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.