As Canada prepares to legalize marijuana for recreational use, concerns are being raised about how the government plans to ensure the product is safe – particularly after the recent discovery of banned pesticides in federally regulated medical cannabis.

Several companies within the medical-marijuana sector, which will soon be tapped to supply the recreational market, too, have raised concerns to The Globe and Mail about Health Canada's ability to enforce product quality and safety under existing rules.

A lack of inspectors at Health Canada, insufficient knowledge of the types of banned chemicals used by unscrupulous growers and an inability to effectively clamp down on problems are among the key issues companies have raised.

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While two companies agreed to talk only on background, fearing they could fall on the regulator's bad side for speaking out, others have been more vocal. Aphria Inc., one of Canada's largest licensed marijuana growers, called upon the government on Thursday to set "clear and enforceable rules to ensure that customers are protected and have access to clean and safe product."

Another licensed producer, CanniMed Therapeutics, questioned Health Canada's oversight of product safety, saying the industry's current production standards are not at "an appropriate level."

The comments came as the federal government tabled legislation on Thursday to legalize cannabis for recreational use in 2018. In announcing the proposed legislation, the government said it would "protect public health through strict product requirements for safety and quality," but did not elaborate on whether new, stricter testing measures would be introduced to prevent the use of dangerous chemicals.

Health Canada's oversight of the industry was called into question in December when three federally licensed medical-marijuana producers were found to have sold products containing banned pesticides that can cause serious health problems when inhaled. Mettrum Ltd. and Organigram Inc. issued product recalls as a result, while Aurora Cannabis recalled a bulk batch it purchased from Organigram for resale.

Prior to the tainted cannabis problems, Health Canada said it would take a "zero-tolerance" approach to dangerous pesticides if they were discovered, and that companies could lose their licenses. But after the banned chemicals were discovered – including myclobutanil, which is known to produce hydrogen cyanide when heated – Health Canada officials acknowledged they hadn't been testing for such pesticides, and were therefore taking companies at their word. The chemicals were discovered only when Aurora tested the bulk shipment it purchased from Organigram for resale.

In January, Health Canada announced it would begin randomly testing marijuana growers for such chemicals as a way of policing them. However, some companies believe that doesn't go far enough – particularly as the government prepares to open the recreational market, worth billions of dollars, which could entice some of producers to cut corners on safety.

"We firmly believe that there is a critical need to set clear and enforceable rules to ensure that customers are protected and have access to clean and safe product," Aphria chief executive officer Vic Neufeld said. "This is of utmost importance, given the expected demand that will come with the introduction of a recreational market. As such, Aphria is calling for a strict product-testing regime across the sector."

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Mr. Neufeld, who used to run vitamin giant Jamieson Laboratories Ltd., said Health Canada should require the sector to foot the bill for mandatory independent testing if the government is too strapped to do it on its own.

Brent Zettl, CEO of Saskatoon-based CanniMed Therapeutics, also said a more stringent system is needed. Last year, a Federal Court ruling forced Ottawa to overhaul the previous medical-marijuana system. That, he said, led to the government watering down its inspection standards in an effort to increase the amount of medical cannabis produced by the industry and avoid shortfalls in supply.

Before the ruling, Mr. Zettl said federal inspectors were up to speed on each producer's standard operating procedures, and verified that safety standards were being met and toxicological tests were proven. Now, a site visit from Health Canada usually focuses on which employees have cleared security screening and whether each room has enough security cameras, he said.

"They put patient access [to products] as a priority over patient safety, whereas, before, it was the opposite," Mr. Zettl said. Current production standards are not at "an appropriate level to produce a pharmaceutical-grade product that is going to patients," he said.

Some marijuana producers resort to dangerous chemicals to battle pests, such as mildew or mites, to save crops and avoid financial damage, even though such pesticides are known to be harmful.

Thomas McConville, a former Mettrum employee who says he witnessed banned pesticides being used as far back as 2014, questions Health Canada's ability to clamp down, partly because the department may not be up to speed on the problem.

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Mr. McConville said that when he brought his concerns to Health Canada, a senior government official in the Office of Medical Cannabis was unfamiliar with the names and uses of chemicals the department was supposed to be policing.

In a recent statement, a Health Canada spokesman told The Globe, "The Department will continue to monitor and inspect [Mettrum's] production facilities, and will take any additional actions if and when warranted."