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

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.

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.

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.

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.

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.