Teenagers who smoke, drink alcohol, use drugs, and engage in other risky behaviors are more likely to struggle with drugs and mental health problems as adults, according to new study findings.

People who had problem behaviors before age 15 were particularly at risk of reporting additional problems as adults. For instance, 90 percent of men and 60 percent of women who reported at least four problem behaviors before age 15 abused drugs, alcohol or nicotine as adults.

Study author Dr. Matt McGue of the University of Minnesota in Minneapolis noted that teenagers who act out may "change the course of their development in a way that increases the likelihood that they will have substance abuse and mental health problems in adulthood."

Teenagers who take risks may become connected to "deviant peers," McGue noted, and less connected to schools, parents, religious groups, and other helpful networks.

Alternatively, some children may inherit an "impulsive personality style," and as a result, often ignore the long-term consequences of their behavior. This in turn causes them to have problems both as teenagers and adults.

During their study, the researchers interviewed 578 male and 674 female twins at ages 17 and 20 about their behavior as teenagers, and their issues as adults.

The participants reported if, as teenagers, they had ever smoked, drank alcohol, used drugs, got in trouble with the police, or had intercourse. As adults, the participants answered questions designed to determine if they abused or were addicted to drugs, alcohol or nicotine, were depressed, or had developed antisocial personality disorder, a mental illness associated with deviant behavior.

The investigators found that people who said they engaged in dangerous behaviors as teenagers were significantly more likely to report additional problems as adults.

For instance, 90 percent of men and 35 percent of women who acted out in at least four ways as teenagers developed antisocial personality disorder.

Another 30 percent of men and 55 percent of women who reported multiple problem behaviors as teenagers developed depression as adults, the team reports in the American Journal of Psychiatry.

McGue noted that these results suggest that teenagers who engage in problem behaviors need "early intervention," to ensure that their youthful experimentation doesn't snowball into long-term problems.

"Adolescent experimentation - at least when expressed early - with drugs and sex, may not be as benign as the broader culture sometimes seems to suggest," McGue added.

SOURCE: American Journal of Psychiatry, June 2005.