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Combining Cannabis and Alcohol Will Impair Your Driving

Dragana Komnenov PhD
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Combining THC and alcohol was more dangerous for drivers than alcohol alone or THC alone.

The interactions between different cannabinoids in the cannabis plant remain largely an unexplored topic. Research is getting there, however. For example, it has shed some light how CBD can slow the intoxicating effects of THC. But, how do cannabinoids interact with alcohol? There is considerable evidence that the cannabis-consuming population also partakes in alcohol and this could have serious implications for DUI.

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Cannabinoids and Alcohol

Given the hundreds of different compounds that we know exist in the plant, and those we have yet to uncover, the number of possible interactions (and their impact on human health) is staggering. One of the most pressing concerns as legalization spreads is how cannabis and alcohol combine to increase impairment for DUI.

One study of interest focused on figuring out the effects of THC, CBD, CBN and alcohol on a series of motor, cognitive and perceptual function tests in human subjects. The study employed university students (122 males and 39 females) aged 18-36 years. Each was identified as a social drinker and non-naïve for cannabis.

Cannabinoids were dissolved into sesame oil and formulated to deliver 215 mg/ kg of THC, and 320 mg/kg of CBD and CBN. The subjects consumed cannabinoids or placebo capsules. Subjects then consumed 0.54g/kg of alcohol within 20-minutes.

Following this was a battery of tests every hour for three consecutive hours. The tests measured speed in visual reaction time as well as auditory.  ‘Standing steadiness factor’ was also evaluated, with eyes opened and then closed.


What Happens When You Combine Alcohol and Cannabis?

THC, on its own, impaired reaction speed and standing steadiness.  The amount of alcohol tested, although impacting psychomotor coordination, did not reach statistical significance overall. CBD and CBN caused no systemic effects.

Alcohol caused the greatest negative change in performance at 20 minutes post consumption, but the effects wore off by the end of the trial (approximately 3 hours later).

In contrast, the onset of performance change was slower for THC, reaching its peak 2 hour into the tests, and did not wear off by the end the experiment.

The combination of THC and alcohol further deteriorated performance compared to either substance taken alone. And there was no measurable recovery at the end of three hours.

The results of these tests are also in line with the subjects’ perception. Each felt incapacitated at the same time points as listed above. The results suggest that, for the THC + ethanol combination, there is an additive effect. This is despite the fact that blood alcohol content was the same in individuals consuming alcohol alone or with THC. CBD and CBN did not affect impairment levels.

keys bud and a joint

Results Show Combined Alcohol and Cannabis is Deadly on the Roads

Recently, a European Driving Under the Influence of Drugs, Alcohol and Medicines (DRUID) study reported a risk associated with cannabis consumption similar to that faced by drivers with a blood alcohol concentration between 0.01 g/dL and 0.05 g/dL. However, more recently, it was found that although cannabis increases risk for crash by 50% when studied alone, once adjusted for the presence of alcohol, the crash risk associated with cannabis became nonsignificant.  These inconsistencies likely stem from different study design and/or the quantity of alcohol consumed.

Another study investigated the contribution of cannabis crash risk alone when drivers consumed no to minimal alcohol. The results blamed crash responsibility on alcohol. Cannabis consumption, however, compounded the problem. Cannabis alone does not contribute to crash potential with any significance.

It was suggested that cannabis may be a larger contributor to impairing highly automatic driving functions but a less damaging effect on complex tasks that require conscious control. This is the opposite of the pattern for alcohol impairment. This may explain the additive effects of alcohol and cannabis on impairing reaction speed, standing steadiness, and psychomotor function.


Dragana Komnenov
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