LOS ANGELES (CBS) A new study of patients who use medical cannabis to treat chronic pain finds more than half say they've driven under the influence in the past six months.
"We know that using marijuana can affect things like your coordination or your reaction time and those are critical functions for driving," study author Erin Bonar.
The study at the University of Michigan Addiction Center surveyed nearly 800 medical cannabis users. It found 51 percent reported driving a "little high" while 21 percent drove "very high."
Researchers say the findings are concerning as more states legalize medical and recreational marijuana. Medical marijuana is currently legal in more than 30 states.
Bonar recommends no driving at all after using cannabis or waiting several hours before getting behind the wheel.
"We just don't know how long the amount a person uses is going to be in their system and affect their driving. We know things about how alcohol affects our body. How much I can drink at this body weight over what period of time and that is pretty standardized," Bonar said. "We just don't have that information yet for marijuana."
With no gold standard yet for determining how high a person is or how impaired they are from marijuana, researchers are developing marijuana breathalyzers and other roadside tests.