People consuming THC could be safe to drive in a matter of hours according to new research which will boost calls for reform to drug-driving laws in Australia.

An analysis of 80 scientific studies by the Lambert Initiative for Cannabinoid Therapeutics identified a ‘window of impairment’ of between three and 10 hours caused by moderate to high doses of THC.

The study found the exact duration of impairment depends on the dose of THC, whether it is inhaled or taken orally, frequency of use and the demands of the task being undertaken while intoxicated.

The researchers found for those using THC in oils, sprays or capsules, the impairment took longer to appear, but lasted significantly longer than for those inhaling.

The team said the findings have implications for drug-driving laws.

Lead author Dr Danielle McCartney from the Lambert Initiative for Cannabinoid Therapeutics at the University of Sydney said: “Legal cannabis use, both medical and non-medical, is increasingly common across the world.

“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.

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

“This impairment may extend up to six or seven hours if higher doses of THC are inhaled and complex tasks, such as driving, are assessed.”

Academic Director of the Lambert Initiative Professor Iain McGregor added: “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.

Lambert Initiative
Professor Iain McGregor from the Lambert Initiative: legal framework needs to catch up

“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.

“Laws should be about safety on the roads, not arbitrary punishment. Given that cannabis is legal in an increasing number of jurisdictions, we need an evidence-based approach to drug-driving laws.”

Published in Neuroscience & Biobehavioral Reviews, the study distilled the results of 80 separate scientific studies into THC-induced impairment conducted over the past 20 years.

For the purposes of the study a moderate THC dose is considered to be about 10 milligrams, but the researchers said what is moderate for a regular user could be high for an occasional user.

Co-author Dr Thomas Arkell, also from the Lambert Initiative, said: “We found that impairment is much more predictable in occasional cannabis users than regular cannabis users. Heavy users show significant tolerance to the effects of cannabis on driving and cognitive function, while typically displaying some impairment.”

The authors noted that regular users might consume more cannabis to achieve an effect, leading to an equivalent amount of impairment.

Previous research by the Lambert Initiative showed CBD does not cause impairment in driving.

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  1. How is that going to be policed? This is not the same as alcohol where a specific concentration is used to differentiate impaired driving from unimpaired driving. Without some discriminating criteria I can’t see the police or politicians changing the law!

    1. Just mandate that if a person tests positive for THC, that they do various tests (walking a straight line, balance and so on). I mean really? Is it so hard to come up with a solution? This is 2021 not 1959.