A meta-analysis of 80 studies found THC significantly impairs lateral control, tracking, and divided attention at peak effect, with impairment from inhaled cannabis largely resolving by 5 hours but persisting longer with oral consumption.
Cannabis users who drive, policy makers setting impaired driving standards, and anyone interested in cannabis and road safety.
At least 5 hours recommended after inhaled cannabis before driving
What the researchers found
At peak effect, THC significantly impaired lateral control, tracking, and divided attention. The duration of impairment from inhaled THC depended on dose, time since use, and skill assessed, but generally resolved within 3-5 hours. Oral THC-induced impairment took longer to subside. The researchers suggested waiting at least 5 hours after inhaled cannabis before performing safety-sensitive tasks.
Why it matters
As cannabis legalization expands, evidence-based guidelines for how long to wait before driving are essential. This meta-analysis provides the most comprehensive synthesis to date of how long THC-related driving impairment actually lasts.
The numbers in context
80 publications reviewed; 1,534 outcomes analyzed; impairment significant at peak for lateral control, tracking, divided attention; recommended wait time: at least 5 hours after inhaled use
How the study worked
Systematic review and meta-analysis of 80 publications encompassing 1,534 outcomes examining acute THC effects on driving performance and driving-related cognitive skills, with particular focus on duration of impairment.
What this study cannot tell us
Laboratory and simulated driving conditions may not fully replicate real-world driving. Most studies used moderate, controlled doses. Individual variation in tolerance and metabolism is not captured. Limited data on high-potency products.
How to read the evidence
Large systematic review and meta-analysis of 80 publications with robust quantitative synthesis of impairment duration.
When this study was published
Published in 2021.
The bigger picture
The 5-hour recommendation for inhaled cannabis provides a concrete, evidence-based number that can inform policy, clinical guidance, and public health messaging, similar to how blood alcohol guidelines inform alcohol-related driving decisions.
Questions still open
- How do high-potency products affect the duration of impairment? Should different wait times be recommended for frequent versus occasional users? How do combined THC and CBD products affect driving impairment duration? What about the day after heavy use?
Common questions
How long should you wait after using cannabis before driving?
Which driving skills are most affected by THC?
Read the original research
Determining the magnitude and duration of acute Δ9-tetrahydrocannabinol (Δ9-THC)-induced driving and cognitive impairment: A systematic and meta-analytic review.
Neuroscience and biobehavioral reviews, 126, 175-193
Citation
McCartney, Danielle; Arkell, Thomas R; Irwin, Christopher; McGregor, Iain S. (2021). Determining the magnitude and duration of acute Δ9-tetrahydrocannabinol (Δ9-THC)-induced driving and cognitive impairment: A systematic and meta-analytic review.. Neuroscience and biobehavioral reviews, 126, 175-193. https://doi.org/10.1016/j.neubiorev.2021.01.003