Nearly 80% of Spanish drug-positive drivers tested positive for THC, with half also positive for other drugs
In Spain's national roadside testing program, THC was found in 79.5% of all drug-positive tests, with 50.8% of THC-positive drivers also testing positive for cocaine or amphetamines, with polysubstance use most common at lower THC levels.
Quick Facts
What This Study Found
Of 65,244 confirmed drug-positive tests (2011-2016), 51,869 (79.5%) were THC-positive. In 50.8% of THC-positive tests, cocaine and amphetamines were also detected. Polysubstance use predominated at lower THC levels (58.6% of those under 25 ng/mL). Mean age was 29.6 years; men accounted for 96.3% of THC-positive drivers.
Key Numbers
179,645 roadside tests. 65,244 drug-positive. 51,869 THC-positive (79.5%). 50.8% of THC-positive also had cocaine/amphetamines. THC-only decreased with age (OR 0.948). Polysubstance increased with age (OR 1.021). Men: 96.3% of THC-positive. Men had higher THC concentrations (OR 1.394).
How They Did This
Analysis of national administrative data from Spain's roadside drug testing program, 179,645 oral fluid tests between 2011 and 2016, with confirmation analysis for driving-impairing substances.
Why This Research Matters
The massive scale of this dataset (nearly 180,000 tests) reveals that cannabis-positive driving rarely occurs in isolation. Half of THC-positive drivers also had cocaine or amphetamines, complicating attribution of impairment risk to cannabis alone.
The Bigger Picture
Studies attributing crash risk to cannabis may be overestimating its independent effect, since so many THC-positive drivers also have other drugs on board. Enforcement and policy focused solely on cannabis may miss the bigger polysubstance picture.
What This Study Doesn't Tell Us
Administrative testing data, not a random sample of all drivers. Only drug-positive tests analyzed, not the full 179,645. Spanish driving culture and drug use patterns may not generalize. Cannot determine impairment level from substance detection alone.
Questions This Raises
- ?How much of the cannabis-crash risk association is actually driven by polysubstance use?
- ?Would targeting polydrug combinations be more effective for road safety than targeting THC alone?
- ?Why were older THC-positive drivers more likely to use multiple substances?
Trust & Context
- Key Stat:
- 80% of drug-positive Spanish drivers had THC; 50.8% had THC plus cocaine or amphetamines
- Evidence Grade:
- Strong: massive national dataset (179,645 tests) over 6 years with confirmed laboratory analysis.
- Study Age:
- Published in 2019, covering 2011-2016.
- Original Title:
- Drivers who tested positive for cannabis in oral fluid: a longitudinal analysis of administrative data for Spain between 2011 and 2016.
- Published In:
- BMJ open, 9(8), e026648 (2019)
- Authors:
- Herrera-Gómez, Francisco, García-Mingo, Mercedes, Colás, Mónica, González-Luque, Juan Carlos, Alvarez, F Javier
- Database ID:
- RTHC-02071
Evidence Hierarchy
Looks back at existing records to find patterns.
What do these levels mean? →Frequently Asked Questions
How common is THC-positive driving?
In Spain's roadside testing program, 79.5% of all drug-positive drivers had THC in their system. However, over half of those also had cocaine or amphetamines, meaning pure cannabis-impaired driving was less common than polysubstance-impaired driving.
Who are the typical THC-positive drivers?
Mean age was 29.6 years. Men accounted for 96.3% of THC-positive drivers and tended to have higher THC concentrations. Younger drivers were more likely to test positive for THC alone, while older drivers more often had multiple drugs.
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Cite This Study
https://rethinkthc.com/research/RTHC-02071APA
Herrera-Gómez, Francisco; García-Mingo, Mercedes; Colás, Mónica; González-Luque, Juan Carlos; Alvarez, F Javier. (2019). Drivers who tested positive for cannabis in oral fluid: a longitudinal analysis of administrative data for Spain between 2011 and 2016.. BMJ open, 9(8), e026648. https://doi.org/10.1136/bmjopen-2018-026648
MLA
Herrera-Gómez, Francisco, et al. "Drivers who tested positive for cannabis in oral fluid: a longitudinal analysis of administrative data for Spain between 2011 and 2016.." BMJ open, 2019. https://doi.org/10.1136/bmjopen-2018-026648
RethinkTHC
RethinkTHC Research Database. "Drivers who tested positive for cannabis in oral fluid: a lo..." RTHC-02071. Retrieved from https://rethinkthc.com/research/herrera-gomez-2019-drivers-who-tested-positive
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.