A new study suggests taking aspirin or ibuprofen regularly could help protect women against breast cancer. But experts caution it’s too early to start raiding the medicine cabinet.

“There are approximately 12 studies of aspirin-like drugs in relation to breast cancer which have inconsistent findings,” said Michael Thun, MD, vice president of epidemiology and surveillance research for the American Cancer Society. “This study doesn’t clinch the story.”

The most recent research was conducted by Randall Harris, MD, PhD, of Ohio State University, and colleagues from three other universities. The work was to have been presented at the 94th meeting of the American Association for Cancer Research, but because the meeting was cancelled, Harris presented the findings in a teleconference. The work has not yet been published in a peer-reviewed journal.

The researchers studied nearly 81,000 women who were taking part in the Women’s Health Initiative, a national investigational study sponsored by the National Institutes of Health. All the women were past menopause, and between the ages of 50 and 79; during the course of the study, 1,392 of them developed breast cancer.

The researchers compared the incidence of breast cancer among women who said they regularly had taken non-steroidal anti-inflammatory drugs (NSAIDs) like aspirin and ibuprofen, and women who said they did not take the drugs. They found a protective effect from taking the pills.

“Women who took aspirin, ibuprofen or related compounds for five to nine years had a 21% reduction in breast cancer risk,” Harris said in the teleconference. “And women who took these compounds for at least 10 years had a 28% reduction in risk.”

When the researchers looked at the drugs individually, ibuprofen seemed to confer even greater benefits than aspirin. Women who took ibuprofen for 10 years or longer reduced their breast cancer risk by 49%, while women who took aspirin for that length of time saw a 22% reduction in risk.

Even women already at higher risk for breast cancer – those with a family history, or who took estrogen replacement therapy, or who were overweight or obese -- benefited from taking NSAIDs, Harris said.

The protective effect was seen in women who took standard doses of the drugs – 325 milligrams of aspirin, or 200 milligrams of ibuprofen – at least every other day. Women who took acetaminophen (a painkiller that is not an NSAID) or low-dose aspirin (81 milligrams) did not see any reduction in risk.

The researchers looked at women who took aspirin, over-the-counter ibuprofen, prescription NSAIDs (such as indomethacin and piroxicam), or acetaminophen. They did not study women taking celecoxib and rofecoxib (Celebrex and Vioxx) because those drugs were not available until 1999.

Harris said the NSAIDs work by blocking COX-2, a compound that triggers inflammation and which is thought to play a role in several key aspects of cancer development, such as cell division, metastasis (the spread of cancer cells), and angiogenesis (the formation of blood vessels that feed the tumor).

He said that might be why acetaminophen didn’t deliver the same results in the study: it doesn’t act on inflammation and on COX-2 the way NSAIDs do.

Harris said he takes ibuprofen daily because of the “compelling and converging” evidence that the compound can prevent cancer.

But Thun said any recommendation to take daily ibuprofen would be overstating the evidence of its effectiveness.

Although two recent studies have suggested NSAIDs may protect against colon cancer by preventing the formation of colon polyps, Thun noted that the drugs are approved for this use only in people who have familial adenomatous polyposis, a rare syndrome that causes the growth of numerous colon polyps.

The evidence for breast cancer prevention is also unclear. Thun said studies larger than this one and with a “stronger design” have not found the same protective effect from NSAIDs.

Thun noted that because Harris and colleagues did not control which women took the drugs or how often they took them, it is possible some factor other than NSAID use accounted for the difference in risk. And, it is possible some women overestimated or underestimated their use of NSAIDs when reporting to the researchers.

Thun also said the doses of painkillers the women took were too low to produce a sustained inhibition of COX-2.

“Aspirin-like drugs, and particularly the newer selective COX-2 inhibitors, do have potential as anti-cancer agents,” Thun said, “But it’s important that recommendations not get ahead of the evidence because these drugs can cause serious side effects in some individuals.”

NSAIDs can cause stomach ulcers and bleeding, liver problems, kidney problems, allergic reactions, and fluid retention.

Thun said evidence from randomized clinical trials – where women are assigned randomly to take either an NSAID or a placebo – is needed before any recommendations can be made about whether to take NSAIDS.

Until then, he said, women should stick with proven methods for reducing the risk of breast cancer: maintaining a healthy weight, minimizing alcohol consumption, avoiding or minimizing estrogen replacement therapy after menopause, and getting regular exercise.