Sure, your friend is being nice by sharing, but she may also be doing herself a disservice in the process. Certain states have strict rules on how soon patients can refill prescriptions, so if your friend runs out as a result of sharing and needs to refill, she may not be able to--some pharmacists may even suspect she's abusing her medication. 

• "Women commonly share their back pain medicine, which is especially unwise," says Bertka. "A pill for back pain could be a narcotic, a muscle relaxer, or an anti-inflammatory--they're not all the same. Taking too much anti-inflammatory medication can cause blood clots, ringing in the ear, even an ulcer. Or, you can dangerously mix back pain medicine with meds you're already taking. And you could have allergies to certain meds you're not aware of."

What about sharing birth control? You and a pal may both be on the same one, so what's the harm? "Certain medication like Advil, Motrin, or Tylenol can mix dangerously with birth control, and you can never be sure what other medication someone is taking," he says. 

• Another common blunder: Borrowing antibiotics for a sick kid. "The reason someone has excess antibiotics is because they didn't have their child finish them--another bad idea," he says. 

Plus, you may think your kid has a bacterial infection when it's really just viral--and an antibiotic won't help. 

"Always check with your doctor before taking any medicine you weren't prescribed," Dr. Bertka says. "He or she can always suggest at-home alternatives if you can't get a refill at a moment's notice."