Teenage girls who experiment with sex and illicit drugs are more at risk of depression than teen boys involved in similar behaviors, researchers say. For example, girls who occasionally use alcohol, tobacco, or other drugs were two and a half times more likely to be depressed than those who abstain.

The Wisconsin State Journal reported July 5 that researchers say that while these behaviors may be associated, there's not necessarily a cause-and-effect relationship. "Adolescents do the things adolescents do," says Martha Waller of the Pacific Institute for Research and Evaluation (PIRE).

Waller said that most adolescents will try smoking, drinking, using marijuana, or engaging in sexual activity; some such experimentation may even be "developmentally appropriate," she said, while those who abstain from these activities are in the minority by the time they reach their late teens.

General abstinence, however, may help prevent depression among teen girls. "The longer you can hold them off, the longer they have for emotional and physical development, the better," Waller said.

"At puberty, we see that the prevalence of depression starts to really increase in girls but not boys, and this gender difference has always been attributed to developmental changes in girls such as hormonal changes," Waller said. "But in this study, we find that for girls who have not experimented with smoking or drinking or become sexually active, their risk for depression is no greater than males, regardless of their developmental status."

Biological or social factors may be behind the differing impact of sex and drugs on teen mental health.

Waller said that more research is also needed on how to treat teens. "If we treat mental health, does the substance abuse stop?" she asked. "If we treat the substance abuse, does the depression go away? That's where we really need to go now. Possibly, we need to treat both at the same time."