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.]