Drivers With Cannabis Markers in Urine Had 8x Higher Odds of Crash Involvement, Even Without Active THC in Blood | Research

The takeaway

Analysis of 1,236 drivers in Rome-area crashes found cannabis urinary markers were associated with 8-fold higher odds of crash involvement, even when blood THC levels were below impairment thresholds.

Traffic safety researchers, policymakers setting impairment thresholds, cannabis users who drive.

8x crash odds with cannabis markers even when blood THC was below impairment levels

What the researchers found

Among 1,236 drivers in crashes, alcohol was most common (19% non-fatal, 32% fatal), followed by cannabinoids (12% non-fatal) and cocaine (9% non-fatal, 20% fatal). While urinary cannabis metabolite levels were high (THCCOOH 15-270 ng/mL), blood THC was often below 1 ng/mL. Cannabis crash odds ratio: 8.13. Cocaine OR: 5.32.

Why it matters

The finding that crash risk was elevated even when blood THC was very low challenges the assumption that impairment ends when active drug levels drop.

The numbers in context

1,236 drivers. Alcohol: 19% non-fatal, 32% fatal. Cannabis: 12% non-fatal. Cocaine: 9% non-fatal, 20% fatal. Cannabis crash OR: 8.13. Cocaine OR: 5.32. Blood THC often below 1 ng/mL.

How the study worked

Cross-sectional analysis of biological fluids from 1,236 drivers involved in road accidents in the Rome area, analyzed for alcohol and psychotropic drugs. Blood and urine results compared. Odds ratios calculated against a control population.

What this study cannot tell us

Cross-sectional with crash-involved drivers only. Urinary metabolites persist for weeks. Cannot determine if cannabis caused the crash. Italian context may differ.

How to read the evidence

Large sample from judicial authority data, but cross-sectional design limits causal inference.

When this study was published

Published 2023.

The bigger picture

If crash risk is elevated even without detectable active THC in blood, per se legal limits based on blood THC may be insufficient for road safety.

Questions still open

Are regular cannabis users at elevated crash risk even when not acutely impaired?
Should roadside testing focus on urinary metabolites rather than blood THC?

Common questions

Can cannabis increase crash risk even when you are not high?

This study found 8-fold higher crash odds for drivers with cannabis metabolites in urine even when blood THC was very low.

How does cannabis compare to other substances for crash risk?

Cannabis (OR 8.13) had higher crash odds than cocaine (OR 5.32). Alcohol was most common, found in 32% of fatal crashes.

Read the original research

Drug-impaired driving and traffic collisions: Study on a cross section of the Italian population.

Drug testing and analysis, 15(5), 477-483

View on PubMed ↗Publisher’s paper ↗

Citation

Odoardi, Sara; Biosa, Giulia; Mestria, Serena; Valentini, Valeria; De Giovanni, Nadia; Cittadini, Francesca; Strano Rossi, Sabina. (2023). Drug-impaired driving and traffic collisions: Study on a cross section of the Italian population.. Drug testing and analysis, 15(5), 477-483. https://doi.org/10.1002/dta.3366