Blood THC Levels Did Not Reliably Predict Driving Impairment in Most Studies | Research

The takeaway

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.

Traffic safety officials, law enforcement, cannabis policy makers, defense attorneys, drivers who use cannabis

10 of 12 studies found no linear THC-driving relationship

What the researchers 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.

Why it 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 numbers in context

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 the study worked

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.

What this study cannot 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.

How to read the evidence

Strong: systematic review with rigorous inclusion criteria, though limited by the small number of qualifying studies (12) and reliance on simulator-based measures

When this study was published

Published in 2025

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.

Questions still open

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?

Common 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 the original research

Association of driving with blood delta-9-tetrahydrocannabinol: a systematic review.

The international journal of neuropsychopharmacology, 28(4)

View on PubMed ↗Publisher’s paper ↗

Citation

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