Study calculates duration of driving impairment after smoking cannabis

How long is a person’s ability to safely drive impaired after consuming cannabis? A world-first meta-analysis looking at 80 different studies suggests cannabis intoxication spans anywhere from three to 10 hours, raising questions over some roadside tests detecting traces of THC days after consumption.

“Legal cannabis use, both medical and non-medical, is increasingly common across the world,” explains lead author on the new study, Danielle McCartney. “THC is known to acutely impair driving and cognitive performance but many users are unsure how long this impairment lasts and when they can resume safety-sensitive tasks, such as driving, after cannabis consumption.”

The new research extracted data from 80 studies looking at the effects of cannabis intoxication on driving. A meta-analysis was then performed to calculate an average duration of impairment. The results revealed a significant spectrum of impairment timing, depending on a broad variety of factors, from mode of consumption (oral vs inhalation) to frequency of use.

“Our analysis indicates that impairment may last up to 10 hours if high doses of THC are consumed orally,” says McCartney. “A more typical duration of impairment, however, is four hours, when lower doses of THC are consumed via smoking or vaporization and simpler tasks are undertaken (e.g., those using cognitive skills such as reaction time, sustained attention and working memory).”

Duration of driving impairment was more predictable in those who only occasionally use cannabis, whereas regular or heavy users displayed greater tolerance to the drug’s effects. Although in general the study concludes most driving-related cognitive skills recover between three and five hours after inhaling a moderate volume of THC, the researchers do point out the extreme variability in their findings suggests there is no easy answer to the question of cannabis impairment and driving.

“Overall, our results confirm that Delta-9-THC impairs aspects of driving performance and demonstrate that the magnitude and duration of this impairment depends on the dose provided, route of administration and frequency with which cannabis is used,” the researchers conclude in the newly published study. “There appears to be no universal answer to the question of “how long to wait before driving?” following cannabis use: consideration of multiple factors is therefore required to determine appropriate delays between Delta-9-THC use and the performance of safety-sensitive tasks.”

Of course, the huge challenge facing lawmakers right now is how to accurately measure impairment levels. Compared to cannabis, alcohol impairment is relatively easy to police. Common breathalyzers reliably measure blood alcohol levels, and these levels have been found to, for most people, correspond reasonably with intoxication and impairment.

Cannabis, on the other hand, is an entirely more complicated drug. Although researchers are racing to bring accurate THC breathalyzers to market, there are serious concerns blood THC levels do not objectively correspond with driving impairment. In fact, an independent report commissioned by the US congress in 2019 concluded there is, “no scientifically demonstrated correlation between levels of THC and degrees of impairment.”

“THC can be detected in the body weeks after cannabis consumption while it is clear that impairment lasts for a much shorter period of time,” says Iain McGregor, another author on the new study. “Our legal frameworks probably need to catch up with that and, as with alcohol, focus on the interval when users are more of a risk to themselves and others. Prosecution solely on the basis of the presence of THC in blood or saliva is manifestly unjust.”

The new study was published in the journal Neuroscience & Biobehavioral Reviews.

Source: University of Sydney