A top U.S. toxicologist is questioning Canada's response to a tainted-cannabis problem in the medical-marijuana sector, saying patients aren't being given accurate information on the risks associated with a banned pesticide thousands of people may have consumed.

Warren Porter, a specialist in molecular and environmental toxicology at the University of Wisconsin-Madison, says company phone calls and e-mails, approved by Health Canada, to patients after a series of recent product recalls are misleading, and appear to be based on faulty science.

Mettrum Ltd. and OrganiGram Inc. have sent messages to customers over the past two months informing them that products they had purchased were being recalled because of the presence of banned pesticides, including myclobutanil – a fungicide known to emit hydrogen cyanide when heated.

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In those messages, both companies have played down the risk to consumers, saying they were only exposed to small amounts of the banned chemical. "Rest assured, this material is not deemed to present a health risk," a letter to Mettrum's clients said.

OrganiGram sent a letter to clients informing them "the probability of serious adverse health consequences is remote," while Health Canada has referred to the amounts of banned pesticide detected as "trace amounts" that are "low risk."

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Dr. Porter questions how they could come to that conclusion so easily.

"They have no idea whether or not that's true. There is no data I am aware of that would give those assurances," he said in an interview.

Dr. Porter raised alarms about myclobutanil and other chemicals last year at a high-level drug policy conference in New York, where he spoke about the impacts that even minuscule amounts of dangerous chemicals can have on the body. Health Canada sent several representatives to that conference.

He said that even though the amount of chemical involved in the recalls can be classified as small, it does not mean that the risks can be dismissed.

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"Ultra-low doses can have all kinds of biological effects, especially over longer periods of exposure," he said. "So when these companies say 'Oh, there's no problem,' the first thing I would ask them is have you looked at the effects on the nervous system, the endocrine system, the immune system, and epigenetics?"

Recordings of phone messages to Mettrum clients, which were obtained by The Globe and Mail, give the impression of safety, saying that myclobutanil "is widely used in Canada and around the world on food crops, including lettuce, fresh fruit and berries."

However, although myclobutanil is approved for use on some foods to control mildew, it is designed to be washed off, while any remaining residue is metabolized by the digestive system so that it is not a threat to the body. The reason it is banned for plants that are smoked, including tobacco, is that the chemical enters the bloodstream directly through the lungs, without being metabolized.

Symptoms of low-level hydrogen cyanide poisoning include dizziness, trouble breathing and vomiting. However, Dr. Porter said the long-term effects of myclobutanil on the body are not known, because they have not been studied.

"The bottom line is, nobody really knows," he said.

Both Mettrum and OrganiGram told The Globe that their communications with clients were approved by Health Canada. In a background briefing with The Globe this week, a senior government official said Health Canada determined the recalled product presented a low risk by calculating the amount of the chemical a person would have been exposed to if he or she had smoked the product. Although the presence of the banned pesticide was unacceptable, the senior official said: "In this particular case, the risks are low."

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However, Dr. Porter figures it is impossible to know that. "These people don't know their biochemical pathways, and they especially don't know low-dose effects."

U.S. jurisdictions that have legalized cannabis have also been confronted with myclobutanil problems in recent years, since the substance – sold as Nova 40 and Eagle 20 – is an easy shortcut when faced with mildew infestations, particularly when the pest threatens crops worth hundreds of thousands of dollars.

Andrew Freedman, who oversaw Colorado's fight against the illegal use of myclobutanil when he was director of marijuana co-ordination for the state, said the government took a zero-tolerance approach, because there were no studies saying it could be safely used on cannabis.

"Is there a tolerance level for it? No, there is no tolerance level," said Mr. Freedman, who is now a consultant on cannabis policy at Freedman & Koski, a firm he co-founded this year. "It's banned outright."

In Mettrum's communications to clients, the company referred to myclobutanil as "not currently approved for use on cannabis," suggesting that it may eventually be permitted.

However, in a statement issued this month in the United States, the maker of Nova 40 and Eagle 20, Dow AgroSciences, indicated otherwise.

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"Dow AgroSciences, without exception, will not seek regulatory approvals or support the use of its products on marijuana," the company told CBS News. "[Myclobutanil] is not approved for use, nor should it be used under any circumstances on marijuana."

Since the myclobutanil problem in Canada first came to light in December, Mettrum has since been purchased by Canopy Growth Corp., in a $430-million deal that closed Dec. 31. All Mettrum questions have now been referred to Canopy.

Canopy chief executive officer Bruce Linton said the company plans to fix the problems at Mettrum, and said the wording chosen in that communication to customers was unintentional.

"In no way did we intend to imply that the pesticide found in [Mettrum's] product could potentially become approved for use," Mr. Linton said in an e-mail to The Globe. Mettrum's CEO, Michael Haines, is no longer with the company and has not responded to questions about the recalls.