If you’re about to reach for that bottle of antacids after the Christmas blow-out - think again. New research suggests that the world’s most popular drugs, used for heartburn and indigestion, can double the risk of pneumonia.

This is because these drugs suppress gastric acids, thereby allowing viruses and bacteria in the upper gastrointestinal tract to migrate up into the respiratory tract.

In a study of 364,683 people, Dutch scientists found that 5551 of them had their first bout of pneumonia during the study period (from 1995 to 2002). After eliminating other factors, the researchers concluded that the risk of pneumonia was just 0.6 among those who had never taken an acid-suppressant compared with 2.45 for those who had taken at least one at some time during the study.

Their findings were similar for proton-pump inhibitors and H2 antagonists. The risk of developing pneumonia with an H2 antagonist was 1.63 and, with a proton-pump inhibitor, 1.89.

The research team, from Nijmegen, noted that pneumonia can be fatal among the elderly, who are more likely to regularly take an antacid for mild stomach disorders. As gastric-acid secretion is reduced in those over 60 anyway, they are already more liable to develop pneumonia, even without the help of antacids.

Antacids are thought to have no serious side-effects and, so, are freely available in pharmacies and supermarkets. Popular brands include Andrews, Bisodol, De Witt’s, Pepto-Bismol, Setlers and Tums. The few that contain magnesium are available only on prescription.

Any side-effects are considered mild. Those made with magnesium may have a laxative effect, while those with aluminium can cause constipation. Magnesium carbonate can also lead to belching.

Similarly, proton-pump inhibitors such as Losec (omeprazole), Nexium (esomeprazole) and Zoton (lansoprazole) are also regarded as benign, although they require a prescription. The most common side-effects are diarrhoea, nausea, constipation, flatulence, abdominal pain and headache.

H2 antagonists include some of the world’s most successful drugs, including Tagamet (cimetidine) and Zantac (ranitidine). These are used to treat stomach and duodenal ulcers, especially those due to NSAID (non-steroidal anti-inflammatory drug) painkillers. Also prescribed to relieve indigestion and heartburn, again they are believed to cause only minor side-effects such as diarrhoea, dizziness and headache. Tagamet is also associated with hair loss.

Doctors are already aware of restrictions on the use of acid-suppressants. Of the antacids, sodium bicarbonate shouldn’t be taken by people on a salt-restricted diet, aluminium-containing antacids should be avoided by those with low blood phosphate and those containing magnesium shouldn’t be taken by people with kidney problems. Proton-pump inhibitors and H2 antagonists should never be prescribed to people with liver or kidney problems, or to pregnant or breastfeeding women.

Now the Dutch researchers want a new set of further restrictions, and are urging doctors to think carefully before prescribing an antacid to elderly patients, particularly those who have a history of chronic respiratory problems, such as asthma or obstructive lung disease (JAMA, 2004; 292: 1955-60).