A new scientific review is challenging the idea that there’s a marijuana “hangover” effect the day after use, raising questions about policies that punish drivers and people in safety-sensitive positions for cannabis consumption that occurs weeks prior to drug tests being administered.
Researchers at the University of Sydney reviewed 20 studies that looked at the effects of marijuana eight hours after use, focusing on performance assessments. Their findings are set to be published in the journal Cannabis and Cannabinoid Research.
“Most studies didn’t detect ‘next day’ effects of cannabis use, and the few that did had significant limitations,” study author Danielle McCartney said in a press release. “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.”
A total of 350 performance assessments were administered throughout the 20 studies that were reviewed. Only 12 of those tests (or 3.5 percent) found a significant hangover effect—and none of those involved randomized, double-blind, placebo-controlled methods. They were also all more than 18 years old.
“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.”
“We can’t really comment on the magnitude of these effects because they weren’t all that well reported,” McCartney said. “They didn’t appear to be associated with a specific dose of THC, route of THC administration or type of assessment.”
The researchers said that their findings are notable in the context of evolving policies on driving and employment for cannabis consumers.
There have been some who’ve argued that a person shouldn’t drive or work in a safety-sensitive position for at least a day after using marijuana, but the study “found little evidence to support this recommendation.”
“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,” the study states. “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.”
A related issue that the researchers noted is that drug tests are only able to detect inactive THC metabolites that don’t reflect intoxication and can stay in a person’s system for weeks or months after use.
This issue has become a focus of policymaking as the legalization movement continues to spread. Certain sectors like the trucking industry have identified THC screening as a major contributing factor for labor shortages, for example.
The head of the American Trucking Association (ATA) recently discussed the problem with a congressional committee, arguing that lawmakers need to “step up” to address the federal and state cannabis policy conflict as the industry faces these shortages.
Tens of thousands of commercial truckers are testing positive for marijuana as part of the federally mandated screenings, recent data from the Department of Transportation (DOT) shows.
Meanwhile, a senator sent a letter to DOT last year seeking an update on that status of a federal report into research barriers that are inhibiting the development of a standardized test for marijuana impairment on the roads. The department is required to complete the report by November 2023 under a large-scale infrastructure bill that President Joe Biden signed.
Experts and advocates have emphasized that evidence isn’t clear on the relationship between THC concentrations in blood and impairment.
A study published in 2019, for example, concluded that those who drive at the legal THC limit—which is typically between two to five nanograms of THC per milliliter of blood—were not statistically more likely to be involved in an accident compared to people who haven’t used marijuana.
Separately, the Congressional Research Service in 2019 determined that while “marijuana consumption can affect a person’s response times and motor performance…studies of the impact of marijuana consumption on a driver’s risk of being involved in a crash have produced conflicting results, with some studies finding little or no increased risk of a crash from marijuana usage.”
Another study from last year found that smoking CBD-rich marijuana had “no significant impact” on driving ability, despite the fact that all study participants exceeded the per se limit for THC in their blood.
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