According to a study in the May issue of the Journal of Studies on Alcohol and Drugs, teenagers who drink alcohol and smoke marijuana may be at increased risk for unsafe driving.
Researchers said the findings point to a need for education on the risks of "simultaneous use" of alcohol and marijuana.
The study of U.S. high school seniors found that teens who had used both drugs in the past year had higher rates of traffic tickets/warnings and car accidents.
At particular risk were kids who used alcohol and marijuana at the same time: They were about 50 to 90 percent more likely to admit to unsafe driving than their peers who did not drink or smoke pot.
"It's well known that both drinking and other drug use are linked to risky driving," said lead researcher Yvonne Terry-McElrath, of the University of Michigan's Institute for Social Research in Ann Arbor.
"But this suggests that it's not only the frequency of substance use that's important," Terry-McElrath said. "The patterns of drug use are also related to the risk of unsafe driving."
The findings come from surveys of more than 72,000 U.S. high school seniors, conducted yearly from 1976 to 2011 through the Monitoring the Future study, supported by the National Institute on Drug Abuse.
The good news: Both drinking and marijuana use declined over time. In 2011, one third of high school seniors said they hadn't used either drug in the past year - versus only 12 percent in 1979.
Still, a "significant number" of students were using both drugs in 2011, Terry-McElrath said. That included 21 percent who said they used the drugs at the same time, at least occasionally. And those kids were at heightened risk for reporting unsafe driving - even compared with their peers who only drank, and those who used alcohol and marijuana but not at the same time.
Roughly 40 percent of teens who used both drugs together had received a traffic ticket or warning in the past year. And about 30 percent had been in an accident.
It's not clear why those teenagers were at increased risk, according to Terry-McElrath. One possibility, she said, is that they are bigger risk takers in general. But it's also possible that using both drugs together impairs teenagers' driving - and judgment - to a greater degree.
Terry-McElrath said it's important to make kids - and the public, in general - aware of the risks of simultaneous drug use.
"Driver's education needs to talk more about the risks, in believable ways - not using inaccurate scare tactics," she said.
More broadly, she added, media messages about unsafe driving should go beyond alcohol.
"We often hear the message 'Don't drink and drive,'" Terry-McElrath noted. "But we don't hear much about the risks of using additional substances, either alone or simultaneously with alcohol."