Despite the increased legalization of marijuana, many people are still skeptical of its adverse effects on users. However, new research shows that cannabis plus alcohol is actually more risky and makes for a more dangerous high.

According to new findings published in the journal Clinical Chemistry, these two substances are one of the most frequently detected drug combinations in US car accidents. Even so, until now the interaction of these two compounds was still poorly understood. Researchers have shown 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 marijuana, while Colorado, Washington, Oregon and Alaska have decriminalized recreational cannabis use. As cannabis becomes more widely accessible, researchers are starting to wonder whether cannabis intoxication increases the risk of car accidents across the nation.

Experts agree, however, that the combination of cannabis and alcohol ups the risk of crashing more than either substance by itself. In a study of 1,882 motor vehicle deaths, for example, the US 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 help shed light on how this drug combination may negatively impact driving, a group of researchers studied 19 adult participants who drank placebo or low-dose alcohol 10 minutes before inhaling 500 mg of placebo, low-dose (2.9% THC), or high-dose (6.7% THC) vaporized cannabis.

They found that with no alcohol in their system, 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, on the other hand, 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 - that's significantly higher than without any 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," lead study author Marilyn A. Huestis, of the National Institute on Drug Abuse (NIDA) in Baltimore, Md., said in a statement. "Our results will help facilitate forensic interpretation and inform the debate on drugged driving legislation."

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