April 20 has become known as a day to celebrate the pleasures of marijuana consumption with parties that traditionally begin at 4:20 p.m.

But a study in JAMA Internal Medicine has found that the high spirits may have a price: a significant increase in fatal car wrecks after the “4/20” party ends.

Researchers used 25 years of data on car crashes in the United States in which at least one person died. They compared the number of fatal accidents between 4:20 p.m. and midnight on April 20 each year with accidents during the same hours one week before and one week after that date.

Before 4:20 p.m. there was no difference between the number of fatalities on April 20 and the number on the nearby dates. But from 4:20 p.m. to midnight, there was a 12 percent increased risk of a fatal car crash on April 20 compared with the control dates. The increased risk was particularly large in drivers 20 and younger.