How frequently older women use aspirin and other pain relievers has nothing to do with their chance of developing breast cancer, a large new study suggests.

Previous research has painted a cloudy picture, with some studies indicating that women who regularly take so-called nonsteroidal anti-inflammatory drugs, or NSAIDs, have a lower risk of breast cancer, but others finding a higher risk with such drugs.

It's still possible that specific groups of women will benefit from certain doses of aspirin, other NSAIDs or acetaminophen (marketed as Tylenol), said one researcher not involved in the new study.

But right now, there's no good evidence any of those painkillers protect women against breast cancer, according to epidemiologist Lynn Rosenberg from the Boston University School of Public Health.

"I don't think they should be taking aspirin to prevent breast cancer," she said.

"If women are looking to decrease their risk of breast cancer, there is, I think, some pretty convincing evidence that high levels of exercise will reduce risk (as will) diets that are high in fruits and vegetables."

NSAIDs, including naproxen (Aleve) and ibuprofen (Advil, Motrin), may block an enzyme that controls cell proliferation and death, therefore possibly affecting cancer risk. And acetaminophen could lower levels of estrogen, a hormone important to certain forms of breast cancer, researchers said.

Studies have found regular use of anti-inflammatories is tied to a lower risk of colon cancer, but it's been unclear whether that is true for other types of cancer as well (see Reuters Health story of April 8, 2011).

Researchers led by Dr. Xuehong Zhang from Brigham and Women's Hospital in Boston used data from a long-term study of more than 100,000 nurses in the U.S., who filled out surveys every other year about their medication use, diet and lifestyle. The women also reported any new medical diagnoses.

Out of 84,600 participants who were past menopause and hadn't had cancer at the start of the study, about 4,700 were diagnosed with breast cancer between 1980 and 2008.

Researchers found that regularly taking aspirin and other NSAIDs - including daily use for more than 10 years - didn't affect women's risk of breast cancer, Zhang and his colleagues reported Monday in the Journal of Clinical Oncology.

Women in the study were typically taking aspirin for headaches, arthritis or other pain, or to protect against heart disease.

Survey participants currently taking acetaminophen were slightly less likely to be diagnosed with cancer than non-users - but there was no clear link between how long women had been taking the drugs, or at what dosage, and their cancer risk. Nor was past acetaminophen use, even for a decade or more, tied to fewer breast cancer cases.

"Our findings suggest that using these analgesics may not decrease women's risk of breast cancer," Zhang told Reuters Health.

"However, women should be reassured that there's no increase in risk with long-term NSAID use," he added.

One in eight American women will be diagnosed with breast cancer in her life, according to the National Cancer Institute.

Rosenberg said she expects any future research on the link between painkillers and breast cancer to focus on whether there might be a benefit for specific groups of women or specific types of breast cancer.

Still, she told Reuters Health, "When you get into high doses of aspirin taken frequently, then you run into the problem of gastrointestinal bleeds - that's a huge, huge problem."

So it's unlikely, Rosenberg added, that any small painkiller-related benefits that might eventually be found for breast cancer would outweigh those risks.

Zhang agreed with her that for women looking to reduce their breast cancer risk, the best strategies are still maintaining an ideal body weight, exercising regularly and avoiding alcohol in excess.

The study is published in the Journal of Clinical Oncology.