More than half of people who take medical cannabis for chronic pain say they’ve driven under the influence of cannabis within two hours of using it at least once in the last six months, according to researchers from the University of Michigan Addiction Center.

For the study, published in the journal Drug & Alcohol Dependence, 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 of participants reported driving within two hours of using cannabis, 51 percent reported they drove while a “little high”, and 21 percent reported driving while “very high”.

“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.”

Boner 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 crash, because they would not be able to respond as quickly. He added that 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.

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