A new study from Johns Hopkins School of Medicine suggests that combining edible marijuana and alcohol may have a more negative impact on driving than using either substance alone. The study also found that standard field sobriety tests often fail to detect impairment caused by cannabis, whether the drug is consumed alone or with alcohol.
The survey results are JAMA Networkhighlights growing concerns about the risks of mixing cannabis and alcohol. Researchers say the results demonstrate the need for increased public awareness and more effective ways to identify impaired drivers on the road.
The study also raises questions about current legal standards. Researchers say the legal alcohol intoxication threshold (0.08% breath alcohol concentration, or BrAC) used in most parts of the United States may not adequately reflect impaired driving when alcohol and marijuana are combined.
“Our findings show that the combination of cannabis and alcohol causes significantly greater impaired driving and subjective intoxication than either drug alone,” said the study’s lead author, Dr. Austin Zamarripa, assistant professor of psychiatry and behavioral sciences at Johns Hopkins University School of Medicine. “Importantly, these findings suggest that the interaction between cannabis edibles and alcohol is not just additive, but may act synergistically in producing impairments that have important implications for real-world risk.”
Test cannabis edibles and alcohol together
To investigate how cannabis edibles and alcohol affect driving, researchers designed a tightly controlled study of 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 25 mg THC) or a placebo brownie. They were also given either an alcoholic drink or a placebo drink. Alcohol dosage was individually adjusted to achieve a breath alcohol concentration of 0.05% or 0.08%.
Thirty volunteers were enrolled in the study and 25 completed all sessions. Participants reported using both cannabis and alcohol together within the past year and binge drinking within the past 90 days. To reduce the effects of cannabis tolerance, participants used cannabis relatively infrequently, less than three times per week, but at least once in the past year.
Researchers screened participants through medical and psychiatric evaluations, physical exams, regular blood tests, and urine drug tests to ensure participants were healthy and had not recently used other illicit drugs.
Simulated driving and obstacle tests
Before the experimental session began, participants completed another training visit. During this visit, they were familiarized with driving simulators and other performance assessments to minimize learning effects during the study.
Each participant then completed seven experimental sessions. Depending on the session, they took cannabis alone, alcohol only, cannabis and alcohol together, or a placebo version of both substances. Session order was carefully balanced across participants to avoid bias.
At the beginning of each session, participants completed baseline testing, including a simulated driving task, a standard field sobriety test, cognitive and psychomotor assessments, a subjective drug effects questionnaire, and a blood draw to measure THC and its metabolites.
One hour after breakfast, participants consumed either a cannabis brownie or a placebo brownie. After 45 minutes, they drank either alcohol or a placebo drink designed to provide similar sensory stimulation and help maintain blindness. Beverages were consumed over 15 minutes.
After brownie consumption, the same driving, cognitive, and impairment assessments were repeated multiple times throughout the day for up to 7.5 hours. Sessions were conducted at least 1 week apart to ensure that the drug was removed from the participants’ systems by the next visit.
There are greater impairments, but they are often missed by breathalyzer tests.
The results showed that combining cannabis edibles and alcohol produced more severe and long-lasting driving impairment than either substance alone. Participants also reported feeling more intoxicated when using both substances together.
Despite these effects, standard field sobriety tests identified significant intoxication only at the highest alcohol state (0.08% BrAC) when compared to placebo. Cannabis-related disorders often went undetected by these tests.
“We designed this study because controlled studies have primarily focused on smoked cannabis, even though people are increasingly using edible cannabis products and alcohol together. This is the first controlled study to examine how edible cannabis and alcohol interact, despite the increased use of edible cannabis products,” said 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. “Typical retail doses of cannabis edibles, taken together with even low doses of alcohol, can result in driving impairments that are equal to or greater than the legal limit of alcohol alone.”
Impact on public safety
As cannabis legalization continues to expand and edible products become more widely available, researchers say the risks associated with the combination of cannabis and alcohol merit increased attention from public health officials, policymakers and regulators.
They also highlight the need for additional research to better understand comorbid driving impairment and develop more reliable biological and behavioral methods to detect cannabis-related impaired driving.
Other Johns Hopkins School of Medicine researchers involved in the study included Ryan Vandry, Ph.D., Elise Wiers, Ph.D., David Wolinsky, M.D., and Denis Antoine, Ph.D.
