Many teenagers may be sharing their prescription medications with their friends, putting them at risk of drug side effects or having a health problem go undiagnosed, a new survey finds.

The survey, of 592 12- to 17-year-olds from across the U.S., found that 20 percent admitted to having lent a prescription drug to a friend, while a similar percentage said they had done the borrowing.

The most commonly shared prescriptions were allergy drugs and narcotic pain relievers like Oxycontin and Darvocet, followed by antibiotics, acne medications like Accutane, and mood drugs such antidepressants and anti-anxiety medications.

What's more, the study found, three-quarters of prescription "borrowers" said they did so instead of seeing a doctor. Some eventually did make a trip to the doctor, but, in 40 percent of cases, failed to mention the borrowed medication.

The findings, published in the Journal of Adolescent Health, also point to the potential safety risks teens face when they share prescriptions.

Less than half said their borrowed medication came with written instructions on how to use it safely. And more than one-third of teens who borrowed prescriptions said they had suffered an allergic reaction or other side effect.

Teenagers are not alone in the practice of prescription sharing.

Previous research has suggested that almost 40 percent of U.S. adults have lent or borrowed a prescription to a family member or friend.

"However, prior to our study, no one had asked adolescents how often they shared prescription medications, which meds they shared and what some of the outcomes were," lead researcher Dr. Richard Goldsworthy, of Academic Edge, Inc., in Bloomington, Indiana, noted in a written statement.

The findings, he and his colleagues conclude, suggest that doctors need to talk to teenage patients about the risks of using other people's prescriptions. Given the high rate of prescription sharing among adults, many parents likely need the same advice, the researchers note.

Wider efforts — like public health campaigns or warnings about medication sharing on product packaging — might also be worthwhile.

SOURCE: Journal of Adolescent Health, online August 3, 2009.