Cannabis exposure is not strongly associated with an increased risk of motor vehicle accident, according to a meta-analysis of 24 epidemiological studies published in the journal Frontiers in Pharmacology.
Investigators at the University of Bucharest, Department of Medicine and Pharmacy reviewed data from two-dozen studies assessing the association between cannabis-positive drivers and unfavorable traffic outcomes, such as motor vehicle collision, injury, or death.
Authors concluded: “Our analysis suggest that the overall effect size of DUIC (driving under the influence of cannabis) on UTEs (unfavorable traffic events) is not statistically significant.”
The researchers also identified a number of limitations in the original research studies, most notably the fact that some studies relied on inaccurate blood tests for THC levels. “A positive test for cannabis (i.e. blood) does not necessarily imply that drivers were impaired, as THC/metabolites might be detected in blood a long time after impairment, especially in chronic cannabis users, which could also induce an important bias in the analysis of the results,” the researchers wrote.
THC can remain in an individual’s system for weeks, and a blood test for THC levels cannot accurately differentiate between a driver who is currently stoned and a driver who is sober but may have used cannabis in the past weeks.
“Simply identifying cannabis use in a driver is not enough to justify the assumption of an increased risk for UTEs,” they concluded. “When such a result is obtained, it should be corroborated with either quantitative data regarding cannabis use, or a clinical assessment of the driver, before establishing his (or her) fitness to drive.”
Due to the limitations of the study, further research will need to be performed in order to definitively determine the effects of cannabis use on driving. In Colorado, researchers are using legal cannabis users as subjects for a variety of new studies on the topic. Several studies are currently being conducted where participants are asked to take a variety of driving-related, motor control, and cognitive tests after hitting their own dab rigs or using other cannabis products of their choice.