Study finds little evidence cannabis ‘hangover’ affects performance

The first scientific review examining the phenomenon of a ‘next day’ hangover caused by cannabis has found little evidence of impairment hours after consumption.

While cannabis has been known to impair cognition and safety-sensitive task performance immediately after use, the question of impairment many hours or days after use has been unclear.

To find out, researchers at the University of Sydney’s Lambert Initiative for Cannabinoid Therapeutics, evaluated 20 published studies investigating the effect the intoxicating component of cannabis, called tetrahydrocannabinol (THC), leaves on users more than eight hours after use.

This window known as the ‘next day’ effect encompasses any impact, including cognition or safety-sensitive task performance, beyond that eight-hour threshold.

Among the 345 performance tests administered across the studies, just 12 performance tests (representing 3.5% of the total) conducted across five papers demonstrated significant ‘next day’ deterioration following THC use.

However, none of those five studies used randomised double-blind placebo-controlled designs and all were dated by more than 18 years.

Credit: Lambert Initiative for Cannabinoid Therapeutics

The research, due to be published in an upcoming issue of Cannabis and Cannabinoid Research, was led by research fellow, Dr Danielle McCartney and colleagues Ms Anastasia Suraev and academic director, Professor Iain McGregor.

Dr McCartney said: “Most studies didn’t detect ‘next day’ effects of cannabis use, and the few that did had significant limitations. 

“We can’t really comment on the magnitude of these effects because they weren’t all that well reported.

“They didn’t appear to be associated with a specific dose of THC, route of THC administration or type of assessment.”

She added: “Overall, it appears that there is limited scientific evidence to support the assertion that cannabis use impairs ‘next day’ performance. Though, further research is still required to fully address this issue.”  

Present day consequences

THC ‘next day’ effects are of significance to drug driving laws and safety-sensitive workplaces, which are strict and lack the nuances of an alcohol breath test, for example. Currently the presence of the THC biomarker in a blood sample is enough to fail a test.

“THC can persist in blood and oral fluid for an extended period of time,” Dr McCartney said.   

“So it is important to find out whether impairment can persist, too. People are being advised not to drive or perform other safety-sensitive tasks for 24 hours after cannabis use. However, we found little evidence to support this recommendation.”

Professor McGregor, who has testified on behalf of people who have failed THC drug tests, said an issue has arisen from various legal cases where the fitness of staff undertaking safety-sensitive tasks has been a problem.

However, as the researchers point out in their review, any ‘next day’ effect from THC is unlikely to be more impairing than a hangover caused by alcohol.

The authors said: “Policy makers should bear in mind that the implementation of very conservative workplace regulations can have serious consequences, such as termination of employment with a positive drug test.

“They can also impact the quality of life of individuals who are required to abstain from medicinal cannabis used to treat conditions such as insomnia or chronic pain for fear of a positive workplace or roadside drug test.

“Studies involving medicinal cannabis users are strongly recommended.”

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