MONDAY, April 28, 2014 (HealthDay News) -- Teenagers who mix alcohol and marijuana are more likely to be dangerous on the road than teens who use one or the other drug, a new study suggests.
Researchers found that previous combined use of alcohol and marijuana further increased a teen's risk of getting a traffic ticket or being in a wreck, as reported in the May issue of the Journal of Studies on Alcohol and Drugs.
"Simultaneous use makes a big difference in your risk for unsafe driving," said lead researcher Yvonne Terry-McElrath, of the University of Michigan's Institute for Social Research in Ann Arbor. "There's a very clear increase in risk for this group of kids, and for the rest of us on the roads."
Teens who used alcohol and marijuana at the same time were about 50 percent to 90 percent more likely to engage in unsafe driving than their peers who didn't drink or smoke pot, the researchers reported.
They also had a higher risk for tickets or wrecks compared with kids who only drank.
"As you go from alcohol use to alcohol and marijuana use concurrently, your risk increased," Terry-McElrath said. "There's a clear stair-stepping up the risk process for tickets and warnings."
Jan Withers, national president of Mothers Against Drunk Driving, called the findings "very concerning."
"It's important for parents to have ongoing conversations with their teens about the dangers of underage drinking," Withers said. "These kinds of open, caring conversations about values, rules and consequences can help kids with a variety of decisions about whether or not to engage in risky behaviors. This issue is too important for parents to leave to chance and hope for the best."
The findings come from surveys of more than 72,000 U.S. high school seniors, conducted annually from 1976 to 2011 through the Monitoring the Future study funded by the U.S. National Institute on Drug Abuse.
Teens were asked to report their alcohol or drug use during the previous 12 months, as well as whether they had been in a car wreck or had gotten a ticket for a moving violation.
Teens who drank were 40 percent more likely to admit they had gotten a traffic ticket and 24 percent more likely to admit involvement in a traffic crash, the researchers found.
But teens who drank and smoked pot at the same time were 90 percent more likely to get a ticket and 50 percent more likely to be in a wreck.
Previous studies have reported that teens who frequently use alcohol or marijuana are more likely to be involved in a crash, the authors noted in background information.
"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."
Both drinking and marijuana use among teens have declined over time. In 2011, one-third of high school seniors said they hadn't used either in the past year, compared with 12 percent in 1979.
About 31 percent of students in 2011 reported that they just drank, while 13 percent said they alternated alcohol and marijuana use. Another 21 percent reported combining pot and alcohol, at least occasionally.
"The study shows that a significant number of teens reported using alcohol but not marijuana, although almost none of them reported using marijuana but not alcohol," Withers said.
The combined use of pot and alcohol could impair teenagers' driving and judgment to a greater degree, Terry-McElrath said.
It also could be that kids who use both substances at once are more inclined to take risks throughout their lives.
"As the parent of a 15-year-old, I would take away the keys if I found out she were using either alcohol or marijuana, let alone both," Terry-McElrath said.
For more on teen drinking and driving, visit the U.S. Centers for Disease Control and Prevention.