After Effects

New research suggests the cannabis “hangover” is more myth than reality — which should affect drug policies everywhere


One of the most common arguments supporting drug-testing policies is the idea of the cannabis “hangover”: sluggishness, grogginess, confusion and disorientation that supposedly follow cannabis use for a full day afterwards. 

It’s the basis on which doctors, nurses, police officers, military personnel, DMV workers, engineers, construction workers, truck drivers, and so many others are drug tested regularly. If they’re using cannabis at home, they could be hungover on the job the next morning. They could make a mistake in their haze and someone could get hurt. 

Anecdotally, anyone who’s ever used too much cannabis at once knows there’s some truth to this. If you eat a 100 mg brownie before you fall asleep, you’ll wake up in a green fog. If you chain smoke blunts with your friends until you pass out, you’ll probably be moving slower than usual on the other side. That’s true of any substance. 

Nine times out of 10, though, when someone smokes a bowl, burns a joint, or eats a gummy (or two), they will not feel anything the next day. Regular users know this: The “hangover” from cannabis is negligible. Especially when it’s being used in moderation (and especially when you have a tolerance).

Now there’s science to back that up. 

New research from the University of Sydney reviewed 20 scientific studies, all exploring the performance effects of cannabis up to eight hours after use. Across the studies, more than 350 performance assessments were administered to test subjects. Only 12 of those indicated any kind of hangover effect — and of those, none used double-blind, randomized, placebo-controlled methods.  

The review, published in Cannabis and Cannabinoid Research, concludes, “A small number of lower-quality studies have observed negative (i.e., impairing) ‘next day’ effects of THC on cognitive function and safety-sensitive tasks. However, higher-quality studies, and a large majority of performance tests, have not.”

The study’s author, Daniel McCartney, a research fellow at the University of Sydney, thinks these findings could have implications for the way lawmakers approach policies about cannabis use as legalization spreads across the U.S. and abroad.

“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,” McCartney says. 

These findings could be significant in the context of labor shortages. Here in the U.S., recent data from the Department of Transportation shows that tens of thousands of commercial truckers are testing positive for cannabis use and losing their jobs as a result, contributing to one of the biggest shortages in American truck-transit history. 

President of the American Trucking Association (ATA) Chris Spear recently told Congress this was an issue that “keeps him up at night.” Not that truckers want to drive while impaired, but rather that they can use cannabis legally in a state they’re traveling through when they’re off-shift, and then still test positive for it weeks later and lose their job. It’s creating a bottleneck in our already-strained supply chain. 

This extends to nurses, teachers, scientists, engineers, construction workers, government employees and anyone else who stands to lose their job because they used cannabis medicinally or recreationally at home. 

“Policymakers 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,” the study says. “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.”

If these findings are accurate, then the issue becomes one of testing for impairment rather than past use. Currently, there are no means for testing for cannabis impairment like a breathalyzer or blood test measures alcohol impairment. Companies are working on it (Weed Between the Lines, “Windows of impairment and detection,” Sept. 16, 2021), but for now, the best we have is the standard immunoassay —  the urine test. 

McCartney notes that these findings need further research before laws start changing.