When it comes to prescription medications, many people embrace the adage to share and share alike. Armed with good intentions and largely unaware of the dangers, they gladly hand over leftover antibiotics, asthma inhalers, antidepressants, insulin and pain pills. After all, if the drugs worked for them, then perhaps they'll help similarly suffering family members, friends or colleagues. And, considering the drugs' expense, throwing away excess, out-of-date or ineffective pills can seem like a waste.

When the AARP asked seniors last fall about sharing medications, about 4% of Medicare beneficiaries -- or nearly 1.7 million people - - said they had shared prescription medications with family and friends in the last year. By comparison, in a smaller 2002 study, UCLA found that 2% of people shared medications.

Teenagers are also likely to give and take others' medications. A report from the Centers for Disease Control and Prevention in 2003 found that 20% of girls and 13.4% of boys share medications with friends, although cost wasn't usually the motivation. Many share sleeping pills and acne medication such as Accutane, the report found. Sharing Accutane is extremely worrisome; the drug is believed to cause birth defects, and girls and women of child-bearing age are supposed to take a birth control pill while taking the medication.