Cannabis connoisseurs have it pretty good these days, as marijuana continues its steady march into mainstream American culture.
Pot is now fully legal in nine states and decriminalized in another 13. Late last month, district attorneys in San Francisco and San Diego announced they would clear thousands of misdemeanor convictions for marijuana-related crimes, going back 40 years. And marijuana-related business is booming: Legal sales of the leaf topped $6.7 billion in 2016, a figure that's growing at a double-digit rate.
But a study published this week in JAMA Internal Medicine is bound to harsh a few mellows.
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Researchers have discovered that the risk of fatal car crashes in the United States jumps 12 percent on April 20th, or "4/20," pot smokers' widely celebrated—if unofficial—national holiday.
The team examined data collected by the National Highway Traffic Safety Administration between 1992—the year 4/20 was popularized in High Times magazine, entering the national consciousness—and 2016.
They flagged all deadly crashes between 4:20 p.m. and midnight on April 20th, and compared those numbers to fatalities in the same afternoon time period on "control days" exactly one week earlier and later.
From the 25 years of data, the researchers were able to compare 1,300 fatal crashes on April 20th with 2,400 fatal crashes on control days.
The results were unambiguous: The public-health threat of "drugged driving" on 4/20 is similar to that of drunk driving on America's other unofficial national holiday, the Super Bowl.
"We examined a quarter-century of national data and found a 12 percent increase in the relative risk of a fatal traffic crash after 4:20 p.m. on April 20th compared with identical time intervals on control days," the authors write. "Although the vast majority of Americans do not celebrate 4/20, the observed association was comparable in magnitude to the increase in traffic risks observed on Super Bowl Sunday."
Sub-group analysis showed that men and women faced similar risk increases, while, perhaps unsurprisingly, younger drivers showed a much larger jump than other motorists. Among drivers under 21, for example, the risk of deadly crash was 38 percent higher on April 20th than on control days.
The states with the highest risk increases were not California or Colorado but Maine, North Dakota, and Hawaii; indeed, a state's level of cannabis use, high or low, seemed not to dramatically alter its risk profile.
"I work at a hospital close to the site of a large 4/20 celebration," lead author John Staples, a clinician at the University of British Columbia, tells Pacific Standard. "We've sometimes seen a surge in patient numbers on April 20th due to drug use at the festival." (Staples is perhaps understating the matter; in 2015, the gathering, at the plaza of the Vancouver Art Gallery, drew a crowd of more than 25,000 and sent 64 smokers to the emergency room.)
"It occurred to me that 4/20 was an annual natural experiment on cannabis use and crash risk," he continued. "This seemed like an important issue since over 65 million Americans now live in states with legalized recreational marijuana, and many jurisdictions are trying to figure out what that means for traffic safety."
"The simplest interpretation of our findings is that more drivers are impaired by cannabis on 4/20 and these drivers contribute to fatal crashes," Staples says. "Other factors may contribute to the observed increase in crash risk, including the possibility that some 4/20 festival participants are also consuming alcohol and other drugs in addition to cannabis, or the possibility that police are emphasizing drug enforcement rather than traffic enforcement on 4/20."
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Staples points out, too, that the 12 percent uptick in risk isn't spread evenly across the driving public. "An assumption that 6 percent of Americans celebrate 4/20 implies that participation triples the risk of involvement in a fatal crash," he says. "We thought this was a surprising and important finding."
Epidemiologists, like all scientists, must be impartial in their research—but they can't afford to be dispassionate the results. Their work is, quite often, a matter of life and death.
"I want the public to know that impairment with drugs and alcohol increases the risk of crash," Staples says. "My message to them: Don't drive high."
Social change is never easy. Even as cannabis use becomes increasingly legal—and, in the public gaze, increasingly normal—Attorney General Jeff Sessions has made marijuana enforcement a priority at the Department of Justice (DOJ), a departure from Obama-era DOJ policy. (Sessions once said that "good people don't smoke marijuana," and last week argued that pot is as much to blame for the current opioid crisis as prescription pills.) His prohibitionist stance seems unlikely to win out in the long run, but that doesn't mean the transition will be a smooth one.
The conditions under which Americans should use cannabis are still very much open to debate, and Staples and Redelmeier's study adds important, empirical evidence to a discussion that is often laced with emotion and dodgy science. Whether Americans will listen is another question entirely.