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New Research Sheds Light on How Long Cannabis Impairment Lasts

By Alexander Beadle

Published: Apr 16, 2021   
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In a new meta-analysis of past cannabis impairment studies, researchers at the University of Sydney have identified a “window of impairment” following medium and high-dose cannabis use, during which users should avoid complex and/or safety-related tasks such as driving.

While the exact duration of impairment will vary with dosage, consumption method, and the specific task that a user is trying to carry out, the researchers say that their findings will have implications for the application of drug-driving laws and the development of related evidence-based policy.

Cannabis intoxication lasts between 3 to 10 hours

Traces of tetrahydrocannabinol (THC), the main intoxicant in cannabis, can be found in the body for weeks after the cannabis product has actually been used. By comparison, the actual impairment time caused by that cannabis product is fleeting. This raises problems in terms of how laws such as drug-driving restrictions are enforced; a simple positive test for presence of THC in saliva might not actually tell a traffic law enforcer very much at all about whether a driver is impaired.

“Legal cannabis use, both medical and non-medical, is increasingly common across the world,” Dr Danielle McCartney from the Lambert Initiative for Cannabinoid Therapeutics at the University of Sydney, said in a statement. “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 University of Sydney researchers conducted a meta-analysis on 80 studies from the past 20 years that dealt with THC-induced impairment. The results of the meta-analysis confirmed that acute THC use can impair aspects of driving performance and identified a clear “window of impairment” lasting for between three-to-ten hours depending on the dose and type of skill being tested.

“Our analysis indicates that impairment may last up to 10 hours if high doses of THC are consumed orally,” McCartney said. “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 (e.g., 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.”

The meta-analysis found that most driving-related skills would be able to recover from impairment roughly five hours after consuming a 20 mg dose of THC, with almost all skills fully recovered by the seven-hour mark.

Co-author Dr Thomas Arkell, added: “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.”

Cannabis intoxication and law enforcement

As more countries adopt legislation legalizing cannabis for medical and recreational use, it is important that the proper impacts of cannabis on driving are well-assessed. This is particularly pertinent for medical cannabis patients who might need to be using cannabis regularly to manage their symptoms. Such a patient would not be intoxicated at all hours, but a simple THC detection test using the patient’s saliva or blood would likely register as positive given the frequent use. To avoid criminalizing patients, or recreational users with a similar usage pattern, it is important that scientists better understand cannabis intoxication and its effects.

Academic Director of the Lambert Initiative Professor Iain McGregor said: “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. 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,” McGregor continued. “Given that cannabis is legal in an increasing number of jurisdictions, we need an evidence-based approach to drug-driving laws.”

These University of Sydney researchers have collaborated on other cannabis-related studies on driving in the past, including a randomized trial investigating the specific effects of vaped cannabidiol (CBD) – another major cannabinoid present in cannabis – on driving performance. Working with colleagues from Maastricht University, the Netherlands, the researchers found CBD to exert no negative effects on driving ability, however other researchers in the field have pointed out that only low doses of CBD were examined in the study, and so more work on this topic is likely needed in the future.


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