Skip to main content

Cannabis Is there a better way to measure how cannabis causes impairment?

Part of Cannabis and your health

Dr. Tom Schweizer demonstrates the use of MRI equipment that performs brain scans while people use a driving simulation, after consuming cannabis, in Toronto on Oct. 22, 2018.

Mark Blinch

Police who suspect people are driving under the influence of cannabis may ask them to perform a roadside sobriety test, or have them give blood or saliva samples. But even policy-makers recognize these methods, which work reasonably well for alcohol, are unreliable when applied to cannabis.

Now in labs across North America, the race is on to find a way to objectively measure cannabis-induced impairment. Researchers are trying to develop a tool for cannabis, akin to the Breathalyzer test – whether it is by identifying some yet-unknown compound in bodily fluids, tracking brain signals, or measuring how well people perform on tests taken on a mobile digital device.

The current system is imperfect because while drug tests can measure how much cannabis someone has in their system, they cannot tell how the drug is affecting that person’s ability to function, since impairment depends on many factors, says Dr. Ryan Vandrey, an associate professor of psychiatry and behavioural sciences at Johns Hopkins University School of Medicine in Baltimore. People have varying tolerance of the drug and different modes of consumption can produce different results: When given the same amount of cannabis, for example, long-time users tend to be less impaired than first-time users, and people tend to be more impaired when they vapourize the drug than when they smoke it.

Story continues below advertisement

“There’s no cut-off value for THC [the main psychoactive compound in cannabis] in blood or a metabolite in saliva or urine that can reliably determine whether someone is acutely impaired,” Dr. Vandrey says. “You can miss it on both sides: You can have people who are not impaired test positive, and you can have people who are impaired test negative.”

So Dr. Vandrey and his collaborators are now studying ways of measuring how high people are, which includes searching for alternative markers in bodily fluids that may more reliably determine impairment. Whether these markers may be other components from the cannabis plant, or some kind of protein that changes in blood or urine, the researchers do not yet know. They are analyzing biological specimens, and comparing them with data from behavioural tests of research participants to see if any patterns arise.

“It’s very exploratory,” he says.

At Harvard Medical School, Dr. Jodi Gilman is taking a different approach. Instead of looking for anything traceable in bodily fluids, she is looking for distinct patterns in brain activity.

Over the past two years, Dr. Gilman and her colleagues have been testing a “brain cap,” a hood equipped with sensors. Using a technology called functional near infrared spectroscopy, the cap monitors the concentration of oxygenated blood in various parts of the brain. Dr. Gilman says its use shows promise, so far, in distinguishing research participants who have been given THC from those who have not. But it may be years before her work translates into a simple, portable, roadside test.

One of the things that still needs to be sorted out in the quest for a reliable cannabis impairment test is what the brain looks like while high. At Toronto’s St. Michael’s Hospital, Dr. Tom Schweizer is scanning the brains of volunteers on different doses of prescription cannabis, while they perform a driving simulation test in a functional magnetic resonance imaging (fMRI) system.

He hopes his small pilot study, involving 12 participants, will shed some much-needed light on how the brain works under the influence of the drug. Existing studies have tended to narrow in on the effects of cannabis on the brain while performing simple tasks, such as memory or attention exercises. But when it comes to driving, an activity that engages multiple areas of the brain, Dr. Schweizer explains, it is yet unknown how different parts of the organ activate and shut off, and whether certain areas are able to compensate for others, when under the influence of the drug.

Story continues below advertisement

“It could be that cannabis has a preferential effect on certain areas of the brain, which we don’t know, because nobody has looked at this whole vast network,” he says.

In the meantime, other researchers, like Dr. Michael Milburn in Boston, are betting on the promise of mobile apps.

Milburn, a professor of psychology at the University of Massachusetts, has developed an app called DRUID to measure cognitive and behavioural impairment based on a series of simple, computer game-like tasks that test the user’s reaction time, hand-eye coordination, balance, decision-making and attention. The tasks take minutes to complete.

Similarly, DriveABLE, an Edmonton-based start-up, has also created a digital cognitive assessment tool to test whether individuals are at risk of impairment.

Currently, neither DRUID nor DriveABLE can tell whether someone is impaired due to cannabis or other reasons. But as more people use these tools and generate more data, researchers believe they may be able to eventually distinguish specific ways users respond when they are on the drug.

Report an error Editorial code of conduct
Due to technical reasons, we have temporarily removed commenting from our articles. We hope to have this fixed soon. Thank you for your patience. If you are looking to give feedback on our new site, please send it along to feedback@globeandmail.com. If you want to write a letter to the editor, please forward to letters@globeandmail.com.

Welcome to The Globe and Mail’s comment community. This is a space where subscribers can engage with each other and Globe staff. Non-subscribers can read and sort comments but will not be able to engage with them in any way. Click here to subscribe.

If you would like to write a letter to the editor, please forward it to letters@globeandmail.com. Readers can also interact with The Globe on Facebook and Twitter .

Welcome to The Globe and Mail’s comment community. This is a space where subscribers can engage with each other and Globe staff. Non-subscribers can read and sort comments but will not be able to engage with them in any way. Click here to subscribe.

If you would like to write a letter to the editor, please forward it to letters@globeandmail.com. Readers can also interact with The Globe on Facebook and Twitter .

Discussion loading ...

Cannabis pro newsletter