Driving After Combined Use of Cannabis and Alcohol

In a recent blog post, I explored data proving how cannabis users’ sense that they were ready to drive safely often occurred sooner than driving impairment waned (see “Self-Perception Doesn’t Predict Readiness to Drive After Cannabis Use”). This left the question of what is known about possible driving impairment after even small amounts of cannabis and alcohol are combined.

A 2015 study reported detection of both alcohol and THC in drivers involved in car crashes rose from below 2% in 1991 to above 10% in 2008, a 5-fold increase.(1) Since detection of THC can occur days after a single use, long after its effects have waned, the question needing answering is whether the combination truly caused these accidents, or whether THC was merely an incidental finding.

The research literature pursuing this question is voluminous, complex, and sometimes contradictory. I have been down a rabbit hole for several days sorting out the most reliable research to summarize what is known to date. As a result, I can report that enough facts about combining cannabis and alcohol are established to guide cannabis users toward safe driving.

Fatal accidents increase when alcohol and cannabis use are combined

A study in France found 21% of drivers involved in accidents have above the national legal limit of blood alcohol, while 6.8% are positive for cannabis. Of those positive for cannabis, 40% are also above the legal limit for blood alcohol content.

These data permitted researchers analyzing 10,000 fatal crashes to calculate the odds ratio of a fatal crash for drivers testing positive for cannabis alone, for alcohol alone, and cannabis combined with alcohol. Odds ratios represent the odds that an outcome will occur given presence of a particular variable, compared to the odds of the outcome occurring in the absence of that variable.

The study calculated the odds of a fatal accident is increased 2.3 times for cannabis alone, 9.4 for alcohol alone, and 14.1 for the alcohol-cannabis combination.(2)

Driving simulators have been shown to closely approximate on-road driving ability, and several studies have looked at the impact of various amounts of alcohol and cannabis on the driving abilities of both regular and occasional cannabis users. All find the risk from driving under the influence of both alcohol and cannabis is greater than the risk of driving under the influence of either alone. The most common effect noted was an increase in weaving behavior, leading to difficulty remaining securely in the center of one’s lane. A detailed list of impairments is available in Pearlson’s excellent review, “Cannabis and Driving."

The best explanation of the different ways cannabis and alcohol affect driving lies in the words of Sewell et al: “Detrimental effects of cannabis use vary in a dose-related fashion, and are more pronounced with highly automatic driving functions than with more complex tasks that require conscious control, whereas alcohol produces an opposite pattern of impairment.”(3) This makes sense, since alcohol diminishes consciousness and alertness more than cannabis.

Sewell concludes that marijuana smokers have an increased awareness that they are impaired and “… tend to compensate effectively while driving by utilizing a variety of behavioral strategies.” Such compensatory strategies were comically illustrated by Cheech and Chong’s driving barely 5 MPH on a busy highway in one of their movies.

However, “combining marijuana with alcohol eliminates the ability to use such strategies effectively … and results in impairment even at doses which would be insignificant were they of either drug alone.”(3) When the authors of one driving simulator study were surprised to find regular cannabis users actually made more errors under the influence of both cannabis and alcohol compared to occasional users, they suggested that “The consumption of alcohol—which has previously been associated with over-confident driving behavior, decreased inhibitions, and greater risk-taking—may have led regular users to underestimate the effects of consuming THC with alcohol.”(4)

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In other words, adding alcohol to cannabis adds the impairments unique to alcohol to those inherent in cannabis use. Perhaps the worst of these additions is to impair cannabis users’ judgment of the degree of their impairment and eliminate their tendency to use compensatory strategies. Altogether, adding alcohol to cannabis further diminishes the accuracy of self-perception regarding readiness to drive safely.

Cannabis users are strongly advised not to mix alcohol and cannabis if they intend to drive any time soon.