A controlled Johns Hopkins Medicine study found that taking cannabis edibles with alcohol can produce greater and longer-lasting impairment on a driving simulator than using either substance alone, while standardized field sobriety tests frequently failed to flag cannabis-related impairment unless alcohol levels were high.
Researchers at Johns Hopkins Medicine tested how cannabis edibles, alcohol and their combination affected driving-related performance in a tightly controlled, within-participant crossover trial involving 25 healthy adults ages 21 to 55. (hopkinsmedicine.org)
Across seven outpatient sessions separated by about a week, participants received either a THC-containing brownie (10 mg or 25 mg) or a placebo brownie, along with either an alcoholic drink calibrated to reach target breath alcohol concentrations (BrAC) of 0.05% or 0.08% or a placebo beverage. Participants completed simulated driving tasks and other assessments before and after dosing. (hopkinsmedicine.org)
The study found that combining edible cannabis with alcohol led to significantly greater driving impairment and higher self-reported intoxication than using cannabis or alcohol alone. The researchers reported that pairing THC with alcohol at 0.05% BrAC could produce impairment comparable to—or in some comparisons higher than—alcohol alone at 0.08% BrAC, the per se legal limit used in most U.S. states. (jamanetwork.com)
However, the researchers said standardized field sobriety tests often did not identify cannabis-related impairment in the study: the tests most consistently indicated intoxication primarily in the highest alcohol condition (0.08% BrAC), even as the driving simulator measures showed impairment in cannabis and combination conditions. (hopkinsmedicine.org)
