Regular aspirin use appears to reduce the risk of breast cancer, according to a new study, but the drug only seems to protect against tumors that are hormonally sensitive.

The report, published in the Journal of the American Medical Association (Vol. 291, No. 20: 2433-2489) is not the first to suggest aspirin can help prevent breast cancer. But it is the first to find a difference in protection according to tumor type.

"That [aspirin] reduced the risk of breast cancer was not surprising," said study co-author Alfred I. Neugut, MD, of Columbia University. "I think the more novel finding is that it seems to be focused on hormone-receptor positive breast cancer."

He and his colleagues suspect that aspirin spurs a process that decreases the production of aromatase, which in turn suppresses the production of estrogen, the female hormone that can fuel the growth of breast cancer.

Neugut and his colleagues studied nearly 3,000 women on Long Island, New York, of whom about half had breast cancer. They asked the women about their use of aspirin, ibuprofen, and acetaminophen, and about breast cancer risk factors like hormone use, menopausal status, reproductive history, and family history of the disease.

Women who used aspirin daily reduced their risk of breast cancer by nearly 30%. But when the researchers looked at what types of tumors were affected, aspirin only seemed to protect against tumors that expressed the hormones estrogen or progestin (or both). These types of tumors tend to have a better prognosis than hormone-receptor negative tumors because they respond to hormonal treatments like tamoxifen.

Ibuprofen, which belongs to a class of drugs similar to aspirin called non-steroidal anti-inflammatory drugs (NSAIDs), had only a very weak effect on breast cancer risk. That may be because fewer women in the study used ibuprofen compared to aspirin, so any effect was not noticeable.

Acetaminophen, which is not an NSAID, did not have any appreciable effect on breast cancer risk. That was expected, Neugut said, because acetaminophen relieves pain differently than either aspirin or ibuprofen, in a way that does not ultimately affect estrogen. That fact adds weight to the theory that aspirin acts against breast cancer by suppressing estrogen.

But the researchers didn't take any measurements that might back up their theory; they didn't collect samples of breast tissue, for instance, to measure levels of hormones and enzymes. More research is needed to determine whether they're right about how aspirin works.

If they are right, though, it raises the possibility that aspirin could one day be combined with aromatase inhibitors to provide an even higher degree of protection from breast cancer, Neugut said. Aromatase inhibitors have been shown to dramatically reduce the risk of recurrence in women who have already had breast cancer.

Because there are so many unanswered questions, Neugut and other experts said it's still too soon to give women the all-clear to take aspirin or NSAIDs as a means of preventing breast cancer.

"This relationship is only now being tested in randomized clinical trials, and there is conflicting information about the optimal dose, duration, and drug," said Michael Thun, MD, vice president of epidemiology and surveillance research at the American Cancer Society. "The evidence remains incomplete and these drugs have side effects [like stomach bleeding] that are uncommon but can be serious."

Still, millions of American women already take aspirin daily because it has been proven to reduce the risk of heart disease. For these women, the study could provide some hopeful news, Neugut said.

"While I'm not suggesting that anyone take aspirin for breast cancer, women who already take it for cardiac prophylaxis might be getting a little icing on the cake," he said.