Cannabis with THC impairs driving for about 4-5 hours after inhalation, and adding alcohol makes it worse | Research

The takeaway

A review of recent controlled studies confirms that THC impairs driving within the first hour and remains detectable for 4-5 hours, CBD does not impair driving, and combining cannabis with alcohol amplifies impairment.

Traffic safety policymakers, cannabis consumers, law enforcement, impaired driving researchers

4-5 hours of detectable driving impairment after inhaling THC

What the researchers found

THC acutely impairs driving performance consistently within the first hour of use, with impairment remaining for approximately 4-5 hours post-inhalation. CBD at doses below 300 mg does not impair driving. Alcohol and THC together produce additive impairment. No significant residual deficits were found after the acute window. Notably, participants were willing to drive shortly after using cannabis despite measurable impairment.

Why it matters

With legal cannabis markets expanding, the question of how long impairment lasts has direct policy implications for per se limits, waiting-period guidelines, and public safety messaging. The disconnect between subjective readiness to drive and objective impairment is particularly concerning.

The numbers in context

Impairment consistently observed within 1 hour of THC inhalation. Detectable impairment lasting approximately 4-5 hours post-inhalation. CBD doses below 300 mg showed no driving impairment. Alcohol plus THC produced additive effects.

How the study worked

Narrative review of controlled experimental driving research from the past five years (through approximately 2025), summarizing findings on acute and residual effects of cannabis, CBD, combined THC-CBD, and cannabis plus alcohol on driving performance.

What this study cannot tell us

Narrative (not systematic) review. Most studies used inhaled cannabis; oral and sublingual formulations were underrepresented. Laboratory driving simulators may not fully replicate real-world conditions. Studies used relatively low CBD doses.

How to read the evidence

Synthesizes multiple controlled experimental studies showing consistent results across labs and methods, though the narrative review format lacks systematic search methodology.

When this study was published

2026 review covering studies from approximately 2020-2025

The bigger picture

Unlike alcohol, cannabis impairment cannot be reliably measured with a simple roadside test. This review highlights that while the science of when impairment occurs is increasingly clear, the tools to detect it in real-world enforcement settings are still lacking.

Questions still open

How does impairment differ with oral cannabis products that have delayed onset? Can biomarkers be developed that reliably indicate current impairment rather than recent use? At what CBD dose, if any, does driving impairment begin?

Common questions

How long should you wait to drive after using cannabis?

Based on this review, measurable impairment from inhaled THC lasts approximately 4-5 hours. The research on edibles and other forms is still limited.

Does CBD affect driving?

At doses below 300 mg, CBD did not impair driving in the studies reviewed. Higher doses have not been adequately tested.

Read the original research

Recent Advances in the Science of Cannabis-Impaired Driving.

Current addiction reports, 13(1), 8

View on PubMed ↗Publisher’s paper ↗

Citation

Metrik, Jane; Bush, Nicholas; Gunn, Rachel L; McCarthy, Denis M. (2026). Recent Advances in the Science of Cannabis-Impaired Driving.. Current addiction reports, 13(1), 8. https://doi.org/10.1007/s40429-025-00712-0