I read with great interest the article by Lira et al.1 assessing the involvement of cannabis and alcohol in motor vehicle crash (MVC) fatalities in the United States. The authors used data from the National Highway Traffic Safety Administration Fatality Analysis Reporting System (FARS) to examine trends in cannabis- and alcohol-involved MVC deaths from 2000 to 2018. The authors should be commended for their efforts in producing a significant step in our understanding of cannabis involvement in relation to alcohol involvement in MVC fatalities in the United States. However, I do have some comments regarding the analysis.
The authors note that FARS reports a maximum of three drugs involved in MVCs based on hierarchy of importance (narcotics, depressants, stimulants, hallucinogens, and then cannabinoids).2 Based on the availability of three involved drugs, it is not justified why the authors chose not to include analyses on the presence (or absence) of a third drug reported in cases in which cannabinoids and alcohol were already present. It was previously observed in a 2014 study by Wilson et al. on fatal MVCs using FARS data that three or more drugs were detected in a significant number of individuals.3 While the authors mention controlling for the presence of a third drug as a confounder as part of an adjusted model, it was not selected as a predictor, which prevents the identification of significant associations that may have explained the observed trends.
When one is assessing the implications of cannabis prevalence in MVC fatalities, the drug testing procedures and classification also warrant consideration. The authors describe the impairing effects of Δ9-tetrahydrocannabinol (THC) in cannabis on driving ability as important support for cannabis being a risk factor for MVCs; however, FARS only reports collapsed data on cannabinoids, with individual cannabinoids not being analyzed. In addition, FARS currently has no standardized drug testing protocol across states.4 This may limit the generalizability of these findings, as it cannot be determined whether THC was involved.
Polysubstance use is highly prevalent in the United States,5–7 yet the effects of driving under the influence of multiple substances are not fully understood. It is critical to consider the extent of the decedents’ polysubstance use when one is assessing cannabis and alcohol involvement trends. By not considering the full extent of decedents’ polysubstance use, Lira et al. may have missed significant associations that would help further solidify the study as an important step toward our understanding of cannabis- and alcohol-involved MVC fatalities.
ACKNOWLEDGMENTS
A. McCabe was supported by the University of Florida Substance Abuse Training Center in Public Health from the National Institute of Drug Abuse of the National Institutes of Health under award T32DA035167 (PI: Cottler).
Note. The content is solely the responsibility of the author and does not necessarily represent the official views of the National Institutes of Health.
CONFLICTS OF INTEREST
The author reports no conflicts of interest.
REFERENCES
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