Health & Wellbeing

Study calculates duration of driving impairment after smoking cannabis

Study calculates duration of d...
New research suggests smoking cannabis can generally impair one's driving ability for between three and five hours
New research suggests smoking cannabis can generally impair one's driving ability for between three and five hours
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New research suggests smoking cannabis can generally impair one's driving ability for between three and five hours
New research suggests smoking cannabis can generally impair one's driving ability for between three and five hours

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

So if my friend smokes pot that won't affect my driving right?
I am always blown away (pun intended) by how many clouds of marijuana smelling vape and smoke pours out of car windows here in California.
What ever happened to the notion that cigarette and marijuana use leads to the more powerful drugs? We see the big issues already caused by the above, and if one decides to turn to more addictive drugs it’ll present even greater issues.

I guess that depends on what one calls impairment.
After smoking pot in 2 hrs regular users are able to drive well, much better than someone with .07 alcohol yet still legal to drive.
Perhaps they should just do a roadside sobriety test. If they fail and have significant enough THC, then it's DWI. If they pass the sobriety test, they are fine.
Brian M
If not sure then air on the side of caution - Using cannabis is not essential, so just don't do it and drive - Not difficult, so perhaps close to zero THC level is the answer, until a clear, reliable and provable answer is found.

Same can be argued for alcohol and any other dugs that effect the ability to drive. If you think this is wrong then you have not suffered or seen the horrific results of a drug or alcohol impaired driver.
I hope this article was not aiming for scaring the public against marijuana legalization!
Unlike alcohol, which is legal, marijuana is overdose-safe, for example!
Perhaps they should just do a roadside sobriety test. If they fail and have significant enough THC, then it's DWI. If they pass the sobriety test, they are fine.[/quote]

In Australia we don't have sobriety tests, we have supposed random breath tests which locally means every car on the road is pulled over and every driver tested, absolutely NOTHING random about it.

I live in a town which is a gateway town to many other regions, everyone coming from Sydney heading west MUST pass through the town/region, whether on the highway or through the town itself and, cops set up supposed random breath tests on both roads.

Plus, they have some BS drug test where they drag a thing down your tongue (the driver does this) and ANY traces of MJ and you are going to lose your license. Regardless of manufacturers data on the validity of the tests or how long after smoking they detect pot.

Of course this isn't random testing, it's blatant revenue gouging by the Australian government and state police departments, we are the Nanny capital of the world.

I don't smoke pot, I have in the past before my son was born but I couldn't afford pot/alcohol and my son so I decided something had to go and the kid was extremely cute.

I am however in the unfortunate position of suffering from chronic pain where medical MJ may help (chronic migraine headaches daily for 50 years) but to access medical MJ I need to hand in my license or end up in court with huge fines and loss of license.
My licenses is extremely important.
'Gateway drug' argument has not actually been backed up with any data.

Very curious to see how 'impairment' is calculated and determined.

'Zero tolerance' is useless in all aspects of life. "until a clear, reliable and provable answer is found"...this never happens...with anything. Everything is 'good enough' and we always learn more, change our understandings, and adapt. If we waited until 'THE answer' is found humans would never exist.

Lack of sleep can impair driving. The test should be an 'impairment test', not a 'substance test.'