Doctors who prescribe anti-anxiety or sleep medications to teenagers may be inadvertently creating a new generation of illegal, recreational drugs users, according to a recent study.

Researchers from the University of Michigan School of Nursing found that teens prescribed anxiety or sleep medications are up to 12 times more likely to abuse those drugs than those who had never had a prescription, either by using someone else's prescription pills or to get high or experiment.

Nearly 9 percent of the 2,745 adolescent study participants had received a prescription for anxiety or sleep medications during their lifetime, and more than 3 percent received at least one prescription during the three-year study period.

"I recognize the importance of these medications in treating anxiety and sleep problems," Carol Boyd, the study's first author, said in a statement.  "However, the number of adolescents prescribed these medications and the number misusing them is disturbing for several reasons."

Researchers found that teens prescribed anxiety or sleep medications during the study period were 10 times more likely to abuse them within two years, to get high or to experiment, than those without prescriptions.

Anxiety and sleep medications can be addictive or even fatal when mixed with narcotics or alcohol, said Boyd, who is also a professor of women's studies and research professor at the Institute for Research on Women and Gender and at the university's Addiction Research Center in the Department of Psychiatry.

"What happened to [actor] Heath Ledger could happen to any teen who is misusing these medications, particularly if the teen uses alcohol in combination with these drugs," Boyd said.

Examples of anti-anxiety medications include Klonopin, Xanax and Ativan; sleep medications include Ambien, Restoril and Lunesta. These are controlled substances partly because of the potential for abuse, and it's a felony to share them, Boyd said.

Researchers said this is the first longitudinal study to determine whether teens' recent medical use of anxiety or sleep medications is associated with later taking somebody else's prescription medication illegally, either for self-treatment or recreational use.

The study recommends better education for parents and adolescents prescribed these medications, monitoring refills and making it standard practice to give teens a substance use assessment before prescribing these drugs.