Health Canada, which is facing a growing controversy over tainted medical marijuana, cannot say with certainty how widespread the use of banned pesticides is within the industry. Instead, the regulator has been leaving it up to the growers to police themselves on the use of potentially harmful chemicals.

In a background briefing with The Globe and Mail, a senior Health Canada official acknowledged that even though the government prohibits the use of potentially harmful chemicals such as myclobutanil, – which is known to emit hydrogen cyanide when heated –the department has not been testing cannabis growers to ensure the 38 federally licensed companies were, in fact, not using it.

"Up until this point, we have not required licensed producers [LPs] to test for any unauthorized pesticides, nor have we been testing all LPs, and it is because we expect their companies to be pro-actively watching and taking the appropriate measures to ensure non-authorized products aren't used," the senior official said.

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In recent weeks, three of those companies have been forced to recall product after it was found to contain myclobutanil, angering customers, including cancer patients and others with compromised immune systems.

The Globe revealed in December that a recall by Mettrum Ltd. was due to the discovery of myclobutanil – a fact neither the company nor Health Canada mentioned when first announcing the recall to the broader public.

The potentially harmful chemical, which has been outlawed for use on cannabis in several U.S. jurisdictions, was only discovered after another banned pesticide was found in Mettrum's product, and subsequent tests were performed.

A few days after the Mettrum problem emerged, two more companies – OrganiGram and Aurora Cannabis – announced recalls due to myclobutanil. The chemical was discovered after Aurora tested a bulk shipment of cannabis it purchased from OrganiGram.

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Health Canada is now preparing to introduce random testing on the licensed producers in an effort to clamp down on the problem. The department is sending out letters to each of the 38 companies this week informing them of the new system, and is scheduling a conference call with the industry to discuss the matter.

"In response to these events, Health Canada … will begin conducting random testing of medical-cannabis products produced by licensed producers, to provide added assurance to Canadians that they are receiving safe, quality-controlled product," the letter states.

However, the new measures do not make regular testing mandatory for the companies. Though licensed producers are required to test for mould, bacteria and heavy metals, the government official said testing for harmful pesticides is still something that companies "have the option" of doing.

Asked how patients could have confidence the product was not exposed to banned chemicals, given the lack of scrutiny by the government, Health Canada said it believed the system works.

Myclobutanil is notorious within the cannabis industry as an easy shortcut to saving crops that are overcome with mildew. Marketed as Nova 40 or Eagle 20, it is approved for use on some fruits and vegetables, since it is designed to be broken down by the digestive system, meaning it's not a threat to the body.

However, myclobutanil is not approved for plants that are smoked, such as tobacco or cannabis, since the chemical is passed directly into the bloodstream through the lungs, rather than being metabolized. California considers it carcinogenic, while legislators in Colorado, Oregon and Washington, where cannabis has been legalized, acted swiftly to ban it a few years ago, in some cases enacting emergency legislation and performing raids on companies to clamp down.

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Health Canada gave no clear answer in its briefing as to why it wouldn't make testing mandatory for the licensed producers. One reason given by the senior official was that he believed there is only about three labs in Canada that could perform such testing, and there would be a backlog. The senior official said the department is hoping the companies themselves begin testing.

"Certainly mandatory testing for myclobutanil and other unauthorized pesticides by all the LPs would be something that we would consider and explore further," if additional problems persist, the official said. "This might not ultimately be necessary if industry begins to implement testing as a best practice of sorts."

However, Rodger Voelker, lab director at OG Analytical in Oregon, who is credited with discovering the myclobutanil problem among growers in the United States, said leaving it up to companies to police themselves is a bad idea.

Cannabis crops can be worth hundreds of thousands of dollars, so if a mildew outbreak occurs, there is a financial incentive for growers to use shortcuts such as myclobutanil to save the crop, with little regard for the consumers eventually using the product. No company in Oregon ever actually admitted to using the banned chemical before they were caught, Mr. Voelker said in an interview last summer.