More than a million dollars of medical marijuana has been destroyed so far amid the fallout from a tainted cannabis scare hitting the government-regulated sector, as patients' fears grow that Health Canada has no way of knowing how big the problem actually is.

Canopy Growth Corp., which recently acquired Mettrum Ltd., said on Tuesday that it has written off about $800,000 of costs owing to a series of product recalls Mettrum announced starting in November.

Those recalls came after Mettrum was caught with a banned pesticide and known carcinogen in products that were sold in 2016. A former Mettrum employee told The Globe and Mail that he witnessed staff spraying plants with myclobutanil as far back as 2014, despite knowing the chemical is prohibited, because it emits hydrogen cyanide when heated.

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To evade detection, he said, staff hid the pesticide in the ceiling tiles of the company's offices when Health Canada inspectors visited the site, aware that the department wasn't testing the plants for banned chemicals.

Mettrum's recall is one of several to hit the sector after OrganiGram Holdings Inc. was also caught selling products containing myclobutanil, forcing it to recall and destroy several hundred thousand dollars worth of product.

While Health Canada has since attached new conditions to the licences of Mettrum and Organigram, requiring them to test products to prove they are clean, the federal department told The Globe it is not placing those same restrictions on the broader industry, which is made up of 38 companies – and is essentially left to police itself.

Health Canada announced last week it would subject the industry only to random tests, but that has left patients wondering which companies they can trust– since Health Canada has no way of proving which products are clean, without requiring more stringent and consistent testing.

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The Globe and Mail has asked Health Canada for an explanation as to why it doesn't feel regular testing is necessary, particularly in light of the recent recalls, and is awaiting a response.

Most of the $800,000 writeoff at Mettrum is related to marijuana that needed to be destroyed, Canopy chief executive officer Bruce Linton said; however, some of that money was to cover the costs of testing products to see which had been exposed.

OrganiGram revealed a few weeks ago that its recalls have cost slightly less than $500,000. OrganiGram CEO Denis Arsenault couldn't be reached for comment Tuesday, but one analyst said the bulk of that figure is also believed to be from product that had to be destroyed.

After The Globe revealed in late December that Mettrum's recall was due to myclobutanil, a fact that neither Health Canada nor Mettrum included in their official public announcements, OrganiGram issued a recall days later that was also due to myclobutanil. A third company, Aurora Cannabis Inc., which purchased a bulk shipment from OrganiGram, also issued a recall.

At a cost of nearly $1.3-million dollars so far, the combined recalls at OrganiGram and Mettrum make up the most costly quality-control issue to hit the nascent sector. The federal government launched the medical marijuana industry in 2014 as a way to provide clean and safe pharmaceutical-grade marijuana to patients, including to those prescribed it by their doctors to treat conditions such as pain associated with cancer, complications from multiple sclerosis, and post-traumatic stress disorder

The recalls have exposed a glaring lack of oversight by Health Canada, and raised questions about the government's response to the problem. Myclobutanil is approved for use on some foods, because the chemical is safely metabolized by the digestive system. However, when smoked, it enters the bloodstream directly through the lungs. Known to emit hydrogen cyanide when burned, myclobutanil is banned on plants that are smoked, including tobacco and cannabis. Its manufacturer, Dow AgroSciences, recently issued a statement saying the product is unsafe for such crops.

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Health Canada said in September it had a zero-tolerance policy for contaminants such as myclobutanil, known among growers as an illegal shortcut to saving crops from mildew infestations.

But when the pesticide was discovered at Mettrum and OrganiGram, Health Canada and the companies played down the problem. Mettrum and OrganiGram told clients the chemical was approved for use on fruit, giving the impression of safety, while Health Canada referred to the residue detected as "trace amounts" that are "low risk."

A top U.S. toxicologist questioned Health Canada's response, saying it and the companies "have no idea whether or not that's true," because there is no scientific data to support those assurances.

"Ultra low doses can have all kinds of biological effects, especially over longer periods of exposure," said Warren Porter, a specialist in molecular and environmental toxicology at the University of Wisconsin-Madison.

Patients of both companies have contacted The Globe in recent weeks describing symptoms that include persistent nausea, vomiting and difficulty breathing. All are similar to the known symptoms of hydrogen-cyanide poisoning.

"When you are given a licence by the Government of Canada, you are entrusted by the government, and us, to make sure that your product was safe," said Patty Wade, a Mettrum client in Trenton, Ont. "I used to be a nurse. Mistakes were not acceptable, you're dealing with people's health."

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"I see it as an astonishing lack of oversight on the part of Health Canada," said Dawn Rae Downton, a client of OrganiGram in Halifax.