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.
Read this if you want to understand the driving risks of combining alcohol and cannabis.
Combined alcohol+THC prevalence in fatal crashes rose from <2% to >10%
What the researchers 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.
Why it 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 numbers in context
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 the study worked
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.
What this study cannot 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.
How to read the evidence
Case-control study using national fatal crash data spanning 17 years. Large dataset but limited by the nature of FARS reporting.
When this study was published
Published in 2015 using data through 2008. Both cannabis and alcohol-impaired driving patterns may have changed since.
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.
Questions still open
- 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?
Common questions
Is it more dangerous to drive after using both alcohol and cannabis?
How much more common has combined impaired driving become?
Read the original research
The combined effects of alcohol and cannabis on driving: Impact on crash risk.
Forensic science international, 248, 94-100
Citation
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
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