Discussing the Impact of Marijuana on Driving

February 25, 2014

I think it's really interesting when hot topics in the news coincide with questions raised in my clinical practice, such as last week when the New York Times published an article about the effects of marijuana on driving. Since I see a fair number of teens in my office, I've had some conversations regarding the impact of different substances (e.g., alcohol, nicotine, marijuana) on various developmental tasks, including driving.

The New York Times article raised a few interesting points:

  • Although the limited body of work exploring the impact of THC (tetrahydrocannabinol, the active ingredient in marijuana) on driving might be described as "mishmash," generally accepted results reveal a two-fold risk of a motor vehicle crash among drivers with any THC in the bloodstream.
  • While laboratory studies of people with THC in their bloodstream do not support significant impairment on single tasks, such as memory, addition, or subtraction, there may be more significant impact on multitasking and handling unexpected events (which are critical components of safe driving).
  • Detecting clinically significant levels of THC in the body can be complicated. Urine levels, which in many states are equated to blood levels (and are both illegal), can reflect past use and may not reliably detect people who are actually "high".
  • Some researchers suggest that resources are better directed toward reducing drinking and driving or reducing the mixing of alcohol and marijuana while driving.

The bottom line for me: 

Marijuana is considered illegal for essentially all of my patients, so there's really no question of whether they should be using it or not. As for the science, there is still much more work needed to understand the full impact of THC on driving, though current studies do suggest at least some increased risk for motor vehicle accidents. I always counsel that any driver should be wary of taking any substance that may be mind-altering, as the tasks required for driving are complex and require much higher order thinking than many may realize.