Driving under the influence of cannabis is common among patients prescribed medical marijuana for chronic pain, in findings investigators describe as “concerning.”
A study of nearly 800 individuals prescribed medical marijuana showed more than half reported driving within 2 hours of use. About the same proportion said they drove “a little high,” and about one in five reported driving “while very high.”
“I was particularly interested in understanding how people using [cannabis] medically may also be engaging in this risky behavior,” Erin E. Bonar, PhD, University of Michigan Addiction Center and assistant professor of psychiatry at the University of Michigan School of Medicine in Ann Arbor, told Medscape Medical News.
“I wanted to see what people are doing [and] how often they are doing this,” Bonar added. “We found a pretty concerning rate.”
A meta-analysis published in 2016 called attention to the public health risks associated with driving under the influence of cannabis (DUIC), including a potential increase in motor vehicle accidents.
However, another meta-analysis published last year revealed that cannabis use was not significantly related to “unfavorable traffic events,” including motor vehicle accidents.
“Although prior literature regarding DUIC is mixed, rapidly shifting cannabis policies in the US warrant greater attention to this important public health issue,” the current researchers write.
“There is an urgent need to better understand DUIC in order to inform future prevention efforts,” they add.
Bonar and colleagues recruited participants 21 years and older seeking certification or recertification for medical marijuana. The 790 patients (mean age, 46 years; 52% men; 81% white) came from three medical cannabis centers in Michigan between February 2014 and June 2015.
Most of the participants were unemployed (61%), and 32% were receiving disability benefits.
The current study is the first in which people using medical cannabis were asked about their driving, Bonnar said. It is also part of a larger, long-term study of patients taking medical marijuana.
The investigators asked about cannabis use, for example, in terms of hours high per day and average quantity per week in the previous month.
They also asked participants the number of times in the past 6 months they drove “within 2 hours of using marijuana,” “while a little high on marijuana,” or “while very high on marijuana.” Possible responses ranged from “never” to “more than 10 times.”
Majority Report DUIC
Nearly three quarters of participants (73%) reported using cannabis daily or almost daily in the previous 6 months. At the same time, 56% drove within 2 hours of cannabis use, 51% drove while a little high, and 21% drove while very high.
White individuals had a greater likelihood of DUIC within 2 hours or while a little high compared with other participants, logistic regression analyses indicated.
Age also played a factor, with younger adults at greater risk for DUIC. In addition, higher pain scores were associated with lower odds of DUIC.
“The thing that most piqued my interest or concerned me was not just the amount of people who said ‘Yes, I drove while a little high’ or ‘I drove while really high,’ but [that] some people did that frequently,” Bonar said.
In the group who drove under the influence of medical cannabis more than 10 times in the previous 6 months, 22% reported driving within 2 hours of cannabis use, 19% drove while a little high, and 7% drove while very high.
Patients who take medical cannabis may be at particularly high risk for DUIC because of their high frequency of use, the researchers note.
In addition, 4% of all participants reported a lifetime history of arrest for DUIC.
Although the study focused on risks associated with medical cannabis, participants were also asked about alcohol use. One goal was “to see if there was any overlap” between DUIC and driving under the influence of alcohol, Bonar said.
The study showed that 8% of the total group reported driving after having four or more drinks in the previous 6 months. The prevalence of this finding was about 13% for those who drove within 2 hours and for those who drove while a little high. The prevalence was higher (21%) among participants who drove very high.
Prevention Efforts Needed
“Results point to relationships between some demographic (eg, age, race) and substance use factors, particularly heavy episodic drinking and quantity of cannabis consumption as markers of DUIC risk,” the researchers write.
“Prevention efforts may specifically need to target simultaneous use of alcohol and cannabis,” they add.
The setting in which participants were asked about driving behaviors — the clinical visit for medical cannabis certification or recertification — could be an ideal opportunity to counsel patients about risks, the investigators note.
“Some physicians are already doing this, but we do encourage them to let their patients know they could be at risk if they get behind the wheel of a car after they use their medical marijuana,” Bonar said.
For example, patients may be unaware they could experience impaired reaction times, including reacting to the unexpected while driving, she added.
Risk counseling for medical marijuana is also essential because, unlike other prescriptions, medical cannabis does not carry a warning label.
“If you got a prescription today for an opioid, there would be a little label on it warning you to avoid operating heavy machinery, that it might make you drowsy,” Bonar said. “But we don’t have that type of regulation for marijuana.”
Counseling is not without its challenges, however, she said. Without a large evidence base on the potential public health risks associated with medical marijuana, “we don’t know how much affects you or how the way you’re using it affects you. So the best guideline to use it is to avoid driving — get a ride from someone else, use a ride share app, that sort of thing.”
Evolving regulations on the state level make it difficult for clinicians to keep pace with the number of states where medical cannabis and recreational cannabis are considered legal, adding more complexity to the DUIC issue, Bonar said.
“It’s such a messy issue with the differences between state and federal regulations, too,” she added. “We don’t know what is going to happen. People like to talk about this world like it’s the ‘Wild West’.”
It becomes a struggle for science to keep up with new regulations when “our society, our country, and our policies are rapidly changing,” she said.
Future research addressing a recommended standard ‘dose’ of medical cannabis across conditions would be helpful, the investigators note.
Incorporation of driving records and/or motor vehicle crash data could also enhance the findings of subsequent studies, they add.
In terms of next steps in their research, “We will continue to look at the outcomes and what happens to people over a couple of years while they are using medical marijuana,” Bonar said.
She added that it would be interesting to know whether people drive more or less over time, and whether those getting medical cannabis certification versus recertification behave differently.
“Part of what we seek to do in our research here at the addiction center is identify the best ways we can help people avoid these risky situations, the best ways to get them information, and the best ways to motivate people to change their behavior,” she said.
“Significant Clinical Value”
Commenting on the findings for Medscape Medical News, Jeramy Peters, DO, a fourth-year resident in psychiatry at Oregon Health Sciences University in Portland, said the study “is of significant clinical value” for those who treat patients using medical cannabis.
“Educating patients about the potential risks, benefits, and interactions of prescribed, ‘over-the-counter’ herbal or traditional medicines is an important aspect of high-quality medical care,” he said.
“As such, physicians should be able to have an informed discussion with their patients regards these substances. These data provide a preliminary understanding of the risk of driving under the influence of cannabis in those who use medical cannabis,” said Peters, who was unaffiliated with the current study but has coauthored a primer for clinicians on cannabis consumption.
Peters added that although the current study’s participants were not necessarily representative of the population at large, it did demonstrate that more than half of the medical cannabis users reported DUIC.
“Given that the CDC reports more than 32,000 deaths and 2 million injured in motor vehicle accidents annually, the clinical value of utilizing this article’s findings to provide evidence-based information to patients likely outweighs the risk of overgeneralizing its findings to those of different demographics,” he said.
The study was funded by the National Institute on Drug Abuse (NIDA), including a NIDA career development award, a career development award from the Department of Veterans Affairs, and a training grant from the National Institute on Alcohol Abuse and Alcoholism. Bonar and Peters have reported no relevant financial relationships.
Drug Alcohol Depend. Published online January 9, 2019. Abstract