Effects of cannabis on driving can last up to 5 hours, warns international study
An international study shows that the effects of cannabis on driving can last up to 5 hours, increasing the risk of accidents even when drivers are aware that they are under the influence.
Published on 08/14/2025
THC can affect driving for more than five hours, says study | CanvaPro
A study recently published in the Journal of Psychopharmacology revealed that the effects of cannabis on driving ability can extend for hours, even when users are aware that they are under the influence. The research evaluated 38 adults between 18 and 40 years old, subjected to vaporized cannabis doses of 5.9% and 13% THC, as well as a placebo, in a randomized, double-blind, cross-over study.
Detailed Methodology
Each participant underwent three days of eight-hour evaluations, with simulated driving tests conducted at four different times. Tasks such as lane maintenance, vehicle tracking, and overtaking were analyzed, totaling 19 behavioral metrics. The statistical analysis used mixed linear models in SPSS, evaluating the main effects of dose, time, and dose × time interaction.
Key Findings
Lane maintenance: participants showed a reduction in the lane departure rate for up to 5.5 hours after the 13% THC dose and 3.5 hours with 5.9%.
Vehicle tracking: pedal changes, including reduced range of motion and lower correction rates, persisted for 1 to 3 hours, only with the higher dose.
Overtaking: after 13% THC, drivers maintained a shorter distance from overtaken vehicles, had less time for potential collision, and spent more time in the opposite lane, increasing the risk of accidents.
Perception vs. Actual Performance
Despite being aware of being under the influence of the drug, about 66% of participants stated they would be willing to drive, revealing a concerning discrepancy between subjective perception and actual performance behind the wheel. Additionally, the study found no significant correlation between THC metabolite concentration in the blood and driving performance, questioning the reliability of blood tests as the sole risk indicator.
Implications for Traffic Safety
The authors highlight that the prolonged effects of cannabis require broader approaches to assess driving risk, beyond simple THC blood measurement. The study also extends the attention window on driving under the influence of the drug, showing that the impacts can last longer than the three hours typically explored in previous research.
