Proton pump inhibitors (PPIs) increase patients' risk of developing pneumonia by around a third, a US study has found.

Previous research has suggested that use of acid-suppressive medicines such as PPIs may be linked to pneumonia. However, this new study found no significant increase in pneumonia risk for patients treated with other acid-suppressive medication (H2 antagonists).

Dr Shoshana Herzig and colleagues from Harvard Medical School examined data from almost 64,000 hospital admissions.

Acid-suppressive medicines were given to around half of patients and 3.5 per cent of patients developed pneumonia.

After adjusting for possible confounding factors, treatment with PPIs was associated with a 30 per cent increased risk of developing pneumonia while in hospital.

Based on these findings, the researchers calculated that for every 111 patients treated with PPIs, one extra patient will develop pneumonia, which has a mortality rate of around 18 per cent.

The researchers said that up to 70 per cent of PPI use is for indications that have not been properly investigated.

'Despite recent evidence for adverse outcomes in the outpatient setting and inappropriate prescribing practices in the inpatient setting, these medications continue to be frequently prescribed,' they said.

The researchers argue that their findings, together with research showing that pneumonia risk is most elevated during initial treatment, suggest that cause of the increased risk needs rethinking.

'These findings should therefore prompt consideration of alternative explanations, such as impairment of white blood cell function associated with PPI therapy, which has been demonstrated to occur within hours,' the researchers said.

They added: 'Further studies are necessary to elucidate the mechanism of increased pneumonia risk in patients prescribed acid-suppressive medications in general and PPIs in particular.'

tom.moberly@haymarket.com

• JAMA 2009; 301: 2,120-8.

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