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A new study is adding to the debate about whether common painkillers such as aspirin and ibuprofen protect against breast cancer. The latest research suggests they don't -- and that long-term use may even raise the risk of developing this disease.

The findings come from the University of Southern California's Keck School of Medicine and were published Tuesday in the Journal of the National Cancer Institute.

But many other studies have found either no increase in breast cancer risk or a decrease in risk in women who use these painkillers, say the researchers and other experts. The bottom line, they say: Women shouldn't be overly worried by the results of a single study.

"We wouldn't say at this stage that ibuprofen or aspirin would cause breast cancer," said lead study author Sarah F. Marshall, MA, a researcher in Keck's Department of Preventive Medicine. "There's no reason for women to change their pain medication based on the results of the study."

An American Cancer Society expert agreed.

"There have been at least 20 previous studies on breast cancer risk in relation to [drugs like aspirin and ibuprofen]," said Michael Thun, vice president of epidemiology and surveillance at ACS. "None of the previous studies have reported an increased risk, so this could be a chance finding."

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Aspirin and ibuprofen belong to a class of drugs called nonsteroidal anti-inflammatory drugs, or NSAIDs. Millions of Americans take them regularly to relieve pain and prevent heart disease. For years, scientists have been curious whether NSAIDs can reduce the risk of certain cancers.

There's good evidence that the drugs may help protect against colorectal cancer, said Thun, who was not involved in the new research. But the jury is still out on their benefit for breast cancer. Some studies have found a protective effect, while others found no link at all.

"The results have been inconsistent, but leave open the possibility that NSAIDs may provide a small -- up to 10% -- reduction in the risk of developing breast cancer," Thun explained.

Marshall and her colleagues tried to clarify the relationship by studying more than 114,000 women between the ages of 22 and 85 in California. The women answered questionnaires about how often they took aspirin, ibuprofen, or acetaminophen (a non-NSAID painkiller) and how long they'd been using the drugs. They also gave information about other lifestyle factors that could influence breast cancer risk, like family history of the disease, weight, alcohol use, and use of hormone therapy.

The women were tracked from 1995 until 2001; in that time, 2,391 of them developed breast cancer.

When the researchers looked at NSAID use in general, they saw that women who used the drugs had the same breast cancer risk as women who did not. Marshall said the group considers that finding especially important, since many of the earlier studies have found a benefit from NSAID use.

But when they analyzed each drug individually, they began to notice some differences.

Women who took aspirin every day for 5 years or more had an 81% greater risk of developing hormone-negative (ER- and PR-) breast cancer. These types of tumors are less common than hormone-sensitive tumors (ER+ or PR+), but are more difficult to treat because they do not respond to hormone therapies like tamoxifen or aromatase inhibitors. Aspirin did not significantly affect the risk of developing hormone-sensitive tumors.

Ibuprofen had an effect on both types of breast cancer. Women who took this drug daily for 5 years or more had about a 50% greater risk of developing any kind of tumor.

Although the increases in risk were statistically significant, Marshall said the overall risk to a woman would still be low. And it's not clear whether it's actually the NSAIDs that are having an effect.

"There may be something else going on with this group of women -- women who are high users of NSAIDs," she said. "It could be they have some other risk factor in common that we have not been able to measure."

Another possibility: Women who use pain relievers regularly may also visit a doctor more frequently, and thus have a better chance of finding breast cancer. Indeed, the women in the study who regularly used NSAIDs also had higher rates of mammography screening, Marshall noted.

Nevertheless, the findings do raise concerns about the potential dangers in using these drugs regularly and for a long period of time, Thun said. And they underscore the need for more research before doctors can recommend NSAIDs for cancer prevention.

In the meantime, Thun noted, women can take other steps to reduce their risk of breast cancer:

• maintain a healthy body weight
• get regular physical activity
• limit alcohol consumption
• minimize use of hormone therapy after menopause
• follow ACS guidelines on breast cancer screening with mammography.

Citation: "Nonsteroidal Anti-Inflammatory Drug Use and Breast Cancer Risk by Stage and Hormone Receptor Status." Published in the Journal of the National Cancer Institute (Vol. 97, No. 11: 805-812). Lead author: Sarah F. Marshall, MA, of the Keck School of Medicine, University of Southern California.