A McMaster University study shows recent cannabis users are more likely to show some memory problems and decreased reaction time compared to people who haven’t used the drug.

“There are clear lingering effects,” said McMaster researcher James MacKillop, who led the study. “Not huge and not everywhere, but lingering effects nonetheless.

“If the stakes are very high and split-second performance really matters, these small effects could be meaningful,” said MacKillop, a psychiatry professor who is also director of the Michael G. DeGroote Centre for Medicinal Cannabis Research.

The McMaster study used data collected as part of a large ongoing U.S.-based research study called the Human Connectome Project, which has amassed a huge database of information from more than 1,000 people.

MacKillop’s team looked at the neuropsychological test results from 135 people in the database, who had positive urine screens for tetrahydrocannabinol (THC), the psychoactive chemical in cannabis.

The team then compared the results on a wide battery of cognitive tests, between those who had screened positive for THC and those who hadn’t.

There were statistically significant differences noted for episodic memory — piecing together the what, where and when of a specific episode — and reaction time.

For other types of cognition — attention, working memory, impulsivity — there were no real differences seen between those who tested positive for THC and those who didn’t.

MacKillop said the deficits for episodic memory and reaction time could have an occupational significance.

“We think this is important because there are certain professions where their employment may have safety-sensitive situations, or require split-second decision-making,” said MacKillop.

Some examples could include police officers, firefighters, air-traffic controllers or even surgeons.

“In those professions, any kind of detriment in cognitive performance could be really significant,” said MacKillop.

Unlike alcohol, which is metabolized relatively quickly, cannabis can stay in tissues much longer. It’s absorbed into fatty tissues and can circulate in the bloodstream in detectable levels long after consumption.

“It can stay in your system for a month if you’re a heavy user,” said MacKillop.

“On the other hand, if you smoke once a month and you’re extremely active, the window it would be detectable would be much shorter.”

Hamilton Police Service is working on a draft form of a new recreational cannabis use policy, which will require officers to be “fit for duty.”

“Fit for work/duty means that a member is mentally, emotionally and physically able to safely and competently perform assigned duties, without any limitations attributable, but not limited to, illness, injury, fatigue, mental stress or the use and/or after-effects of alcohol or drugs,” according to an earlier statement by police spokesperson Jackie Penman.

MacKillop also said the evidence suggests that once THC has cleared the body, the cognitive results return to normal.

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The McMaster study was published in the Journal of Psychiatry and Neuroscience last month.

Correction - July 25, 2019: This article was edited from a previous version that misstated the name of the U.S.-based research study, the Human Connectome project.

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