Michael Cleveland of the Prevention Research Center at Pennsylvania State University said influences on what teens do often come from their friends, but parents have big roles.
Cleveland and colleagues interviewed high school freshmen and sophomores, asking them about their substance use, their friends -- and their parents' discipline behaviors and knowledge of their social lives.
They also grouped students based on their friends.
"Substance abuse was lowest for those teens whose parents were knowledgeable and consistent, and who also had friends whose parents were knowledgeable and consistent," Cleveland said in a statement. "Parents should be aware of their teens' whereabouts. They also can keep track of who their teens are friends with, and get to know the parents of those other kids."