Effects of Combining Alcohol and Cannabis on Driving, Breath Alcohol Level, Blood THC, Cognition, and Subjective Effects: A Narrative Review

Alcohol and cannabis are the two most commonly found intoxicating substances in fatally injured drivers. Epidemiological studies have demonstrated that the use of alcohol or cannabis can lead to an increase in the risk of a motor vehicle collision. Reducing the risks associated with driving under the influence of alcohol or cannabis is achieved partly through roadside detection of breath alcohol concentrations (BrAC) or blood delta-9-tetrahydrocannabinol (THC) levels. The purpose of the present review is to compile the laboratory studies on the combined effects of alcohol and cannabis on simulated driving as well as those evaluating combinations of these drugs on BrAC or blood THC. Given that driving can be affected by a number of cognitive processes, the literature on the cognitive effects of combinations of alcohol and cannabis is also reviewed, along with a discussion of a potential additive effect on the subjective qualities of these drugs. In sum, it is concluded that alcohol and cannabis have additive effects on driving skills, cognition and subjective effects. Public Health Significance Alcohol and cannabis are the two most commonly found intoxicating substances in fatally injured drivers. In the present review, we present evidence that the use of these two drugs at the same time produces additive effects on driving and other measures. Despite this, evidence that breath alcohol levels or blood delta-9-tetrahydrocannabinol (THC) are impacted by co-use of these drugs is inconclusive.