A new prospective study has reported that regular aspirin use is associated with an approximately 20% lower risk of postmenopausal breast cancer and that this reduction in risk does not vary according to estrogen (ER) and progesterone (PR) receptor status. The study was designed to examine whether the associations of aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs) with the risk of postmenopausal breast cancer vary according to the ER and PR receptor status of the tumor. Aspirin and other NSAIDs have been reported to inhibit aromatase activity (in which androgen is converted to estrogen in the body) and could thereby lower the incidence of hormone receptor positive tumors in women who take them regularly. The study included data concerning 26,580 postmenopausal women. A self-administered questionnaire mailed in 1992 provided information concerning use of aspirin and other NSAIDs.

As of year-end 2005, 1,581 new breast cancers had been found. Compared to never users of aspirin, women who regularly consumed aspirin were found to have a 20% lower risk of breast cancer. A trend of lower risk of breast cancer for increasing frequency of use was found, with women who used aspirin at least six times per week having a 29% lower risk of breast cancer than women who never used aspirin. Similar inverse associations between breast cancer risk and regular aspirin use were found regardless of the ER and PR status of the tumors. On the other hand, use of non-aspirin NSAIDs was not found to be associated with breast cancer risk overall, or with breast cancer according to ER or PR status. The lack of association between aspirin use and hormone receptor positive breast cancer suggests that the apparent protective effects of aspirin may either be through cellular pathways that are independent of estrogen or progesterone signaling.