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Under-reporting of side effects ‘contributing to over-use of antibiotics’

Amoxicillin and the combination of amoxicillin with clavulanic acid often lead to the side effects diarrhoea and candidiasis, but harms appear to be under-reported, according to a study in the Canadian Medical Association Journal.

The study authors said the under-reporting could be contributing to over-use of these antibiotics – and therefore increased antibiotic resistance.

Amoxicillin is one of the most commonly prescribed antibiotics for respiratory infections in primary care, and is often prescribed in combination with clavulanic acid.

The Australian team conducted a systematic review of controlled trials – involving 10,519 adults and children – to assess the harms of amoxicillin. Of the participants, 4,280 received only amoxicillin, 1,005 received amoxicillin-clavulanic acid, and 5,234 received placebo.

Diarrhoea was over three times as likely in people taking amoxicillin and clavulanic acid as in people on placebo, while amoxicillin in combination with clavulanic acid was associated with nearly an eightfold increased risk of candidiasis. The number of courses needed to harm was 10 for diarrhoea with amoxicillin-clavulanic acid, and 27 for candidiasis with amoxicillin, with or without clavulanic acid.

However, of the 45 studies included, only 25 included information on harms, suggesting under-reporting of adverse effects.

‘Reported harms were fewer than we expected from clinical anecdotal experience and observationally-derived data, which have primarily reported common harms as rashes (at rates of 5%-8% of those treated and even higher, up to 20%, among those with mononucleosis treated with amoxicillin) and gastrointestinal disturbance,’ the authors wrote.

‘Under-reporting of harms in trials remains widespread, and until that problem is addressed, under-reporting will flow to systematic reviews and other evidence syntheses such as guidelines.’

Co-author Dr Christopher Del Mar, an evidence-based medicine specialist at Bond University, Queensland, commented: ‘The important consequence of under-reporting of harms is the tilting of the balance of benefits and harms towards amoxicillin.

‘The root cause of antibiotic resistance is the over-use of antibiotics, and therefore these drugs should not be prescribed when the benefits do not outweigh the harms.’

CMAJ 2014; available online 17 November

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