NIDA Examines Effects of Marijuana With or Without Alcohol on Driving Performance

By John Lavitt 07/28/15

Researchers found that marijuana and alcohol make for a dangerous combination.


Highlighted by the NIDA Science Spotlight, a recent study conducted by researchers from the U.S. Department of Health and Human Services examined the effects of marijuana, with or without alcohol, on driving performance.

The study specifically inspected the effects of marijuana use on lateral motion. In order to mirror real-life situations, the most sophisticated driving simulator available was used. The research shows that marijuana use impairs this measure of driving performance. People driving with blood concentrations of 13.1 µg/L THC showed increased weaving within the lane.

THC is the main psychoactive ingredient in marijuana. The blood concentration examined was similar to those with a blood alcohol level of 0.08, the threshold for impaired driving in many states. Drinking alcohol and smoking marijuana had a much greater effect. The drivers using both substances weaved within lanes even if their blood THC and alcohol concentrations were below the impairment thresholds for each substance alone.

Alcohol increased the number of times the car actually left the lane and the speed of weaving. In contrast, marijuana use alone did not produce this negative effect. The National Institute on Drug Abuse, the Office on National Drug Control Policy, and the National Highway Traffic Safety Administration funded the study.

THC concentrations drop rapidly during the time required to collect a blood specimen. Within two to four hours, the shift is significant. Like breathalyzers for alcohol, saliva (oral fluid) tests for THC can be performed roadside without any delay. The problem is that saliva testing of THC showed a two to five-fold greater variability than blood tests. Although oral fluid may be an effective screening tool for detecting recent marijuana use by a driver, it may not be a precise measure of the level of impairment.

In the study, more than 50% of participants controlled their marijuana inhalations, known as titrations. As a result, the participants maintained consistent blood THC peak concentrations, regardless of the percentage of THC in the marijuana. The THC percentages of the marijuana used for the study ranged from 2.9% to 6.7%. This shows that past driving studies based on cannabis dose rather than blood THC may have missed the importance of dose titration.

More significantly for the results of the study, it was found that low amounts of alcohol significantly increased peak THC concentrations. Without question, there is a definitive ongoing interactive effect between alcohol and THC in the human body.

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Growing up in Manhattan as a stutterer, John Lavitt discovered that writing was the best way to express himself when the words would not come. After graduating with honors from Brown University, he lived on the Greek island of Patmos, studying with his mentor, the late American poet Robert Lax. As a writer, John’s published work includes three articles in Chicken Soup For The Soul volumes and poems in multiple poetry journals and compilations. Active in recovery, John has been the Treatment Professional News Editor for The Fix. Since 2015, he has published over 500 articles on the addiction and recovery news website. Today, he lives in Los Angeles, trying his best to be happy and creative. Find John on Facebook, Twitter, and LinkedIn.