As marijuana becomes increasingly legalized in states across the United States, it has created new challenges to policing the impairment level of drivers on the road. While recent studies have identified new biomarkers that can be used to estimate an individual’s cannabinoid intake, a commentary published by toxicologists in the journal Trends in Molecular Medicine says that using these markers to judge cognitive and behavioral impairment can be complex.
“There is no one blood or oral fluid concentration that can differentiate impaired and not impaired,” explains Marilyn Huestis, who spent over 20 years leading cannabinoid-related research projects at the National Institute on Drug Abuse. “It’s not like we need to say, ‘Oh, let’s do some more research and give you an answer.’ We already know. We’ve done the research.”
Whether the primary psychoactive agent in cannabinoids – ∆9-tetrahydrocannabinol (THC) – is inhaled or consumed, there are a variety of factors that can influence how impaired an individual may be. And although alcohol can impair a driver more than cannabis, impairment is still a problem when it comes to driving while high.
One problem with trying to use blood tests for THC is that it quickly leaves the bloodstream. An occasional user may be impaired for 6 to 8 hours, but blood THC concentrations can drop to zero after just 2.5 hours, according to previous research by Huestis. “If someone is driving impaired, by the time you get their blood sample, you’ve lost 90% or more of the drug. So, we have to change what we do at the roadside,” she says.
Furthermore, long-term daily cannabis users – such as those who use medical marijuana – present additional challenges. THC accumulates in body tissues and then slowly releases over time. This means that chronic users can test positive for marijuana even if they’ve gone 30 days without using. Additionally, psychomotor impairment can be observed even three weeks after the last dose. “You want people to be taking medicinal cannabinoids and now you know that their driving is going to be impacted,” says Huestis. “So how do you handle that problem?”
Like most researchers, Huestis doesn’t support a legal driving limit for marijuana, like the one in place for blood alcohol concentrations. Rather, she believes the best solution is to train police officers to identify behavioral signs of impairment, accompanied by less invasive biological marker tests that can be immediately performed at the roadside to confirm the presence of a cannabinoid. Recent research has identified new blood and urine biomarkers to that end. Tests using breath and saliva markers are also under development.
Beyond just catching drivers who are under the influence, these new markers and tests may also be used to help treat drug dependence, as they can determine appropriate therapeutic levels of medical marijuana. They could also be used to help women monitor cannabinoid levels if they’re trying to become pregnant. While potentially tricky and complex, this new research is helping people who use or need medical marijuana, as well as the general public that want to stay safe on the road.