How long should you wait after using cannabis before getting behind the wheel of a vehicle? The answer is not straightforward according to a review and meta-analysis of the effects of cannabis on driving and driving-related cognition.
While the study confirms THC impairs driving, there is no simple formula to determine how long to wait before driving, because the impacts depend on many factors, including whether someone is a regular cannabis user.
The bottom line, according to the authors, is to wait at least seven hours after inhaling cannabis—and longer after ingestion—before driving or engaging in other safety-sensitive tasks.
As more states legalize the use of recreational and medical cannabis, legislators want to know how THC intoxication affects road safety so they can regulate what constitutes safe behavior. Unfortunately, the duration of THC-induced driving impairment has not been well established and, unlike the case with alcohol, there is no accurate measurement of intoxication, because there is no clear relationship between blood levels of THC and impairment.
Analyzing 80 scientific articles, the current study showed THC has significant adverse effects on lateral control and reaction time during driving, as well as on mental skills essential for safe driving, including sustained attention and fine motor function.
Interestingly, maintaining the distance from the vehicle ahead, speed, and speed variability were not affected by THC.
The extent of impairment was less for regular users and depended on how much cannabis was consumed and the length of time since it was used, making hard-and-fast rules impossible. The study suggests impairment may last longer—possibly as much as 10 hours—after oral ingestion of 20 mg of THC.
An earlier review by the National Academies of Sciences, Engineering, and Medicine concluded that driving while under the influence of cannabis increases the risk of motor vehicle accidents.
These recommendations are in keeping with others, including the Health Canada-endorsed Lower Risk Cannabis Use Guidelines, which caution to wait at least six hours and the recommendation from the College of Family Physicians of Canada to wait at least eight hours after inhalation or ingestion if the user experiences euphoria.