Aspirin, the wonder drug that can help prevent heart attacks and strokes, also appears to reduce women's chances of developing the most common type of breast cancer, a study found.
The authors of the study said that the findings are tantalizing but that more research is needed before doctors can recommend that women take aspirin to ward off breast cancer.
The study appears in Wednesday's Journal of the American Medical Association and was led by researcher Mary Beth Terry and Dr. Alfred Neugut of Columbia University.
Previous studies reached conflicting conclusions on whether there is a link between aspirin and breast cancer. This is the first study to examine whether aspirin might influence the growth of specific types of tumors, said Dr. Raymond DuBois, director of cancer prevention at Vanderbilt University's Vanderbilt-Ingram Cancer Center.
"It's a landmark study,'' said Dr. Sheryl Gabram, a breast specialist at Loyola University Medical Center in suburban Chicago.
The reduced risk was found for tumors whose growth is fueled by the sex hormones estrogen or progesterone. About 70 percent of women who develop breast cancer have this type of cancer, called hormone receptor-positive.
Women in the study who used aspirin at least four times a week for at least three months were almost 30 percent less likely to develop hormone-fueled breast cancer than women who used no aspirin. Aspirin had no effect on the risk of developing the other type of tumor, hormone receptor-negative.
Researchers suspect aspirin works by interfering with the body's production of estrogen.
Similar studies have suggested that aspirin might reduce the risks of developing other kinds of cancer, including cancer of the pancreas, cancer of the ovaries, and Hodgkin's disease. But these studies could not say definitively whether other factors might explain the results.
And like the new breast cancer research, many of these studies relied on people's recollections of how often they took aspirin. However, a more rigorous study has linked the use of baby aspirin and a reduced risk of growths that can eventually turn into colon cancer.
That study involved randomly assigning patients to take aspirin or dummy pills - the gold-standard research method.
The researchers in the breast cancer study stopped short of recommending women take aspirin, noting that it can cause side effects such as stomach bleeding.
The researchers analyzed data on 1,442 breast cancer patients age 59 on average and a comparison group of 1,420 healthy women without the disease. The women were asked about their use of three pain relievers: aspirin, ibuprofen and acetaminophen.
The link with aspirin was strongest in women who took seven or more tablets a week and was greater in postmenopausal women than in younger women - which the researchers said makes sense, since hormone-fueled tumors are more common in older women.
Ibuprofen, an anti-inflammatory drug sold under such brand names as Motrin and Advil, was used by fewer women in the study, and the results were inconclusive. No reduced risk was found among users of acetaminophen, the active ingredient in Tylenol.
Aspirin, unlike acetaminophen, blocks the action of an enzyme that produces inflammation-causing substances called prostaglandins, which in turn induce an enzyme crucial to the production of estrogen, said co-researcher Dr. Andrew Dannenberg of Weill Cornell Medical College.
Aspirin thus might indirectly help lower levels of estrogen in the breast, Dannenberg said.
"The thing that's interesting about that is the biology of the process, and what aspirin does makes sense,'' DuBois said.
The study was funded by the National Cancer Institute and the National Institute of Environmental Health.