If confirmed, the study will cause concern among doctors and patients. Ibuprofen is generally regarded as one of the safest painkilling and anti-inflammatory drugs available. Millions of doses are consumed every week.

Recently Boots, the maker of Nurofen — which is based on ibuprofen — has been promoting a range for children.

The study, published this week, says: “We observed a statistically significant increased risk of breast cancer, especially non-localised breast cancer, in long-term daily users of ibuprofen.”

The research followed more than 114,000 Californian women aged 22-85, all of them free from breast cancer at the start of the trials. All were asked to give details of the pills and medicines they consumed.

During the six-year study period 2,391 of the women contracted breast cancer. The researchers then examined what drugs they had taken prior to becoming ill. They estimate that in perhaps a few dozen of the cancer cases ibuprofen use may have played a part.

More than 23,000 of the women in the study used ibuprofen regularly and 8,000 were using it daily, most of them for more than five years. There are no figures for British use of the drug.

The findings came as a surprise. The researchers had expected the study to show a fall in cancers with use of such drugs. This is because ibuprofen and aspirin are among a group of compounds known as non-steroidal anti-inflammatory drugs (NSAIDs), one of the effects of which is believed to include suppression of a gene known as Cox-2, which can stimulate cancer growth.

The study, which appears in the Journal of the National Cancer Institute, suggests ibuprofen may have other unknown biochemical effects that need to be better understood if women are to be safely prescribed the drug for long-term use. One in nine women can expect to contract breast cancer during their lives.

Professor John Toy, medical director of Cancer Research UK, said the research was important. “The development of breast cancer is closely linked to hormone levels and their fluctuations, which can be be affected by a range of things including drugs,” he said.

“Most drugs reach the market without the benefit of long-term trials on humans, so problems may take years to emerge.”

Breast cancer is most common in women aged over 50, such as Lynn Redgrave, the actress, who was diagnosed three years ago at the age of 59. She had a mastectomy and chemotherapy before receiving the all-clear this March. There is no evidence that Redgrave took ibuprofen.

Older women tend to be higher users of ibuprofen due to arthritic and other conditions.The drug and other NSAIDS work by changing the body’s response to pain, swelling and high temperature.

Ibuprofen is considered safe enough to be sold over the counter for relief from short-term conditions such as period pain, headaches, migraine, feverishness and muscular pain. Many people also use it as a lifestyle drug — for example, to prevent a hangover developing. It is also offered widely on prescription for people with long-term conditions.

Sales are soaring. Last year Nurofen sales at Boots grew to £140.3m, up 15.3%. A spokesman for Boots said it had not seen the study but considered the product to be among the safest painkillers on the market.

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