A review of 25 years worth of car crash data has found a link between the so-called "high holiday" celebrated by marijuana users on April 20 (4/20) and a rise in fatal car accidents.
The significance of the date is debated, but has been linked to the story of a group of students in San Rafael, Calif., who used to meet after school at 4:20 pm to smoke pot.
The study conducted at the University of British Columbia looked at U.S. government statistics from 1992 - the year after an article in High Times magazine popularized the informal event - through 2016.
Researchers compared fatalities on that date to those one week before and after it and found there to be a 12 percent increase on average, which represents 142 deaths.
The study's authors hypothesized a connection based on previous research that suggests driving after marijuana use is "surprisingly common" and can increase crash risk.
Other studies have found a similarly elevated risk linked with alcohol and driving on Super Bowl Sunday and New Year's Eve.
Most accidents had no police data on drug testing so there's no way to confirm that marijuana was involved, but researchers think the drug was responsible for some crashes.
The study was published Monday in JAMA Internal Medicine.
"It's a really relevant question to be thinking about now since legalization seems to be progressing across the United States and in Canada," Staples said.
Marijuana is legal for medical use in most U.S. states and in Canada; recreational use has been legalized in nine states plus Washington, D.C. and is expected to become legal in Canada in July.
According to the National Institute on Drug Abuse, marijuana can impair driving ability. However, the U.S. agency also notes that directly linking marijuana with car crashes can be difficult because it's often used while drinking alcohol.
An earlier survey of college freshmen found that driving after marijuana use and riding with someone using pot was common, especially among young men, and more common than alcohol-linked driving. And a 2014 study in six states found that marijuana was increasingly detected in drivers killed in car crashes during a 10-year period.
It's a recognized risk in Colorado, where voters approved legalizing it in 2012. Last year the state's transportation department partnered with Lyft and a pot industry group to offer discounted rides for pot users partaking in 4/20 festivities. The program runs through April this year. Data show that 15 percent of DUI arrests in Colorado last year involved marijuana, and a 2016 survey found about half of users in the state think driving under the influence of pot is safe.
Dr. Andrew Monte, an emergency medicine physician and toxicology expert at the University of Colorado, said his hospital typically sees a slight increase in marijuana-using patients on April 20, but that vomiting and intoxication are more common reasons than car crash injuries.
Still, he said the study offers an important message.
"We think about cannabis as being ubiquitous in our society," he said. "However we need to realize that people should not be using this and driving."
The Associated Press contributed to this report