Fifty-six percent of those who take medical marijuana for chronic pain admit that in the last six months they've driven under the influence of marijuana within two hours of using it.
About one in five report that they've driven while "very high" at least once in the last six months, and about half say they've driven while "slightly high."
That's according to a recent University of Michigan study of almost 800 Michiganders who were applying for medical marijuana re-certification or a new certification for chronic pain in 2014 and 2015.
Lead author Erin Bonar, Ph.D., assistant professor of psychiatry and a clinical psychologist at the UM Addiction Treatment Services, said, "Those percentages don't add up to 100% because they are separate questions. These people overlap. They could have said yes to all three questions."
Bonar said mixing marijuana and driving is risky.
"Using marijuana can result in changes in your reaction time, how well coordinated you are, and we need those functions when we're driving to respond to unexpected events," said Bonar.
Bonar said more research needs to be done to establish guidelines about when it is safe to drive after using marijuana. She said it is even more important to understand the risks of driving under the influence of marijuana now that recreational, as well as medical, marijuana use has been approved in Michigan.
"We don't know yet how different doses of different products can affect your driving ability," Bonar said. "Is there a safe level and how can you figure out what that level is for yourself depending on the types of marijuana you use, how you use it, and how much you use?"
"The safest strategy would be not to drive at all on a day that you've been using marijuana," said Bonar. "Or at least avoid driving for several hours after using marijuana."
Bonar said there is uncertainty about how marijuana might affect driving for chronic daily marijuana users.