Patients who consumed tainted medical marijuana from government-regulated suppliers are questioning how safe the industry is in the wake of several high-profile recalls due to banned pesticides, which have exposed serious gaps in Health Canada's oversight.

After a string of recent recalls by Mettrum Ltd., OrganiGram Inc. and Aurora Cannabis Inc. because of the presence of myclobutanil – a banned pesticide that produces hydrogen cyanide when heated – a number of patients told The Globe and Mail they don't see how Health Canada can assure them the product can be trusted. Revelations that the government isn't testing regularly to prove all companies aren't using harmful chemicals have left consumers concerned for their health.

"I think this has probably given everybody a wakeup call," said Patty Wade, a Mettrum client in Trenton, Ont., who was prescribed medical cannabis for post-traumatic stress disorder. "When you are trusting a company to be healthy, you would have thought that the government would have ensured this."

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Investigation: What's in your weed?

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Last week, Health Canada acknowledged to The Globe it had not been testing product from the 38 federally regulated medical-marijuana growers to ensure they weren't using banned chemicals.

Instead, the department said the companies knew pesticides such as myclobutanil were banned and the companies had been left to police themselves.

However, Thomas McConville, a former Mettrum employee, told The Globe he witnessed company employees spraying myclobutanil on plants to combat a mildew problem in 2014, even though they knew the chemical – a known carcinogen – was banned. To evade detection when Health Canada inspectors visited the facility, a Mettrum employee hid the pesticide behind the ceiling tiles in the company's offices, knowing the department wasn't testing the plants, Mr. McConville said.

Health Canada has since attached new conditions to the licences of Mettrum and OrganiGram, requiring their product to be tested for banned pesticides. And last week, the department announced random tests for the rest of the industry, which it hopes will ensure other companies aren't breaking the rules.

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Without regular testing, though, there is no way to be certain which companies are producing clean product, patients say. Ms. Wade, a former nurse, said there is little certainty which companies are safe to buy her medicine from.

"What I'm hoping is that this has put such a spotlight on it, that the government is going to step up its processes and look into all of the licensed producers," Ms. Wade said. "I don't understand why they haven't been testing … So what are going to be the safeguards for us, the consumers?"

The licensed producers are already expected to test for mould and bacteria in their products before selling them. Health Canada said last week that, for now, it's not planning mandatory testing for banned pesticides. "We have a couple of cases right now. I wouldn't want to extrapolate that that's an issue that would be happening at all our LPs [licensed producers.]," a senior government official told The Globe.

Still, it's impossible for Health Canada to know how big the problem is. The Globe has talked to more than 20 patients affected by the recalls, and several of them say their confidence in the safety of the industry has been shaken.

"Presumably it's out of the product now – although who can even say that?" said Dawn Rae Downton, a patient in Halifax who was prescribed medical marijuana last March for severe back pain that prevented her from sleeping, and purchased her product from OrganiGram. "I see it as a consumer-protection issue, and I see it as an astonishing lack of oversight on the part of Health Canada."

In a submission to Health Canada, known as an Adverse Reaction Report, Ms. Downton told the department she suffered "severe, intractable nausea, vomiting and anorexia," which "continued relentlessly daily for nine months, resolving to a tolerable degree about five weeks after I stopped using the cannabis."

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OrganiGram announced it was recalling her products in December, because of the presence of myclobutanil. Ms. Downton's symptoms are similar to several of the known effects of low-level hydrogen-cyanide poisoning on the body.

Health Canada has referred to the amounts of banned pesticide it detected as "trace amounts" that are "low risk." However, Warren Porter, a top U.S. toxicologist, questioned that response last week, saying, "There is no data I am aware of that would give those assurances. "Ultra-low doses can have all kinds of biological effects, especially over longer periods of exposure," said Dr. Porter, a specialist in molecular and environmental toxicology at the University of Wisconsin-Madison.

Ms. Downton is now worried about her health. "I inhaled hydrogen cyanide every day for eight months," she said.

Another patient told The Globe he chose to use medical marijuana for pain relief because he didn't want to use opioids. He selected a government-regulated supplier, Mettrum, because he wanted to ensure the product was safe and clean.

"I made a choice to go the medical-marijuana route, despite paperwork, numerous medical appointments and cost, because I wanted to feel safe about what I was putting in my body," said the man, who requested anonymity because he didn't want to disclose publicly he was using medical marijuana. "I felt this was safer."

The man said he is reluctant to purchase new product. "I don't trust the replacement product is safe," he said.

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Only Mettrum and OrganiGram are now required to submit to regular testing for myclobutanil. The third recent recall, at Aurora Cannabis, came after that company purchased a bulk supply from OrganiGram, which it resold to its customers.

Since the recalls were announced in December, Mettrum was sold to Canopy Growth Corp. for $430-million. Mettrum chief executive Michael Haines has not responded to requests for comment. Mr. Haines is no longer with the company.

Canopy CEO Bruce Linton said the company is working to correct the Mettrum problems. Mr. Linton said he is in favour of routine testing to prove to customers the product is free of pesticides, which his company is adopting.

"The supply chain has to be completely trustworthy, and showing that you get that means that you will routinely and actively test and confirm this is on track, this is following the rules, there isn't what you wouldn't want in it," Mr. Linton said.