Drivers positive for both alcohol and cannabis had higher crash risk than those positive for either substance alone
Among drivers in fatal US crashes (1991-2008), combining alcohol and THC increased the odds of unsafe driving actions by 8-10% per 0.01 BAC unit beyond the risk of either substance alone.
Quick Facts
What This Study Found
Researchers examined drivers aged 20+ involved in fatal crashes in the United States from 1991 to 2008 who had been tested for both alcohol and cannabis. Using a case-control design where "cases" were drivers with unsafe driving actions (UDAs) and "controls" had none, they estimated impairment risk.
The prevalence of combined THC and alcohol in fatal crash drivers increased approximately five-fold over the study period, from below 2% in 1991 to above 10% in 2008. THC alone was associated with a 16% increased odds of an UDA. Each 0.01 BAC unit increased UDA odds by 9-11%.
When both substances were present, the odds of an UDA increased by an additional 8-10% per 0.01 BAC unit beyond either substance alone. The combined effect was greater than additive, indicating a synergistic interaction between alcohol and cannabis on driving impairment.
Key Numbers
Combined THC+alcohol prevalence in fatal crashes: <2% (1991) to >10% (2008). THC alone: 16% increased odds of UDA. Each 0.01 BAC: 9-11% increase. Combined: additional 8-10% per 0.01 BAC beyond individual risks.
How They Did This
Case-control study using the Fatality Analysis Reporting System (FARS) for US fatal crashes 1991-2008. Cases were drivers with at least one unsafe driving action recorded; controls had none. Logistic regression estimated adjusted odds ratios for UDAs associated with alcohol alone, THC alone, and both combined.
Why This Research Matters
As cannabis legalization expands, many drivers may use both alcohol and cannabis before driving. Understanding that the combination is more dangerous than either substance alone is critical for public safety messaging and enforcement priorities.
The Bigger Picture
Drug-impaired driving policies often focus on single substances, but this study shows the combination of alcohol and cannabis is particularly dangerous. The five-fold increase in combined-positive drivers over two decades signals a growing public health concern that requires coordinated policy responses.
What This Study Doesn't Tell Us
FARS data only captures fatal crashes, which may not represent all impaired driving. THC detection indicates recent use but not necessarily impairment at crash time. The case-control design using UDAs as the outcome may misclassify some cases. Confounders like other drug use may be present.
Questions This Raises
- ?Should impaired driving laws specifically address polysubstance impairment?
- ?Would public messaging about the synergistic risk change behavior?
- ?At what BAC level does adding cannabis no longer meaningfully increase risk?
Trust & Context
- Key Stat:
- Combined alcohol+THC prevalence in fatal crashes rose from <2% to >10%
- Evidence Grade:
- Case-control study using national fatal crash data spanning 17 years. Large dataset but limited by the nature of FARS reporting.
- Study Age:
- Published in 2015 using data through 2008. Both cannabis and alcohol-impaired driving patterns may have changed since.
- Original Title:
- The combined effects of alcohol and cannabis on driving: Impact on crash risk.
- Published In:
- Forensic science international, 248, 94-100 (2015)
- Authors:
- Dubois, Sacha, Mullen, Nadia, Weaver, Bruce, Bédard, Michel
- Database ID:
- RTHC-00949
Evidence Hierarchy
Compares people with a condition to similar people without it.
What do these levels mean? →Frequently Asked Questions
Is it more dangerous to drive after using both alcohol and cannabis?
Yes. This study found that combining alcohol and cannabis increased the odds of unsafe driving actions beyond the risk of either substance alone. The effect was synergistic, not just additive.
How much more common has combined impaired driving become?
The prevalence of drivers testing positive for both THC and alcohol in fatal crashes increased approximately five-fold from below 2% in 1991 to above 10% in 2008.
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Cite This Study
https://rethinkthc.com/research/RTHC-00949APA
Dubois, Sacha; Mullen, Nadia; Weaver, Bruce; Bédard, Michel. (2015). The combined effects of alcohol and cannabis on driving: Impact on crash risk.. Forensic science international, 248, 94-100. https://doi.org/10.1016/j.forsciint.2014.12.018
MLA
Dubois, Sacha, et al. "The combined effects of alcohol and cannabis on driving: Impact on crash risk.." Forensic science international, 2015. https://doi.org/10.1016/j.forsciint.2014.12.018
RethinkTHC
RethinkTHC Research Database. "The combined effects of alcohol and cannabis on driving: Imp..." RTHC-00949. Retrieved from https://rethinkthc.com/research/dubois-2015-the-combined-effects-of
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.