Adults are commonly prescribed an antibiotic when they have a persistent cough, but it is unlikely to help and could even be harmful, a large new study suggests.

A cough with lower respiratory tract infection symptoms is one of the most common illnesses that family doctors see.

Antibiotics are not very helpful for those with a persistent cough, a new study suggests. (Mario Tama/Getty )

Education campaigns have tried to spread the word that antibiotics don’t work against viruses that often cause lower respiratory infections but there's been little data until now from placebo-controlled trials, the gold standard type of medical research.

"In conclusion, amoxicillin provides little symptomatic benefit for patients presenting in primary care who are judged to have clinically uncomplicated lower respiratory tract infections," Dr. Paul Little, of the University of Southampton in the U.K., and his co-authors concluded in Wednesday's online issue of the medical journal The Lancet Infectious Diseases.

"Any mild, short-term benefits of antibiotic treatment should be balanced against the side-effects and, in the long-term, of fostering resistance."

The researchers randomly assigned 1,038 European patients with a cough lasting less than 28 days to take amoxicillin and 1,023 to take a placebo.

'Refrain from antibiotic treatment'

The findings only applied to adults. The researchers said the findings should not be extrapolated to severely ill older patients. Viruses are thought to cause most lower respiratory tract infections beside pneumonia, but the viral cause is rarely found, Dr. Philipp Schuetz, of the Medical University Department at Kantonsspital in Aarau, Switzerland, noted in a journal commentary accompanying the study.

"Little and colleagues have generated convincing data that should encourage physicians in primary care to refrain from antibiotic treatment in low-risk patients in whom pneumonia is not suspected," he wrote.

Infectious disease expert Dr. Lynora Saxinger of the University of Alberta in Edmonton agreed.

"Patients only go in if they're feeling very unwell and their doctor does want to help them and doesn't really want to run the risk of things getting worse if they don't offer an antibiotic," Saxinger said.

"I think this type of study, which is very practical, helps reassure us that we're not missing the boat," by prescribing just in case.

Similarly in September, newly revised guidelines from the Infectious Diseases Society of America noted that most sore throats are also caused by a virus not streptococcus bacteria, and shouldn’t be treated with antibiotics.

The study was funded by the European Community and medical research granting agencies in the U.K., Spain and Belgium.

Schuetz is a paid speaker for laboratory product manufacturers Thermo Fisher Scientific and bioMérieux

With files from CBC's Kim Brunhuber