Surprisingly, this is a question on which there is very little data — unlike alcohol, there is no accurate measurement of intoxication or clear relationship between blood levels of THC and impairment. However a new review and meta-analysis of the effects of cannabis on driving and cognition helps shed some light.
The research, carried out at the University of Sydney, evaluated 80 studies and confirmed that, as expected, acute administration of THC or tetrahydrocannabinol, which is the main psychoactive component in cannabis, impairs aspects of driving performance.
However the study suggests many different variables make predicting how long someone will be unsafe to drive after cannabis difficult. The researchers found that regular users were less affected than those who used cannabis just occasionally for example, and that while sustained attention and lateral control — the ability to stay in one lane and not swerve across lanes while driving — was affected, maintaining distance from the vehicle ahead, as well as control over speed were not affected by THC.
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 the operation of machinery. Higher doses of 20mg THC may require at least ten hours abstinence from driving or other potentially dangerous tasks.
These recommendations fit with Health Canada’s 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.
However the researchers say there are limitations to the conclusions that can be drawn and more research is needed. “Further research involving regular cannabis users and longer post-treatment time intervals would permit better characterisation of THC’s effects and help inform the development of guidelines and drug-driving legislation to promote safe driving practices following THC and cannabis use,” they wrote.
Sources: Marijuana Business Daily, Science Direct