Ottawa is now requiring the country's licensed medical cannabis producers to screen their products for all banned pesticides after months of product recalls dogged the industry and random spot tests turned up unauthorized contaminants at two more growers.

Health Canada said Friday this next step of a mandatory testing regime was necessary after final results from random tests of seven growers found two, Hydropothecary and Peace Naturals, had products with traces of the banned pesticide myclobutanil or another active ingredient in other pesticides piperonyl butoxide. Myclobutanil, a chemical which is used to kill mildew, is a known carcinogen that is strictly prohibited for use on plants that are smoked because it produces hydrogen cyanide when heated.

The federal department had hinted it might take further action after four recalls in the past six months among the licensed producers, who are expected to eventually provide much of the country's recreational cannabis supply once the drug is legalized as early as next year.

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"This requirement for mandatory testing for the presence of unauthorized pesticides will help ensure that Canadians can continue to have confidence in obtaining safe, quality-controlled medical cannabis from licensed producers," said a statement from Department spokesperson Eric Morrissette sent to The Globe and Mail Friday evening.

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 licences. But after the banned chemicals were discovered – including myclobutanil – 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 licensed producer Aurora tested the bulk shipment it purchased from Organigram for resale.

Though harmful chemicals are a known problem in the black market, this is the fourth time in less than six months that the government-regulated sector has been hit with a tainted cannabis problem. Mettrum Ltd., Organigram Inc. and Aurora Cannabis Inc. each announced recalls in December because of chemicals such as myclobutanil and bifenazate, an insecticide. Both can lead to serious health problems if inhaled directly into the lungs.

When the three recalls came to light in December, Health Canada and some of the companies involved said the levels found (of up to 20 ppm of myclobutanil) were "trace" amounts and that consumers were at low risk for health problems.

But a top U.S. toxicologist disputes that assessment, noting that "trace amount" isn't a scientific term, and is often used subjectively to play down any problems. Minute levels of chemicals can have dangerous effects on the body, said Dr. Warren Porter, at the University of Wisconsin-Madison. Another expert told The Globe that levels above 1 ppm are not considered trace amounts.

Hundreds of patients exposed to myclobutanil and bifenazate through the recalls announced in December have come forward complaining of serious health issues, ranging from breathing problems to persistent headaches and nausea, blistering rashes and muscle pain. Health Minister Jane Philpott will not comment on the matter, despite several attempts to pose questions through her spokesman.