Antibiotics are ineffective in treating patients with persistent coughs caused by mild chest infections, the Lancet Infectious Diseases journal reports.
About 2,000 patients across 12 European countries filled in an 'illness' diary.
The study found that the severity and duration of symptoms in patients treated with antibiotics were no different to those given a placebo.
But experts caution that if pneumonia is suspected, antibiotics should still be used due to the disease's severity.
Prof Paul Little from the University of Southampton, who led the research, said: "Using the antibiotic amoxicillin to treat respiratory infections in patients not suspected of having pneumonia is not likely to help and could be harmful.
Most mild chest infections will settle by themselves with no need for antibiotics - as mainly caused by viruses”
Dr Nick Hopkinson British Lung Foundatio "Overuse of antibiotics, dominated by primary care prescribing, particularly when they are ineffective, can lead to the development of resistance and have side effects like diarrhoea, rash and vomiting.