Cannabis and Driving – New Data

The Wall Street Journal published a piece this week about cannabis and driving. While well meaning, the article provides data from old studies, along with some commentary by cherry picked “experts.” The bottom line from the article is that more studies need to be done on this subject.

A new study published in the April 2024 edition of the journal Accident Analysis & Prevention (https://doi.org/10.1016/j.aap.2024.107459) is an important advance in the understanding of the relationship between cannabis consumption and driving. The authors examined motor vehicle crash data in three cannabis legal states, and assessed the association of alcohol and cannabis on accident rates.

This study found that, as expected, higher blood alcohol levels increased crash risk. Blood levels of THC, on the other hand, showed no association with crash risk. In fact, among regular cannabis users, THC levels decreased the risk of an accident. I quote the Abstract from the article:

Background

The objective of this study was to examine the relationship between cannabis and alcohol use and occurrence of motor vehicle collision (MVC) among patients in the emergency department (ED).

Methods

This was a cross-sectional study of visits to EDs in Denver, CO, Portland, OR, and Sacramento, CA by drivers who were involved in MVCs and presented with injuries(cases) and non-injured drivers (controls) who presented for medical care. We obtained blood samples and measured delta-9-THC and its metabolites. Alcohol levels were determined by breathalyzer or samples taken in the course of clinical care. Participants completed a research-assistant-administered interview consisting of questions about drug and alcohol use prior to their visit, context of use, and past-year drug and alcohol use. Multiple logistic regression was used to estimate the association between MVC and cannabis/alcohol use, adjusted for demographic characteristics. We then stratified participants based on levels of cannabis use and calculated the odds of MVC across these levels, first using self-report and then using blood levels for delta-9-THC in separate models. We conducted a case-crossover analysis, using 7-day look-back data to allow each participant to serve as their own control. Sensitivity analyses examined the influence of usual use patterns and driving in a closed (car, truck, van) versus open (motorcycle, motorbike, all-terrain vehicle) vehicle.

Results

Cannabis alone was not associated with higher odds of MVC, while acute alcohol use alone, and combined use of alcohol and cannabis were both independently associated with higher odds of MVC. Stratifying by level of self-reported or measured cannabis use, higher levels were not associated with higher odds for MVC, with or without co-use of alcohol; in fact, high self-reported acute cannabis use was associated with lower odds of MVC (odds ratio [OR] 0.18, 95% confidence interval [CI] 0.05–0.65). In the case-crossover analysis, alcohol use alone or in combination with cannabis was associated with higher odds of MVC, while cannabis use alone was again associated with decreased odds of MVC.

Conclusion

Thus, in this real-world study, with real accident data, cannabis appears to have minimal effects on driving ability. This study suggests that those with tolerance to THC may be less impaired than control subjects after cannabis consumption.

A surprising conclusion indeed.

This important data needs to be widely disseminated to inform policy on cannabis use.