ASPIRIN may increase the risk of pancreatic cancer if used regularly, a study in the United States has concluded.

The results are a surprise, because aspirin had previously been linked to a reduced risk of cancers of the rectum, stomach and oesophagus. It also has protective effects against heart disease and is therefore taken by many people every day.

The new findings, published in the Journal of the National Cancer Institute, come from one of the longest-running studies in the US, the Nurses’ Health Study, in which almost 90,000 nurses have been followed for 18 years and the diseases they suffer correlated with their diet and lifestyle.

A team led by Eva Schernhammer, of Brigham and Women’s Hospital in Boston, mined the database for evidence of any link between aspirin use and cancer of the pancreas. Over the 18 years, 161 cases of pancreatic cancer had been diagnosed in the women.

Those who took two to three standard aspirins a week showed no increase in risk, compared with those who took fewer than two. But higher rates of use, over a long period, did show a statistically significant effect.

Those who took two aspirins a day over a period of at least four years showed an 86 per cent increased risk, though the small numbers involved in the study suggest no real cause for alarm.

Dr Schernhammer said she could not yet explain the increased risk, and Bayer, the German manufacturers of aspirin, said in a statement that the findings from the study “do not demonstrate a conclusive link between aspirin and pancreatic cancer”.

In a commentary, John Baron of Dartmouth Medical School raised some questions. Could it really be, he asked, that the “wonder child of chronic disease prevention” could be implicated in causing a deadly cancer? There are few known risk factors for pancreatic cancer, he said, except smoking, diabetes and pancreatitis. It remains an understudied cancer.

The most important doubt he raised was whether aspirin was the cause or the consequence of the cancer. If the women had already started to develop pancreatic cancer, they might have taken aspirin to reduce the pain.

If the two years before diagnosis are omitted from the analysis, the measured risk drops sharply, suggesting that there may be something in this explanation. But he said the study’s conclusions were hard to argue away.

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