A meta-analysis of previous studies provides insight into how long the effects of cannabis last

The first-of-its-kind meta-analysis synthesized the results of 80 different scientific research.
Mert Erdemir
Hand holding a roll slim joint of marijuana on organic cannabis farm background.
Hand holding a roll slim joint of marijuana on organic cannabis farm background.

Jikaboom/iStock 

Being exposed to moderate to high doses of tetrahydrocannabinol (THC), the intoxicating component of cannabis, can cause individuals to remain impaired for between three and ten hours, according to an extensive analysis of 80 scientific studies that was published in the journal Neuroscience & Biobehavioral Reviews in 2021.

Unsurprisingly, the duration of the impairment depends on the dose of THC, whether it was inhaled or taken orally, the frequency of the individual's consumption, and the requirements of the work being performed while intoxicated, according to a press release published by the University of Sydney.

A first-of-its-kind meta-analysis

The first-of-its-kind meta-analysis synthesized the results of 80 different scientific research studies on THC-induced impairment conducted over the previous 20 years. And the researchers believe that the findings could have global implications for the application of drug-driving laws.

"Legal cannabis use, both medical and non-medical, is increasingly common across the world," stated Dr. Danielle McCartney, the lead author of the study published a year ago, from the Lambert Initiative for Cannabinoid Therapeutics at the University of Sydney.

THC, the psychoactive compound in cannabis, is what produces high sensation by impairing the driving and cognitive performance of individuals. It can be taken by smoking cannabis or in the form of oils, edibles, tinctures, capsules, and more.

"Our analysis indicates that impairment may last up to ten 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)," said McCartney.

“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.”

A moderate THC dose of 10 milligrams

The moderate THC dose for this study was set at around 10 milligrams, but the experts also stated what is moderate for a frequent user may be high for a casual user.

"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," said co-author Dr. Thomas Arkell.

THC taken in the form of oils, sprays, or capsules, which is the forms used in the medicinal use of cannabis, causes the impairment to take longer to appear and lasts significantly longer than with inhaling.

"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," said Professor Iain McGregor, the 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."

Abstract:

The increasing legal availability of cannabis has important implications for road safety. This systematic review characterized the acute effects of Δ9-THC on driving performance and driving-related cognitive skills, with a particular focus on the duration of Δ9-THC-induced impairment. Eighty publications and 1534 outcomes were reviewed. Several measures of driving performance and driving-related cognitive skills (e.g. lateral control, tracking, divided attention) demonstrated impairment in meta-analyses of “peak” Δ9-THC effects (p’s<0.05). Multiple meta-regression analyses further found that regular cannabis users experienced less impairment than ‘other’ (mostly occasional) cannabis users (p = 0.003) and that the magnitude of oral (n = 243 effect estimates [EE]) and inhaled (n = 481 EEs) Δ9-THC-induced impairment depended on various factors (dose, post-treatment time interval, the performance domain (skill) assessed) in other cannabis users (p’s<0.05). The latter model predicted that most driving-related cognitive skills would ‘recover’ (Hedges’ g=–0.25) within ∼5-hs (and almost all within ∼7-hs) of inhaling 20 mg of Δ9-THC; oral Δ9-THC-induced impairment may take longer to subside. These results suggest individuals should wait at least 5 -hs following inhaled cannabis use before performing safety-sensitive tasks.

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