European researchers found that compared with placebo, amoxicillin did not reduce symptom duration or prevent worsening of illness and gave only a small reduction in severity in chronically ill patients.
Researchers writing in the British Journal of General Practice said the modest benefits must be balanced against side-effects and risks of antibiotic resistance.
Antibiotic prescribing for lower respiratory tract infections is common despite research showing limited benefit for most patients.
NICE guidelines encourage GPs not to prescribe antibiotics for common respiratory infections in most patients.
It advises the use of antibiotics in high-risk patients, such as older patients or those with comorbidities. But it remains unclear whether the moderate benefits of antibiotics found in previous studies apply to all major clinical subgroups.
Researchers led by a team from the University of Southampton randomised 2,061 with acute lower respiratory tract infection to either amoxicillin or placebo.
They then compared symptom duration and severity, as well as the emergence of new or worsening symptoms, using patient symptom diaries.
No patient subgroup was more likely to benefit from antibiotics in terms of symptom duration or worsening of symptoms compared with placebo.
Some patients with significant comorbidities saw a reduction in severity of symptoms between day two and four when they took the antibiotic.
Similarly, non-smokers also benefited from a modest reduction in symptom severity. Those with a short prior illness who took antibiotics also saw a slight reduction in severity and a modest fall in duration.
Researchers said the findings ‘found no clear evidence of clinically meaningful selective benefit from antibiotics among key clinical subgroups of patients with uncomplicated lower respiratory tract infection where prescribing is highest’.
They added: ‘The modest short-term benefits are of questionable clinical significance and must be balanced against the side-effects and the longer-term harm of fostering antibiotic resistance.'