SYDNEY, Australia — As marijuana continues to become more and more accepted by society, it opens up a number of new questions regarding how to efficiently regulate the safe, legal use of cannabis. For example, how long should a cannabis user have to wait before operating a vehicle? A new study finds the answer may be longer than many people think. Australian researchers find the “window of cannabis impairment” can last anywhere from three to 10 hours.
Based on an analysis of 80 prior marijuana studies, researchers from the University of Sydney say their findings are relevant regarding moderate to high doses of tetrahydrocannabinol (THC). This is the psychoactive ingredient in marijuana responsible for giving users their trademark “high.”
What keeps someone ‘high’ longer?
Three and ten hours is quite a difference, but researchers say many factors can influence how long a high lasts. These include the THC dosage, how users consume the marijuana (edibles, smoking), and how often these individuals use marijuana. Researchers add even the difficulty of the task cannabis users are trying to perform plays a role in determining impairment.
“Legal cannabis use, both medical and non-medical, is increasingly common across the world,” says lead author Dr. Danielle McCartney from the Lambert Initiative for Cannabinoid Therapeutics in a university release. “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 vaporization and simpler tasks are undertaken (e.g., those using cognitive skills such as reaction time, sustained attention and working memory),” Dr. McCartney explains. “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.”
Regular cannabis users build up a greater tolerance
For this study, researchers defined a moderate THC dose as 10 mg. However, they note that dose may be very mild for a habitual smoker but enough to significantly affect novice users. Daily smokers often end up using more marijuana than beginners just to attain the same high. Tolerance builds over time and the only way to get it back down is to take a break from cannabis.
“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,” comments study co-author Dr. Thomas Arkell.
Different products deliver their highs at different times
Furthermore, unlike the classic image of a “stoner” with a joint or bong, people now consume cannabis in multiple ways. Many medical marijuana users get their daily dose via oils, edibles, sprays, or capsules. This is important to note because these methods often take longer to “kick in” but last significantly longer than smoking.
“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,” adds Professor Iain McGregor, Academic Director of the Lambert Initiative.
“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,” McGregor concludes.
On that note, a recent study did discover that fatal road collisions do go up in areas with legal marijuana. That report, from Canada, suggests that marijuana users wait at least six hours before driving.
Researchers add that CBD, another popular cannabidiol these days, does not result in any impairment whatsoever. This non-psychoactive ingredient is growing in popularity among patients seeking a natural pain or stress reliever.
The study appears in the journal Neuroscience & Biobehavioral Reviews.