People underestimate how long coughs from bronchitis last, a mismatch that could drive overprescription of antibiotics, a U.S. study suggests.

The average acute cough lasts 17.8 days, Dr. Mark Ebell, an epidemiology professor at the University of Georgia, and his co-authors conclude in the January issue of the Annals of Family Medicine after reviewing the medical literature.

There is a mismatch between patients' expectations on duration of cough and how long it really lasts. (iStock)

The researchers looked at the results of 19 studies on patients in the U.S., Europe, Russia and Kenya on duration of cough and surveyed 493 adults in Georgia on how they expect acute coughs to last. Respondents gave an average duration of cough of 7.2 to 9.3 days.

"There is a mismatch between patients' expectations regarding the duration of acute cough illness and the actual duration based on the best available evidence," the study's authors said.

"Efforts to reduce inappropriate antibiotic use should target this discrepancy."

Acute cough illness accounts for two to three per cent of outpatient visits to doctors, according to the U.S. Centers for Disease Control and Prevention. Over half leave with a prescription for antibiotics, which should be lower, Ebell said in a release.

Clinical trials have found little benefit to treating acute cough with antibiotics since most are caused by viruses that don't respond.

Ebell suggested educating patients about how long the illness lasts to try to adjust their expectations.