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A lack of scientific research on cannabis impairment is driving a rift between employers and workers, particularly in safety sensitive industries, as some unions argue that overly cautious policies amount to an outright ban on the drug.

On June 3, Transport Canada implemented a new policy that prohibits pilots, cabin crew and flight controllers from consuming cannabis for at least 28 days before being on duty. The transportation agency says the change is in line with the “best available science” and is consistent with other government departments, including the Department of National Defence and the RCMP. The policy doesn’t preclude airlines from enacting their own, even stricter rules – Air Canada, WestJet Airlines and Jazz have all imposed total bans on cannabis use for employees directly involved in flight operations.

Researchers, clinicians and cannabis advocates say that much is still unknown when it comes to cannabis impairment – including how long the drug’s effects on the brain last and how to measure whether someone is impaired. That has left many employers operating in the dark when trying to implement policies that will keep workers and the public safe now that new federal laws have made cannabis more readily available.

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“At the heart of this is that we really don’t know," said Scot Purdon, a neuropsychologist with Alberta Hospital Edmonton.

“We don’t have the data, we don’t have the research to clearly back up a precise estimate of how long one would have to wait until the retroactive effects of cannabis are gone," added Dr. Purdon, who is also a clinical professor of psychiatry at the University of Alberta.

And unlike with alcohol, where there is a strong correlation between the concentration in the blood and how impaired a person is, the link for cannabis use is much murkier. Many factors play in to how impaired someone becomes after consuming cannabis, including what they ate, what strain they used, how much experience they have with the drug and whether they smoked or ingested it, said Shelley Turner, a family physician who has prescribed medical cannabis to thousands of patients.

“It is really very individual, and that is one of the challenges for us in doing clinical research," Dr. Turner said.

Drug testing also poses a challenge, because of the drug’s tendency to linger in fat tissue long after its effects have dissipated. The length of time that trace amounts of cannabis can remain in an individual’s body depends on factors such as how frequently the person uses the drug and how much fat tissue they have, but researchers say it can take up to three months for some.

And if that fat tissue suddenly breaks down, such as due to a period of rapid weight loss or a spike in cortisol during a stressful landing, those trace amounts of cannabis could re-enter the blood, said Brent Guppy, a postdoctoral research fellow at the University of Manitoba who specializes in how drugs affect the body.

“Would it be at a high enough concentration to cause impairment or inebriation?” Dr. Guppy suspects not, but adds, “the data on that isn’t out there.”

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Teamsters Canada, a union representing transportation industry workers including those in the air, rail and trucking industries, firmly opposes the 28-day policy, calling it “harsh, unfair and ineffective."

No reasonable person can argue that someone can be impaired 27 days after using cannabis, said Christopher Monette, the union’s director of public affairs.

“In our opinion, it doesn’t strike the right balance between safety and the rights of workers, and it’s frankly an attempt to limit and to regulate what workers do when they’re off the job, when they’re on their own time, which is wrong,” Mr. Monette said.

Some cannabis advocates questioned what scientific research Transport Canada drew from when crafting the policy. The transportation agency said it consulted research on pharmacokinetics, or the movement of drugs through the body, half-life elimination, which is the amount of time that it takes for the concentration of the drug in the blood to be halved, as well as the resolution of cognitive performance effects.

“The work of Dr. Harrison Pope, often cited by other scientists in the field of cannabis study, is part of the collected body of research on cannabis which was reviewed in the development of the 28-day cannabis use policy,” Frédérica Dupuis, a spokeswoman for Transport Canada, said in an e-mail.

Dr. Pope, a professor of psychiatry at Harvard Medical School, published a study in 2001 that looked at how long cognitive deficits lasted in cannabis users. It involved 180 participants divided into three groups: heavy users, formerly heavy users who had decreased their usage, and those who had smoked cannabis no more than 50 times in their lives. All of the participants stopped consuming the drug for 28 days and underwent a series of cognitive tests on Days 0, 1, 7 and 28.

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The study found that long-term, heavy cannabis users had an impaired ability to remember word lists at least seven days after they stopped using their drug. But when they were tested 28 days after their last use, they showed “virtually no significant differences” from the control group on 10 neuropsychological tests. Because the participants weren’t tested between Day 7 and Day 28, it’s impossible to say how long it took for the effects to dissipate.

“These findings suggest that cognitive deficits associated with cannabis use persisted at least seven days, but could not be detected with our measures after 28 days,” the study concludes.

Ultimately, regardless of the lack of scientific research, employers have a right to remove cannabis users – even those with a medical need – from safety-sensitive positions, said Damian Rigolo, a partner and employment and labour lawyer at Osler, Hoskin & Harcourt LLP. A recent decision by the Supreme Court of Newfoundland and Labrador found that because it’s difficult or impossible for an employer to gauge how impaired a worker is, accommodating medical cannabis users would pose an undue hardship.

“May it be subject to challenge later? Maybe, particularly if the science on how to measure impairment and cannabis gets better and gets more accurate and gets more specific and detailed," Mr. Rigolo said. "But for now I’d rather err on the side of being overly cautious than not cautious enough, in respect of industries like airlines and train transportation.”

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