Parents get a lot of mixed messages about alcohol, drugs and their adolescent children. Some experts say curiosity about drugs and alcohol is natural, and it's OK to allow teens to experiment a little. Others say kids shouldn't touch drugs or alcohol at all until they're much older. Now, scientists are getting some insight into the long-term effects of teenage drug and alcohol use. VOA's Rose Hoban reports.

Professor Candice Odgers from the University of California at Irvine says the first clues have come from studies of rats.

"[The studies] find that they are more sensitive to the stimulating effects. They crave more. They become addicted faster," Odgers says. "So animal models are kind of pointing to this period as something that, that might be a critical period in alcohol exposure."

But Odgers says it would be unethical for scientists to intentionally expose young people to alcohol and look at their brains. Instead, researchers have been conducting a large, long-term study in New Zealand.

The Dunedin study has been collecting information about a group of more than one thousand children who were born in the 1970s and are now in their 30s. Among the data collected is how much alcohol and drugs the kids used as adolescents and when they started using it.

Odgers examined some of the Dunedin data that's been collected on how kids respond to alcohol over the long term.

"We found that kids who used substances before the age of 15? they were two to three times more likely to develop substance dependency later on, to develop a sexually transmitted disease, to fail in school or to be convicted of a crime," Odgers reports.

"And for the girls, those girls who were exposed to substances before the age of 15 were two to three times more likely to have an early pregnancy... that is, to have been pregnant before the age of 21," she says.

What Odgers and her colleagues found especially disturbing was that it was impossible to predict which kids would have problems with drugs and alcohol. Teens from so-called "good" families with stable, supportive parents and those from so-called "bad" families with abusive or neglectful adults were equally as likely to have bad outcomes. What did seem to make a difference, Odgers says, was whether the kids began using drugs or alcohol before the age of 15.

"So this tells us something about prevention efforts," she says. "If we are going to try to just target at-risk kids, we're going to miss half of our audience, those 50 percent of kids who don't look like they have risk factors or behavioral problems coming into that window. They're still at risk for developing these problems if they are exposed to substances."

Odgers says parents need to continue to be vigilant about monitoring their children's behavior and habits well into their teens.

Her paper is published in the journal Psychological Science.