof, around medicinal marijuana.

"One thing that was quite consistent was adverse events," said Dr. Mike Allan, a professor of family medicine at the University of Alberta in Edmonton. "And the benefits, even if they're real, are much smaller than what people might anticipate."

Allan co-ordinates the college's biweekly updates, "Tools for Practice," which focus on topical issues and are circulated through professional chapters in every province except Quebec and Newfoundland and Labrador.

The three most recent updates were inspired by physicians wanting to know more about the research behind medical cannabis. Family doctors are facing increasing pressure from patients asking about medicinal pot and the college wants to ensure its members have the information they need to make informed decisions around treatment, Allan said.

"I think this gives (doctors) some comfort, saying, 'Look, here's the evidence. It's actually missing in a lot of places, so I can't give it for conditions X, Y and Z,' " Allan said. He described medical pot as a high-risk product that should only used in rare circumstances when other, safer treatment options have been exhausted.

Health Canada data show the number of clients registered with licensed medical marijuana producers jumped to more than 200,000 as of June 2017, which is about 2.7 times greater than the 75,166 people registered at the same time last year. Registrations more than tripled in both 2016 and 2015.

"The decision to use cannabis for medical purposes is one that is made between patients and their health-care practitioners, and does not involve Health Canada," department spokeswoman Tammy Jarbeau said Tuesday in an email.

Health Canada published a lengthy information bulletin for medical professionals about the benefits and harms of medical marijuana, topped with a disclaimer warning that cannabis is not an approved therapeutic product and the department does not endorse its use. The document was last updated in 2013.

The first advisory prepared by Alberta's family physicians college, which was released Nov. 14, says the evidence is "too sparse and poor" to conclude that marijuana is effective at relieving pain. The second document, released two weeks later, describes "adverse effects" as one of the only consistent findings of the existing studies. These harms included hallucinations, paranoia, dizziness and low blood pressure.

The research likely underestimates the frequency of adverse outcomes because most studies involve patients who have a history of using pot so are less likely than the average person to experience any negative side effects, Allan said.

Some studies indicate marijuana can reduce nausea for chemotherapy patients and control spastic muscle contractions in patients with multiple sclerosis, but there is virtually no evidence of any effect on chronic anxiety or glaucoma, as is commonly cited by industry advocates.

"Some of the pain studies go for nothing more ... than five, six hours. And this is for chronic pain," Allan said. "It's hard to get a great feel for how someone is going to do long term on a medicine after five, six hours."

The only evidence of marijuana being used to treat glaucoma is a single randomized study conducted on six patients, he added.

"We would kind of be putting the cart before the horse if we started to prescribe this without the research," Allan said.

Philippe Lucas, head of the Canadian Medical Cannabis Council, applauded the college for its efforts to inform the medical community about the benefits and risks of medicinal cannabis, but said he was concerned about what he saw as a bias toward highlighting the harms.

"I'd be the last person to suggest that cannabis was entirely safe or entirely appropriate for every individual," said Lucas, who is also an executive with Canadian marijuana producer Tilray.

Many patients use marijuana as a substitute for other pharmaceutical drugs, such as opiates, and limiting access to cannabis could hinder efforts to combat the opioid overdose crisis, he said.

Medical colleges have released some general guidelines for primary-care providers about prescribing marijuana. Documents released by the colleges of physicians and surgeons in both British Columbia and Alberta cite the absence of reliable evidence demonstrating the effectiveness of cannabis as medication.

The Alberta college is helping put together more comprehensive provincial guidelines for prescribing medical cannabis, which it expects to release by March, Allan said.

Ottawa has agreed to give the provinces and territories 75 per cent of tax revenues from the sale of legalized marijuana

The Canadian Press