What a difference a year makes. A study released last May made national headlines with the finding that aspirin can reduce a woman's risk of breast cancer. Now a new study shows little protective benefit and a possible increase in breast cancer among women who take aspirin or ibuprofen.

Researchers downplayed the potential risk but called the failure to find a protective benefit for the anti-inflammatory pain relievers disappointing.

"These drugs are not preventing breast cancer as earlier studies suggested that they might," Sarah F. Marshall of the University of Southern California, Los Angeles, tells WebMD.

Read "WebMD Special Report: Painkillers Under Fire"

Mixed Findings for Daily Use

Marshall and colleagues analyzed data on more than 114,000 California women. None of the women had breast cancer when they entered the study, but nearly 2,400 were diagnosed with the disease between 1995 and 2001.

Their findings are reported in the June 1 issue of The Journal of the National Cancer Institute.

The researchers reported that taking aspirin or ibuprofen more than once a week but not every day neither increased nor decreased breast cancer risk. Other anti-inflammatory drugs, such as naproxen, were not studied.

Daily aspirin use for more than five years, however, was linked to a small increased risk for developing breast cancers called hormone receptor-negative tumors. Breast cancers are classified based on whether or not they grow in response to hormones, such as estrogen. Tumors that do not respond to hormones -- hormone receptor-negative -- are less common and harder to treat.

But the risk from daily aspirin use is small. The researchers say that among the nearly 2,400 women with breast cancer, seven excess cases of hormone receptor-negative cancer may be caused by long-term daily aspirin use. The study did not compare the risks of low-dose "baby" 81 milligram aspirin to the regular 325 milligram dose.

Read WebMD's "What Increases a Woman's Breast Cancer Risk?"

Small Risk With Ibuprofen

Daily ibuprofen use was also linked to an increase in breast cancer risk among women in the study. The risk was higher for women with tumors that had spread to other parts of the body.

But again the actual risk was small. The researchers estimated that 24 of the nearly 2,400 cases of breast cancer could be due to daily ibuprofen use.

Marshall says the findings should not discourage women who are taking a daily low-dose aspirin to lower their risk of heart disease.

"I would not advise women to take these drugs to prevent breast cancer," she says. "But women who are taking them for other good reasons should not stop because of this study."

Regular or daily use of acetaminophen, which is not an anti-inflammatory drug, had no effect on breast cancer risk.

Read WebMD's "The Breast Cancer Gene: What Should You Do?"

Aspirin Evidence Confusing

There have been at least 20 published studies examining the use of aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs) for the prevention of breast cancer and the findings have been inconsistent, says epidemiologist Michael Thun, MD, of the American Cancer Society.

One of the most widely publicized of these trials, reported a year ago this week, showed that aspirin helps protect against more common hormone receptor-positive tumors but not more aggressive hormone receptor-negative tumors.

Thun says the fact that the latest trial also found tumor type to be a factor in the response to aspirin is intriguing and deserves further study.

Taken as a whole, he adds, the research suggests only a very small protective role for aspirin and other NSAIDs against breast cancer, if any benefit exists at all. In addition to over-the-counter NSAIDs, there are more than 20 available by prescription.

"The findings with regard to colon cancer have been much more persuasive," he says. "It is clear that prolonged use of NSAIDs is associated with lower colon cancer risk. But the breast cancer trials have been less convincing."

Visit WebMD's Breast Cancer Health Center

By Salynn Boyles, reviewed by Michael W. Smith, MD

SOURCES: Marshall, S. The Journal of the National Cancer Institute, June 1, 2005; vol. 97: pp. 805-812. Sarah Marshall, department of preventive medicine, University of Southern California at Los Angeles. Michael Thun, MD, head of epidemiologic research, American Cancer Society.