New research from Johns Hopkins Medicine finds that eating cannabis‑infused treats while drinking alcohol makes driving far riskier than using either substance alone. The study also discovered that common field‑sobriety checks often miss the impairment caused by cannabis, whether it is taken by itself or with alcohol.
The results, published in JAMA Network, raise fresh worries about mixing these two popular substances. Scientists say the findings call for better public education and more reliable ways to spot impaired drivers.
They also question current legal limits. In most of the United States, a breath alcohol level of 0.08% is treated as the line for intoxication. The new data suggest that this threshold may not reflect true driving ability when alcohol is combined with cannabis.
How the Study Was Done
Researchers recruited healthy adults aged 21 to 55 who had used both cannabis and alcohol in the past year. Each participant visited the lab several times. In each visit they received one of three options: a brownie with a low (10 mg) or higher (25 mg) dose of THC, a placebo brownie, an alcoholic drink designed to reach a breath alcohol level of 0.05% or 0.08%, or a non‑alcoholic placebo drink.
Thirty volunteers started the trial; twenty‑five completed all the sessions. Before any drug was given, participants went through medical, psychiatric and laboratory checks to confirm they were healthy and drug‑free.
Driving Simulators and Performance Tests
Before the real experiments, everyone practiced on a driving simulator so that learning effects would not skew the data. During each experimental session, participants performed a simulated drive, standard field‑sobriety tasks, and a series of cognitive and motor‑skill tests. Blood samples were taken to measure THC and its breakdown products.
The order of the sessions—cannabis only, alcohol only, both together, or both placebos—was randomized for each person. Sessions were spaced at least one week apart to let the drugs clear the system.
What the Researchers Found
When the edible cannabis and alcohol were taken together, drivers showed the biggest drop in performance, and the effects lasted longer than when either substance was used alone. Participants also reported feeling more drunk when both were combined.
Standard field‑sobriety tests, however, only flagged significant impairment during the highest alcohol condition (0.08% breath level). The same tests often failed to detect the slowdown caused by cannabis, even when it was mixed with alcohol.
"People are using cannabis edibles and alcohol together more often, yet most research has looked only at smoked cannabis. Our controlled study shows that even typical edible doses paired with modest alcohol can impair driving as much as, or more than, legal‑limit alcohol alone," said Dr. Tory Spindle, the study’s lead author.
Why This Matters
They also call for further research to develop better tools—both biological and behavioral—to spot cannabis‑related driving impairment.
Other Johns Hopkins contributors to the work included Dr. Ryan Vandrey, Dr. Elise Weerts, Dr. David Wolinsky and Dr. Denis Antoine.