TUESDAY, April 29 (HealthDay News) -- If you've ever shared your allergy medicines, antibiotics or even painkillers to a family member or friend, you've got plenty of company: A new survey suggests many give away their prescription medicines or borrow them from others.

However, this can be an extremely bad idea, experts say. Prescription drugs, after all, are prescribed for a reason: Because a doctor or pharmacist needs to play a role in their use.

In the case of shared antibiotics, "we've managed to document that this is a real public health risk," said study author Richard Goldsworthy, CEO and director of research and development for The Academic Edge company in Bloomington, Ind.

Goldsworthy's company came up with the idea for their survey while studying whether prescription warning labels should urge some users to not share their medication. Would the labels be effective?

"There wasn't a whole lot of data on it," Goldsworthy said. "So we decided if we were going to ask the question, we needed to look more broadly at the entire issue, find out who's sharing."

In 2006, researchers interviewed 700 people aged 12 to 44 in several large U.S. cities, including Los Angeles, Phoenix, Philadelphia and Atlanta, among others. In the one-on-one interviews, the researchers asked the subjects about their use of medications.

The findings are published in the June issue of the American Journal of Public Health.

Two-thirds of those surveyed said they had never borrowed medications from others or shared their own. However, 23 percent said they'd shared their medications with others, and 27 percent had borrowed them; 16 percent had done both.

About 22 percent reported shared pain medications, and 7 percent said they'd shared mood-altering medications. A quarter said they'd shared allergy medications, and almost 21 percent reported sharing antibiotics.

The latter number is worrisome, because patients shouldn't have any antibiotics left over after a prescribed course of treatment, Goldsworthy said. In addition, he said, overuse of antibiotics is contributing to the rise of germs that are immune to many drugs.

"Don't share antibiotics," Goldsworthy advised. "You shouldn't have any leftover. You should have finished them all yourself."

In some cases, however, sharing drugs may not be very risky, Goldsworthy said, and is done for "pretty reasonable reasons."

"I happen to share some painkillers, because I have a bad toothache, I'm sneezing, and my mother-in-law has a prescription medicine while we're on a trip," he explained.

In general, he said," people share for a variety of reasons. They share because it's convenient, because they want to fix a problem. And they share for thrill-seeking, like when they get pain and mood-altering medications."

Should prescription medications come with warning labels telling users to not share them? Goldsworthy isn't sure, especially considering that medications already have plenty of warning labels. "It would just get loss in the sea of other ubiquitous messaging that ends up on pharmaceutical packaging," he said.

More information

The FDA has details about using medicine properly.