College students who used all three substances had 10 times higher risk of impaired driving

Compared to alcohol-only users, students using all three substances (alcohol, marijuana, nicotine) had dramatically higher odds of DUI (OR=10.33) and riding with an impaired driver (OR=10.22). Marijuana-only users also had elevated DUI risk (OR=5.44 between-person; OR=9.08 on specific occasions). On days when both alcohol and marijuana were used, riding with an impaired driver was nearly 4 times more likely (OR=3.86).

Burst design study of 367 college student drinkers with past-year marijuana and/or nicotine use. Assessed on two consecutive weekends for three semesters. Logistic and multilevel logistic models examined between- and within-person associations of substance combinations with DUI and RWID.

Prevention programs typically focus on alcohol-impaired driving. This study shows that polysubstance use dramatically increases driving risk beyond alcohol alone, suggesting prevention efforts need to address substance combinations rather than single substances.

The finding that marijuana-only occasions carried high DUI risk (OR=9.08) is notable because cannabis-impaired driving receives less attention than alcohol-impaired driving. The multiplicative risk of combining all three substances underscores that polysubstance use creates qualitatively different risk profiles.

Wide confidence intervals reflect small cell sizes for some substance combinations. Self-reported substance use and driving behavior. College student sample limits generalizability. Cannot determine impairment level at time of driving.