What does science actually know about cannabis and driving impairment? | Research

The takeaway

A focused review found that while cannabis impairs driving-related skills, significant research gaps remain around blood THC levels as impairment markers, time courses of impairment, and appropriate legal thresholds, with authors arguing policy has outpaced science.

Cannabis users who drive, policymakers working on impaired driving laws, and anyone interested in the gap between cannabis science and traffic safety policy.

Blood THC levels poorly predict impairment

What the researchers found

Cannabis impairs reaction time, lane tracking, and divided attention, though patterns differ from alcohol impairment. Blood THC levels correlate poorly with actual impairment due to variable pharmacokinetics. Combining cannabis with alcohol produces greater impairment than either alone. Per-se THC limits used in legal settings lack strong scientific basis.

Why it matters

As cannabis legalization spreads, states are setting impaired driving policies often without adequate scientific evidence. This review identifies the specific gaps between what science has shown and what policies assume, including the unreliability of blood THC as an impairment proxy.

The numbers in context

Blood THC levels correlate poorly with impairment; combined alcohol-cannabis produces greater impairment than either alone; per-se THC limits lack strong scientific validation

How the study worked

Focused narrative review examining evidence on cannabis and motor vehicle accidents, impairment patterns, time courses, dose relationships, THC blood level correlations, alcohol-cannabis combinations, and legal per-se limits.

What this study cannot tell us

Narrative review with selective literature coverage. Research gaps identified but not systematically quantified. Rapidly evolving field means some conclusions may already need updating.

How to read the evidence

Focused review identifying critical research gaps. Provides useful synthesis but not a systematic review.

When this study was published

Published in 2021; cannabis-impaired driving research is an active and rapidly evolving field.

The bigger picture

The parallel to alcohol is tempting but misleading. Blood alcohol level reliably predicts impairment, but blood THC does not work the same way due to how THC is metabolized and stored in fat. This fundamental pharmacological difference means that alcohol-style legal frameworks for cannabis-impaired driving may be scientifically unfounded.

Questions still open

What biomarker, if any, could reliably indicate cannabis-related driving impairment? Should per-se THC limits be abandoned in favor of behavioral impairment testing? How do cannabis tolerance effects change the impairment profile for regular users?

Common questions

How does cannabis impair driving?

Cannabis slows reaction time, impairs lane tracking, and reduces divided attention. Unlike alcohol, cannabis users tend to drive more slowly and leave more following distance, but they are worse at responding to unexpected events.

Can a blood test tell if you are too impaired to drive?

Not reliably. Unlike alcohol, blood THC levels do not consistently correlate with impairment because THC is stored in fat and can remain detectable long after impairment has passed. Heavy users may test positive when not impaired.

Is combining cannabis and alcohol worse for driving?

Yes. Research consistently shows that combining cannabis with alcohol produces greater driving impairment than either substance alone.

Read the original research

Cannabis and Driving.

Frontiers in psychiatry, 12, 689444

View on PubMed ↗Publisher’s paper ↗

Citation

Pearlson, Godfrey D; Stevens, Michael C; D'Souza, Deepak Cyril. (2021). Cannabis and Driving.. Frontiers in psychiatry, 12, 689444. https://doi.org/10.3389/fpsyt.2021.689444