Many older Americans lack knowledge about antibiotics, with some admitting to using leftover medication, a new survey reveals.

More than 2,200 adults, aged 50 to 80, were questioned. Nine out of 10 said they're cautious about using antibiotics, and nearly that number knew that overuse of the drugs can lead to them becoming ineffective, according to the University of Michigan National Poll on Healthy Aging (NPHA).

Also, almost 56% agreed that doctors overprescribe antibiotics, but about one-third mistakenly thought that antibiotics could effectively treat colds or the flu. However, these illnesses are caused by viruses, while antibiotics kill bacteria.

The results show the need for health care providers to educate older patients about the proper use and disposal of antibiotics, according to the team from the university's Institute for Healthcare Policy and Innovation.

The poll also found that 13% of the older adults who filled an antibiotic prescription said they had leftover medication, even though patients are usually advised to take all of the antibiotic medication.

Not finishing a prescribed course of antibiotics increases the risk that surviving bacteria will evolve resistance to future treatment by antibiotics, the study authors noted in a university news release.

Nearly two-thirds of respondents with leftover antibiotics said they kept them, and nearly 60% of those older adults said they did so in case they needed the medication in the future.

However, doctors recommend against doing this. People should see a doctor for new signs of infection and get a new prescription for antibiotics if necessary, the study authors said.

According to the survey, one in five respondents said they had taken antibiotics without talking to a health care professional at least once in the past, and most said they had taken their own leftover antibiotics.

Nearly half of the respondents said they received at least one prescription for antibiotics in the past two years, and half of those said at least one prescription was for a respiratory condition.

The study, by NPHA director Dr. Preeti Malani and colleagues, was published online recently in the journal Infection Control and Hospital Epidemiology.