In a study supported by the National Institute on Drug Abuse of the National Institutes of Health, Johns Hopkins Medicine researchers added to evidence that using cannabis edibles and alcohol together worsens driving impairment compared with consuming either substance alone. The study also found that cannabis (alone or with alcohol) did not impair performance on standard field sobriety tests.
These findings, published May 1 in JAMA Network, highlight the urgent need for wider public education about possible augmented effects from combining cannabis and alcohol and for improved roadside driver impairment detection methods. The study also revealed that the legal alcohol intoxication limit in most of the U.S. (0.08% breath alcohol level, or BrAC) is likely too liberal if a driver has used cannabis and alcohol together.
"Our findings indicate that co-use of cannabis and alcohol produces significantly greater driving impairment and subjective intoxication than either substance alone," says the study's lead author, Austin Zamarripa, Ph.D., assistant professor of psychiatry and behavioral sciences at the Johns Hopkins University School of Medicine. "Importantly, these findings suggest that the interaction between cannabis edibles and alcohol is not merely additive, but may be synergistic in producing impairment, which has important implications for real-world risk."
For the study, the research team used a rigorous, controlled design in which participants attended several outpatient experimental sessions. During each session, they were given either a cannabis-infused brownie (10 or 25mg THC) or a placebo brownie, followed by either an alcoholic beverage or a non-alcoholic placebo drink. Alcohol doses were tailored to each participant to achieve BrACs of 0.05% or 0.08%.
Participants were healthy adults ages 21–55, confirmed through medical and psychiatric screening, physical examination and routine bloodwork at the initial visit. Thirty participants were enrolled and randomized in the study and 25 completed all study visits. All participants reported binge drinking within the past 90 days, prior experience using cannabis and alcohol together within the past year and limited cannabis use (fewer than three times per week, with at least one use in the past year) to reduce the influence of tolerance. Participants had no recent use of other illicit drugs, as verified by urine drug testing.
After medical eligibility was confirmed through an initial screening visit, participants completed a second pre-study visit to become familiar with the driving simulator and other performance tests to reduce learning effects in the experiment. After this training visit, participants completed seven experimental sessions where they either ingested cannabis alone, alcohol alone, cannabis and alcohol together or placebo cannabis and placebo alcohol. Session order was carefully balanced across participants to ensure unbiased results.
At each experimental session before dosing, participants completed baseline assessments, including a simulated drive, standard field sobriety tests, cognitive/ psychomotor performance tests, subjective drug effect questionnaires and a blood sample to measure THC and THC metabolite levels. Participants consumed either a cannabis-infused brownie or a placebo brownie one hour after breakfast, and 45 minutes later, they were given either alcoholic beverages or placebo drinks — which provided similar sensory cues as the alcohol drinks to maintain dose blinding — consumed over a 15-minute period. Participants completed the same series of driving and other assessments repeatedly throughout the day for up to 7.5 hours after brownie consumption. Study sessions were spaced at least one week apart to ensure enough time for the drug to leave the body between visits.
Findings showed that combining cannabis edibles with alcohol led to greater and longer-lasting driving impairment and feelings of intoxication compared with either substance used alone, yet standard field sobriety tests only deemed participants as intoxicated in the high alcohol dose condition (0.08% BrAC) relative to placebo.
"We designed this study because people are increasingly co-using alcohol with edible cannabis products, yet controlled research has largely focused on smoked cannabis. This is the first controlled study to examine how cannabis edibles and alcohol interact, despite their growing combined use," says Tory Spindle, Ph.D., the study's principal investigator and associate professor of psychiatry and behavioral sciences at the Johns Hopkins University School of Medicine. "Consuming typical retail doses of cannabis edibles alongside even low doses of alcohol can produce driving impairment comparable to — or greater than — alcohol alone at the legal limit."
As cannabis legalization expands and access to cannabis products increases, the researchers emphasize the importance of considering the heightened risks associated with combining cannabis and alcohol in public health messaging, policy and regulatory decisions. They also note a need for continued research to better understand impairment from co-use and to develop improved biological or behavioral tools for detecting cannabis-related driving impairment.
