Coughs usually take longer to clear up than people think, and the gap between how long people expect then to last and how long it actually takes may drive some patients to the doctor for antibiotics that won't help, according to a US study.

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Researchers in the US state of Georgia write in the Annals of Family Medicine that survey respondents tend to expect their cough to be gone in about a week, but a review of cough studies shows the hacking takes about three weeks to clear up.

The team, led by Mark Ebell from the University of Georgia in Athens, says they are concerned that patients' unrealistic expectations could lead them to ask doctors for antibiotics that won't speed their recovery, but will fuel drug resistance, cost money and increase the risk of side effects.

"Efforts to reduce inappropriate antibiotic use should target this discrepancy," the authors write, referring to unrealistic patient expectations.

"We're not trying to discourage people from getting care if they feel they need it, but at the same time we want to give them the confidence to give themselves care in situations when it's appropriate," says Ebell.

For the study, Ebell and his colleagues quizzed 493 adults in Georgia by telephone about how long they'd expect a cough to last based on a hypothetical situation: if they had a 38 C fever and were bringing up yellow mucous.

Overall, people said they'd expect the cough to take between seven and nine days to clear up.

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That's too optimistic, the research team says, based on 19 previous studies that recorded how long severe coughs actually lasted. In those studies, it took a cough - on average - 17.8 days to subside.

"I think it is important to understand that if you do get a cough you're probably going to be coughing for about three weeks," says Jeffrey Linder, who is not involved in the study but has done similar research.

"Also, there is evidence out there that getting an antibiotic at any point in the course is not going to make it shorter," adds Linder, an associate professor at Brigham and Women's Hospital and Harvard Medical School in Boston.

According to the researchers, about 50 percent of patients with an acute cough in 2006 were prescribed an antibiotic. But most respiratory infections are caused by viruses - while antibiotics only affect bacteria.

Ebell says that patients should call their doctors if they bring up blood when they cough or are short of breath, while Linder says they should also do so if their coughs last longer than a month or get worse.

"There are over-the-counter things I recommend to people to feel better, but the main treatment is time," he says.