Cannabis plus alcohol is one of the most frequently detected drug combinations in car accidents, yet the interaction of these two compounds is still poorly understood. A study appearing online in Clinical Chemistry, the journal of AACC, shows for the first time that the simultaneous use of alcohol and cannabis produces significantly higher blood concentrations of cannabis's main psychoactive constituent, Δ9-tetrahydrocannabinol (THC), as well as THC's primary active metabolite, 11-hydroxy-THC (11-OH-THC), than cannabis use alone.

Currently, 23 states and the District of Columbia have legalized medical cannabis, and Colorado, Washington, Oregon, and Alaska have decriminalized recreational cannabis use. As cannabis becomes more widely accessible, the verdict remains out on whether cannabis intoxication increases the risk of car accidents. Experts agree, however, that the combination of cannabis and alcohol raises the chance of crashing more than either substance by itself. In a study of 1,882 motor vehicle deaths, the U.S. Department of Transportation found an increased accident risk of 0.7 for cannabis use, 7.4 for alcohol use, and 8.4 for cannabis and alcohol use combined.

To shed light on the ways in which cannabis and alcohol interact to negatively impact driving, a group of researchers studied 19 adult participants who drank placebo or low-dose alcohol (with a target peak breath-alcohol concentration of approximately 0.065%) 10 minutes prior to inhaling 500 mg of placebo, low-dose (2.9% THC), or high-dose (6.7% THC) vaporized cannabis. The researchers found that with no alcohol, the median maximum blood concentrations for low and high THC doses were 32.7 and 42.2 µg/L THC, respectively, and 2.8 and 5.0 µg/L 11-OH-THC. With alcohol, the median maximum blood concentrations for low and high THC doses were 35.3 and 67.5 µg/L THC and 3.7 and 6.0 µg/L 11-OH-THC -- which is significantly higher than without alcohol.

"The significantly higher blood THC and 11-OH-THC [median maximum concentration] values with alcohol possibly explain increased performance impairment observed from cannabis-alcohol combinations," said lead study author Marilyn A. Huestis, PhD, of the National Institute on Drug Abuse, Baltimore, Maryland. "Our results will help facilitate forensic interpretation and inform the debate on drugged driving legislation."