Antibiotics are not justified to reduce the risk of complications after upper respiratory tract infection, sore throat or ear infection, finds a study published on bmj.com, the Web site for the British Medical Journal. But they do substantially cut the risk of pneumonia after chest infection, particularly in elderly people.

Most antibiotic prescribing is in primary care, and most of it is for common respiratory tract infections. Guidelines advise against the routine use of antibiotics in patients with upper respiratory tract infection, sore throat and ear infection, but do recommend them for pneumonia.

The BMJ study identified 3.36 million episodes of respiratory tract infection recorded between 1991 and 2001 in the UK General Practice Research Database and determined whether complications were less common in people who were prescribed antibiotics than in those who were not.

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Risk of serious complications in the month after diagnosis were recorded: mastoiditis (infection of the mastoid bone of the skull) after ear infection, quinsy (an abscess at the back of the throat) after sore throat and pneumonia after upper respiratory tract infection and chest infection.

Serious complications were rare after upper respiratory tract infections, sore throat and ear infection. Antibiotics reduced the risk, but over 4,000 courses were needed to prevent one complication.

In contrast, the risk of pneumonia after chest infection was high, particularly in elderly people, and was substantially reduced by antibiotic use. The number needed to treat to prevent one case of pneumonia was 39 for those aged 65 and over and between 96 and 119 in younger age groups.

The authors conclude that general practitioners should not base their prescribing for sore throat, ear infection or upper respiratory tract infections on a fear of serious complications. However, antibiotic prescribing to reduce the risk of pneumonia after chest infection is justifiable, particularly in elderly patients in whom the risk is highest.