A new study says that over half of Michigan's medical marijuana patients have driven while under the influence of cannabis at least once in the past six months.

The study, published by the University of Michigan Addiction Center, surveyed 790 adults at medical marijuana clinics from February 2014 through June 2015.

Some of the key findings indicate that more than half of patients surveyed say they've operated a motor vehicle within two hours of smoking pot. One in five say they've driven while 'very high' at least once in the last six months.

Nearly 270,000 adults in the state of Michigan have state approval to use marijuana for medical purposes.

By the numbers:

  • 56.6% of medical marijuana users reported driving within two hours of using cannabis.
  • 21.1% reported driving while 'very high.'
  • 51.5% reported admitted to driving while 'a little high.'
  • Patients with higher pain levels were less likely to drive while under the influence.

Lead author Erin E. Bonar, Ph.D., assistant professor of psychiatry and a practicing clinical psychologist at the U-M Addiction Treatment Services says that ideally people should not drive the same day when they 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.”