BALTIMORE — Researchers at Johns Hopkins Medicine published a study in May 2026 in JAMA Network Open showing that combining cannabis edibles and alcohol causes greater driving impairment than either substance used alone. The study found that the interaction between the two substances may be synergistic, producing more severe and longer-lasting impairment.

The controlled study involved 30 healthy adults aged 21 to 55, of whom 25 completed all sessions. Participants had used both cannabis and alcohol together within the past year and reported binge drinking in the previous 90 days, though they used cannabis infrequently—fewer than three times per week. Each participant completed seven experimental sessions, consuming different combinations of a cannabis brownie containing 10 or 25mg THC or a placebo brownie, along with an alcoholic beverage or placebo drink. Alcohol doses were individually adjusted to produce breath alcohol concentrations of either 0.05% or 0.08%.

Participants consumed the cannabis brownie one hour after breakfast and the alcoholic beverage 45 minutes later over a 15-minute period. They completed a training visit beforehand to familiarize themselves with the driving simulator and performance assessments. Driving and impairment assessments were repeated for up to 7.5 hours after brownie consumption, and study sessions were separated by at least one week to ensure substances had cleared participants’ systems.

Assessments included simulated driving tasks, standard field sobriety tests, cognitive and psychomotor tests, subjective effect questionnaires, and blood sampling for THC and metabolites. Standard field sobriety tests often failed to detect impairment caused by cannabis, whether consumed alone or with alcohol. The tests only identified impairment during the 0.08% breath alcohol concentration condition compared to placebo; cannabis-related impairment frequently went undetected. The legal alcohol intoxication threshold used across most of the U.S.—0.08% breath alcohol concentration—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,” said Austin Zamarripa, assistant professor of psychiatry and behavioral sciences at the Johns Hopkins University School of Medicine. He added, “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.”

Tory Spindle, associate professor of psychiatry and behavioral sciences at the Johns Hopkins University School of Medicine, said, “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.” Spindle also noted, “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.”