Driver smoking medical cannabis. When people drive under the influence of marijuana their reaction time and coordination may be slowed down and they could have a harder time reacting to the unexpected. (Photo: iStock)

More than half of people who take medical cannabis for chronic pain say they have driven under the influence of cannabis within two hours of using it at least once in the last six months, according to a new study.

One in five said they had driven while “very high” in the past six months, researchers from the University of Michigan Addiction Center reported in a new paper in the journal Drug & Alcohol Dependence.

Hundreds of thousands of Americans have state approval to use medical marijuana, including nearly 270,000 in the state of Michigan, according to Statista, as of May 2018. Michigan is second only to California for the highest number of medical marijuana patients in a state.

Lead author Erin E. Bonar, Ph.D., assistant professor of psychiatry and a practicing clinical psychologist at U-M Addiction Treatment Services, found the results of a survey of 790 Michigan medical cannabis users troubling.

Reaction time & coordination may be slowed down

Bonar said that when people drive under the influence of marijuana their reaction time and coordination may be slowed down and they could have a harder time reacting to the unexpected. If they are in a risky situation, they could be more likely to be involved in motor vehicle crash, because they would not be able to respond as quickly.

Related: Marijuana use could be jeopardizing safety on the roads, survey finds

For the study, the team surveyed adults in Michigan who were seeking medical cannabis recertification or a new certification for chronic pain in 2014 and 2015. The researchers asked about respondents’ driving habits for the past six months.

Fifty-six percent (56%) of participants reported driving within two hours of using cannabis, 51% reported they drove while a “little high,” and 21% reported driving while “very high.”

Safest strategy: Not drive at all on day you use marijuana

“There is a low perceived risk about driving after using marijuana, but we want people to know that they should ideally wait several hours to operate a vehicle after using cannabis, regardless of whether it is for medical use or not,” Bonar said. “The safest strategy is to not drive at all on the day you used marijuana.”

There is uncertainty about how marijuana could affect driving for chronic daily users, who might have even longer-lasting effects that linger in their system, Bonar added.

To add complexity to the issue, in November Michigan voters approved the use of recreational marijuana in the state. In early December, it became legal under state law for any Michigan resident over the age of 21 to use marijuana inside a private residence, and to grow up to 12 plants for personal use. Retail sales are only allowed for those with medical marijuana cards issued by the state.

Related: Fatal crashes doubled after state legalized marijuana, AAA says

In light of this policy change, Bonar added, all cannabis users need a clear understanding of the side effects of this drug.

“When it comes to driving, we haven’t yet figured out the best way to know how impaired marijuana users are at any given time,” she said. “With alcohol, you can do some quick math based on the amount you drank, and take an educated guess at your blood alcohol level. For marijuana, an estimate like this would be complicated. It’s hard to quantify because there is a lot of variation in marijuana dosing, THC potency, and route of administration. We also don’t have specific guidelines yet about when exactly it would be safe to operate a vehicle.”

Bonar said the goal of her team’s study — conducted before the passage of the ballot question that resulted in the change in state law — was to help medical marijuana users to be safer on the roads.

Understand the risks

“We believe more research is needed to inform a larger public education effort that will help individuals understand the risks for themselves, and others, of driving while under the influence of cannabis,” she said. “It is especially needed during this time of rapid policy change as many states are determining how to manage marijuana legalization. We also need clearer guidelines about marijuana dosing and side effects with an understanding of how individual differences in things like sex and body weight interact as well.”

Related: Wide support for driving impairment standards for marijuana