Blood THC Levels Did Not Reliably Predict Driving Impairment in Most Studies
A systematic review found that 10 of 12 studies showed no significant linear relationship between blood THC concentration and driving performance measures, challenging the use of blood THC as an impairment indicator.
Quick Facts
What This Study Found
Of 12 studies meeting inclusion criteria, 10 found no significant linear correlations between blood THC and driving measures. Specifically: 8 of 9 for lateral control/weaving, 4 of 5 for speed, 2 of 3 for car following, 1 of 1 for reaction time, and 3 of 3 for overall driving performance showed no relationship. The two studies that did find associations involved complex driving situations.
Key Numbers
4,845 records searched, 12 included. 10 of 12 found no linear THC-driving correlation. 8/9 for lateral control, 4/5 for speed, 2/3 for car following, 1/1 for reaction time, 3/3 for overall performance showed no relationship.
How They Did This
Systematic review of published studies examining the linear relationship between blood THC levels and driving performance, primarily measured by simulated driving. Searched 4,845 records; 12 met inclusion criteria.
Why This Research Matters
Many jurisdictions use blood THC levels as evidence of impaired driving. If THC levels don't linearly predict driving performance, current enforcement approaches may not accurately identify impaired drivers.
The Bigger Picture
Unlike blood alcohol concentration, blood THC levels do not appear to reliably indicate driving impairment. This has major implications for roadside testing policies and legal per se limits for THC, which may not reflect actual impairment.
What This Study Doesn't Tell Us
Most studies used driving simulators, not real-world driving. Only 12 studies met criteria, indicating limited research in this area. Complex driving situations may reveal relationships that simple tasks miss.
Questions This Raises
- ?Should jurisdictions move away from per se THC limits for driving?
- ?What alternative measures better predict cannabis-impaired driving?
- ?How do increasing cannabis potency levels affect the THC-driving relationship?
Trust & Context
- Key Stat:
- 10 of 12 studies found no linear THC-driving relationship
- Evidence Grade:
- Strong: systematic review with rigorous inclusion criteria, though limited by the small number of qualifying studies (12) and reliance on simulator-based measures
- Study Age:
- Published in 2025
- Original Title:
- Association of driving with blood delta-9-tetrahydrocannabinol: a systematic review.
- Published In:
- The international journal of neuropsychopharmacology, 28(4) (2025)
- Authors:
- Behzad, Danial(3), Zhao, Sampson(2), Besa, Reena, Brands, Bruna, Wickens, Christine M, Huestis, Marilyn A, Le Foll, Bernard, Di Ciano, Patricia
- Database ID:
- RTHC-06035
Evidence Hierarchy
Analyzes all available research on a topic using a structured method.
What do these levels mean? →Frequently Asked Questions
Does higher blood THC mean worse driving?
Based on this systematic review, the answer is generally no. Ten of 12 studies found no significant linear relationship between blood THC concentration and driving performance. The two exceptions involved complex driving scenarios.
Can a blood test prove cannabis-impaired driving?
This review challenges that assumption. Unlike alcohol, where BAC reliably predicts impairment, blood THC levels do not appear to have a consistent linear relationship with driving ability. This raises questions about per se THC limits used in some jurisdictions.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-06035APA
Behzad, Danial; Zhao, Sampson; Besa, Reena; Brands, Bruna; Wickens, Christine M; Huestis, Marilyn A; Le Foll, Bernard; Di Ciano, Patricia. (2025). Association of driving with blood delta-9-tetrahydrocannabinol: a systematic review.. The international journal of neuropsychopharmacology, 28(4). https://doi.org/10.1093/ijnp/pyaf021
MLA
Behzad, Danial, et al. "Association of driving with blood delta-9-tetrahydrocannabinol: a systematic review.." The international journal of neuropsychopharmacology, 2025. https://doi.org/10.1093/ijnp/pyaf021
RethinkTHC
RethinkTHC Research Database. "Association of driving with blood delta-9-tetrahydrocannabin..." RTHC-06035. Retrieved from https://rethinkthc.com/research/behzad-2025-association-of-driving-with
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.