Effects of marijuana on driving ability

April 9, 2014, 6:31 pm



Medical Marijuana and Driving: a Review. Neavyn MJ et al. J Med Toxicol 2014 Mar 20 [Epub ahead of print]


This excellent and comprehensive review of literature related to the possible effects of marijuana on driving skills is a must-read for anyone dealing with forensics and medical-legal cases related to the topic, or consulting with government agencies on regulatory issues. It is also helpful for any clinician who might have to counsel patents about driving and the use of medical marijuana. The 70-item reference list alone will be a great resource for further study.

As the article points out, unlike studies of alcohol, the literature relating use of cannabis, drug levels, and driving impairment has not consistently found a reliable connection. This will make legislating controls difficult and open to challenge.

This entire review is worth reading for those with an interest in the area. Here are some key points made by the authors:

  • The THC metabolite THC-COOH can be present in the urine for weeks after last exposure.
  • Some studies finding no relation between marijuana use and driving impairment used urine drug screens for THC-COOH as a marker for use, a clear invitation to bias.
  • Studies have indicated that marijuana use impairs visual tracking, reaction time, and attention.
  • Blood THC concentrations are only measurable in the first 2 hours after smoking, but cognitive and motor effects may last for 8 hours or longer.
  • A negative blood test for THC does not rule-out impairment from marijuana.
  • Whereas laboratory studies have consistently demonstrated marijuana-induced impairment of vision, coordination, and attention, driving-simulation studies have not been as clear.
  • Studies demonstrate that alcohol impairs driving ability along with the subject’s ability to perceive the impairment; in contrast, cannabis makes the subject feel he is more impaired than he actually is.

Finally, the authors recommend that those with a subjective “high” from medical marijuana refrain from driving for a least 8 hours.

[NOTE: The original post stated that this article would not be that useful for clinicians. On reflection, I believe that was wrong for the reason stated above. As a result, I have also upgraded the paper to 4.5 skulls.]
Related post:

Driving under the influence (of marijuana)


  1. fisolani Says:

    Studies demonstrate that alcohol impairs driving ability along with the subject’s ability to perceive the impairment; in contrast, cannabis makes the subject feel he is more impaired than he actually is.

    The studies in MVA show that Newtonian relationships between the fourth power of small increases or reductions in speed and large increases or reductions in deaths state the case for speed control as the most important factor in reducing traffic fatalities. Alcohol is involved in 40% of motor vehicle accidents in accidents involving only motor vehicles.

    I am fairly confident that by driving more slowly, the stoned drivers out there are actually decreasing fatality rates, not only because they are drinking less, but also because they are driving more slowly. Maybe driving is like playing music, and Willie Nelson can sure play music, and he plays it slow, or fast and damn well for an 80 year old. In cases in which a pedestrian is hit by a motor vehicle, 40% of the time the pedestrian is drunk. I don’t know to what extent THC levels are a factor in pedestrian vs MVA accidents, but I don’t think it is close to 40%. I think the most logical approach would be some kind of real time coordination test via the automobile navigation system. I suspect all but the most neophyte teenage stoner driver post dab would pass such a test at rates no different and perhaps higher than the population at large.
    Anywhoo…love TPR, first time visitor, thank you!

  2. Leon Says:


    Thank you for your comments. You make several interesting points.

    In reply, I’d say that the proper comparison isn’t between on person driving under the influence of marijuana and another significantly drunk, but between a driver who has recently used a cannabinoid and another who is not under the influence of any intoxicant.

    I’d say that driving more slowly than usual has not been shown to be safer than driving at normal speeds while not intoxicated at all. In addition, it is not at all clear that legalizing marijuana will cut down on the use of alcohol or the incidence of alcohol-related crashes. It will be interesting to see that data that come out of the Colorado and Washington experiments.

    I’m very pleased that you’ve discovered and are enjoying TPR!