Cannabis does not cause “next-day impairment” researchers found in a new study published last week in the journal Psychopharmacology. Researchers found little evidence to suggest that THC can impact a person’s performance the next day, though any experienced smoker could tell you the same.

Researchers observed 20 adults with physician-diagnosed insomnia who consumed cannabis irregularly, gathering existing data from a larger study investigating the effects of THC and CBD on insomnia. Since the participants were taking THC and CBD right before bedtime, it was a perfect pool of subjects to examine next-day effects.

People received either a 2 ml dose of cannabis oil containing 10 mg THC and 200 mg CBD, or a placebo, being randomly selected. Within two hours of waking up, participants had to complete cognitive tasks and psychomotor tests.

The study shows that people who took 10 mg of THC, the intoxicating ingredient, showed little to no impairment the next day, suggesting that it’s entirely safe to drive after taking cannabis the night before. People who took THC before bedtime nearly aced the test the next day. Participants showed “no differences in ‘next day’ performance in 27 out of 28 tests of cognitive and psychomotor function and simulated driving tests relative to placebo.”

“The use of cannabis by night as a sleep aid is highly prevalent and there are legitimate concerns that this may lead to impaired daytime (‘next day’) function, particularly on safety sensitive tasks such as driving,” said the researchers at Macquarie University in Sydney, the University of Sydney, Royal Prince Alfred Hospital in Sydney, Gold Coast-based Griffith University and Johns Hopkins University. 

The study was written by Anastasia Suraev,  Danielle McCartney,  Nathaniel S. Marshall, Christopher Irwin,  Ryan Vandrey,  Ronald R. Grunstein,  Angela L. D’Rozario,  Christopher Gordon,  Delwyn Bartlett,  Camilla M. Hoyos and Iain S. McGregor. Many are associated with the University of Sydney’s Lambert Initiative for Cannabinoid Therapeutics, and have studied cannabis impairment in the past. 

“The results of this study indicate that a single oral dose of 10 mg THC (in combination with 200 mg CBD) does not notably impair ‘next day’ cognitive function or driving performance relative to placebo in adults with insomnia disorder who infrequently use cannabis,” the study reads. “Larger studies in patient populations are required to determine the effects of repeated dosing with THC (with or without CBD), and at higher doses of THC, on ‘next day’ function.

Researchers associated with the Lambert Initiative have found data showing that impairment from THC lasts only hours, while drug test can detect THC for weeks if not months after consuming it, long after the effects have worn off.

While researchers noticed minor but insignificant changes in cognitive function, further tests showed that there was likely no significant impairment.

“Importantly, no significant difference in accuracy was observed on the more difficult ‘hard/incongruent condition’ of the Stroop-Word Test, which requires participants to match the meaning of the word presented, not the printed colour of the word,” authors added. “For comparison, the morning after alcohol consumption (i.e., the hangover state) produced significantly greater interference on the Stroop-Word Test, but not the Stroop-Colour Test, relative to the alcohol-free control group (i.e., no hangover state).”

Impairment and Drug Tests

Recent studies have shown that levels of THC detected in the blood or breath of cannabis consumers is not a reliable indicator of impairment. Researchers also found that levels of THC in blood and breath did not provide reliable evidence of how recently a test subject had consumed cannabis.

Neither the detection of THC in blood nor in breath is correlated with impairment of performance or recency of cannabis exposure, according to data published in the journal Nature Scientific Reports.

Researchers wrote that “finding an objective measure of recent cannabis use that correlates with impairment has proven to be an elusive goal.” Some states have enacted laws that set per se legal limits on the amount of THC a driver may have in their blood, similar to the 0.08% blood alcohol concentration limit in effect nationwide.

“These findings provide further evidence that single measurements of specific delta-9-THC blood concentrations do not correlate with impairment, and that the use of per se legal limits for delta-9-THC is not scientifically justifiable at the present time,” wrote the authors of the study.

To conduct the study, the researchers recruited a group of test subjects, most of whom were daily cannabis users. The scientists then determined the THC levels in their blood and breath prior to and after inhaling cannabis.

While any cannabis consumer with experience knows that the effects last only a matter of hours, science to show that impairment doesn’t last that long now backs it up.