A new study by the Lambert Initiative has found “scant evidence” that cannabis consumption causes a next-day hangover.
Researchers evaluated 20 published studies investigating the residual effects of THC on users – including cognition or safety-sensitive task performance – more than eight hours after consumption.
While cannabis has been known to cause impairment immediately after use, its impact hours or days after use has been less clear.
Study lead and Lambert research fellow Dr Danielle McCartney said: “Most studies didn’t detect ‘next-day’ effects of cannabis use, and the few that did had significant limitations.
“Overall, it appears there is limited scientific evidence to support the assertion that cannabis use impairs next-day performance, [although] further research is required to fully address this issue.”
Among the 345 performance tests administered across the studies, just 12 (3.5%) conducted across five papers demonstrated significant next-day deterioration following THC use.
None of those five studies used randomised, double-blind, placebo-controlled designs and all were more than 18 years old.
“We can’t really comment on the magnitude of these effects because they weren’t all that well reported,” Dr McCartney said.
“They didn’t appear to be associated with a specific dose of THC, route of administration or type of assessment.”
Lambert said the findings are of particular significance for drug-driving laws and in safety-sensitive workplaces, with the presence of the THC biomarker enough to fail a drug 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.”
The study authors said their findings suggest any next-day effect from THC is unlikely to be more impairing than a hangover caused by alcohol.
“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 said.
“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.”
The research, due to be published in an upcoming issue of Cannabis and Cannabinoid Research, is available online here.