If rules about cannabis for Canadian police are any measure, we know precious little about how long cannabis impairment lasts. The wait time for police to show up for work afterward from anywhere in the next day to 28 days, with some forces prohibiting members from using the drug altogether

University of Alberta psychiatrist Scot Purdon agrees the science is scant on impairment beyond the first few hours. But what we do know, according to his latest research, is that cannabis use impairs "new verbal learning," or the ability to remember what you have just read, for 12 to 24 hours after.

That means taking up stress relief during exams may not be the best idea. "If you are a high-school or college student trying to study for exams, you'll have problems the next day," he said. "It's a time when students might consider cannabis to relax or reduce stress, but it could compromise their ability to remember what they've studied."

Long cannabis impairment

The good news is that within one to three days, that impairment is cut in half, said Purdon. Between three and seven days after use, it diminishes even more, with test scores returning to near normal.

But I have added that everyone has a different reaction to the intoxicating effects of cannabis, so the residual effects are bound to differ as well. "In some people, memory problems may clear quicker than sensory problems or the motor problems." We just do not know. "

Cameron Wild, a researcher at the U of A's School of Public Health and one of Purdon's collaborators, agreed to perform a cannabis use, tolerance, dosage and strength of cannabis consumed, and mode of use. Edibles, for example, are associated with a slower onset of intoxication and performance deficits. Slower onset of intoxication and performance deficits.

For the past year, Purdon, Wild and Alexander Penney of MacEwan University have been exploring how cannabis duration and frequency use-and the age of the user may contribute to impairment levels. Duration and frequency of cannabis use—and the age of the user—may contribute to impairment levels.

The team examined 120 young people between the ages of 18 and 35 described as heavy users-those who consume at least one gram of cannabis at least eleven a week-to gauge the lingering effects of abstinence after at least 12 hours.

There are more than enough studies to suggest that, in the hours immediately after, "if you are high, you seem to do more poorly on most things," said Purdon. "We have good data on what happens after a few hours, but remove shallow data after a day or two or three."

Subjects in Purdon's study took standardized performance tests to measure verbal learning proficiency and verbal fluency-how fast you can talk as well as memory retention of words and images. Other tests measured executive skills, or how fast you can initiate a plan of action without tripping over yourself.

As for when someone is ready to return to work after using the drug, a safe limit would be somewhere around 72 hours, said Purdon, "but I think it would probably be less than that.

"Some of the longer-term wait times, such as 28 days, are presumably based on how quickly cannabis leaves the body," he said. "First of all, I'm not sure it's clear in 28 days; we certainly have patients testing positive in urine after 28 days."

More importantly, I have added, there is a crucial difference between the presence of tetrahydrocannabinol (THC) in the body and the length of time it remains psychoactive.

"It's not the same thing at all. The THC metabolites we're picking up in urine or in the blood are not the same neuroactive components" that get you high, he said. It is why Canadian Blood Services does not test for THC in blood donations, "because they acknowledge that the blood bank is safe even if there are metabolites present."

Purdon said there is any number of questions about cannabis impairment that need better answers. With legalization, however, those answers are likely to come soon. In the meantime, I cautioned against panic.

"Cannabis has been in widespread use since at least the '70s. We are not going to see this huge flood of,' Now I want to get high and go to work. ' But we will be much more attentive to it than we were before. "