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.

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.