How long-lasting is the typical cough — the kind you get with a cold? If you guessed about a week, you are average, but you are wrong.

In a new study, doctors who reviewed medical studies found that "acute cough," sometimes called "acute bronchitis," lasts an average of nearly 18 days. But when the doctors asked 493 adults in Georgia how long they expected such coughs to last, estimates averaged seven to nine days.

Many people wrongly believe that coughs lingering more than a few days need to be treated with antibiotics, says lead researcher Mark Ebell, a family physician and associate professor of public health at the University of Georgia. The report was published Monday in the Annals of Family Medicine.

"I frequently see patients who come in and have been sick for four or five days and say 'boy I really need an antibiotic — I'm just not getting better,'" Ebell says.

In fact, he and other experts say, most acute coughs are caused by viral illnesses, such as colds and flu, and won't be helped by antibiotics no matter how long they last. That's because antibiotics only treat bacterial illnesses, such as some forms of pneumonia. When they are used for viral illnesses, they can do more harm than good — frequently causing diarrhea, allergic reactions and other side effects and spurring the growth of antibiotic-resistant bacteria that can sicken not only the person who took the antibiotics but other people, as well.

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