How does combining alcohol and marijuana affect driving behavior compared to using either alone? | Research

The takeaway

A daily diary study of 408 young adults found that simultaneous alcohol and marijuana use days carried the highest risk of riding with an impaired driver, while days with non-simultaneous use of both substances had the highest odds of driving after use.

Young adults who use both alcohol and marijuana, public health professionals designing impaired driving campaigns, and researchers studying polysubstance use.

2.22x higher odds of riding with impaired driver on SAM days

What the researchers found

On simultaneous alcohol-marijuana days, odds of riding with an impaired driver were 1.28x higher than alcohol-only days and 2.22x higher than marijuana-only days. Driving after use was 1.25x more likely on simultaneous days vs marijuana-only days, but non-simultaneous use days had the highest driving-after-use odds (1.59x vs simultaneous days).

Why it matters

Simultaneous use of alcohol and marijuana is increasingly common and appears to amplify risky transportation behaviors. The finding that non-simultaneous use days carry the highest driving-after-use risk suggests complex decision-making patterns around impairment.

The numbers in context

408 participants; 14,675 substance use days; riding with impaired driver: SAM vs alcohol-only AOR 1.28, SAM vs marijuana-only AOR 2.22; driving after use: non-simultaneous vs SAM AOR 1.59, SAM vs marijuana-only AOR 1.25

How the study worked

Daily diary study with 408 young adult simultaneous alcohol-marijuana users completing five 14-day reporting bursts, yielding 14,675 substance use days. Within-person analyses using adjusted odds ratios.

What this study cannot tell us

Self-report data on substance use and driving behavior. Participants were recruited as simultaneous users, limiting generalizability. Cannot measure actual impairment levels or accident risk.

How to read the evidence

Strong within-person daily diary design with large number of observation days, but self-report measures and specialized sample.

When this study was published

Published in 2021; impaired driving research continues to evolve with cannabis legalization.

The bigger picture

The distinction between simultaneous and non-simultaneous polysubstance use is often overlooked in public health messaging. This study suggests the two patterns carry different risk profiles, with non-simultaneous use potentially more dangerous for driving because users may feel less impaired when substances do not overlap.

Questions still open

Why is driving after use more common on non-simultaneous use days? Do users perceive themselves as less impaired when substances are used separately? Would targeted messaging about non-simultaneous polysubstance use and driving be effective?

Common questions

Is combining alcohol and marijuana more dangerous for driving?

The study found that simultaneous use days had higher odds of riding with an impaired driver than either substance alone. However, days when both were used at different times actually had the highest odds of driving after use.

Why would non-simultaneous use be riskier for driving?

The authors suggest that when substances are used at different times during a day, users may underestimate their cumulative impairment compared to when both substances peak simultaneously.

Read the original research

Intoxicated driving and riding with impaired drivers: Comparing days with alcohol, marijuana, and simultaneous use.

Drug and alcohol dependence, 225, 108753

View on PubMed ↗Publisher’s paper ↗

Citation

Patrick, Megan E; Graupensperger, Scott; Dworkin, Emily R; Duckworth, Jennifer C; Abdallah, Devon Alisa; Lee, Christine M. (2021). Intoxicated driving and riding with impaired drivers: Comparing days with alcohol, marijuana, and simultaneous use.. Drug and alcohol dependence, 225, 108753. https://doi.org/10.1016/j.drugalcdep.2021.108753