The findings, published in JAMA Network, underscore growing concerns about the risks of mixing cannabis and alcohol. Researchers say the results point to a need for better public awareness and more effective ways to identify impaired drivers on the road.

The study also raises questions about current legal standards. According to the researchers, the legal alcohol intoxication threshold used across most of the United States (0.08% breath alcohol level, or BrAC) may not adequately reflect driving impairment when alcohol is combined with cannabis.

"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."

Testing Cannabis Edibles and Alcohol Together

To investigate how cannabis edibles and alcohol affect driving, researchers designed a tightly controlled study involving healthy adults between the ages of 21 and 55.

Participants attended multiple outpatient study sessions. During each visit, they received either a cannabis brownie containing THC (10 or 25mg THC) or a placebo brownie. They were also given either an alcoholic beverage or a placebo drink. Alcohol doses were individually adjusted to produce breath alcohol concentrations of either 0.05% or 0.08%.

Thirty volunteers were enrolled in the study, and 25 completed all sessions. Participants had previously used both cannabis and alcohol together within the past year and reported binge drinking within the previous 90 days. To reduce the influence of cannabis tolerance, participants used cannabis relatively infrequently, fewer than three times per week, while still having used it at least once during the past year.

Researchers screened participants through medical and psychiatric evaluations, physical examinations, routine blood tests, and urine drug testing to confirm they were healthy and had not recently used other illicit drugs.

Simulated Driving and Impairment Testing

Before the experimental sessions began, participants completed a separate training visit. During this visit, they became familiar with the driving simulator and other performance assessments to minimize learning effects during the study.

Each participant then completed seven experimental sessions. Depending on the session, they consumed cannabis alone, alcohol alone, cannabis and alcohol together, or placebo versions of both substances. The order of the sessions was carefully balanced among participants to avoid bias.

At the start of each session, participants completed baseline testing that included a simulated driving task, standard field sobriety tests, cognitive and psychomotor assessments, questionnaires about subjective drug effects, and blood sampling to measure THC and its metabolites.

One hour after breakfast, participants consumed either the cannabis brownie or the placebo brownie. Forty-five minutes later, they drank either alcohol or a placebo beverage designed to provide similar sensory cues and help maintain blinding. The beverages were consumed over a 15-minute period.

The same driving, cognitive, and impairment assessments were repeated multiple times throughout the day for as long as 7.5 hours after brownie consumption. Sessions were separated by at least one week to ensure the drugs had cleared participants' systems before the next visit.

Greater Impairment, But Sobriety Tests Often Missed It

The results showed that combining cannabis edibles with alcohol produced more severe and longer-lasting driving impairment than either substance alone. Participants also reported feeling more intoxicated when they used both substances together.

Despite these effects, standard field sobriety tests only identified significant intoxication during the highest alcohol condition (0.08% BrAC) when compared with placebo. Cannabis-related impairment often went undetected by those tests.

"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."

Implications for Public Safety

As cannabis legalization continues to expand and edible products become more widely available, the researchers say the risks associated with combining cannabis and alcohol deserve greater attention from public health officials, policymakers, and regulators.

They also emphasize the need for additional research to better understand impairment resulting from combined use and to develop more reliable biological and behavioral methods for detecting cannabis-related driving impairment.

Additional Johns Hopkins Medicine researchers involved in the study included Ryan Vandrey, Ph.D., Elise Weerts, Ph.D., David Wolinsky, M.D., and Denis Antoine, M.D.