You may be taking low-dose aspirin regularly because its blood-thinning powers are known to stave off heart disease, but some California researchers may have just given you another reason. Their findings indicate that taking low-dose aspirin at least three times per week may reduce a woman's risk of breast cancer by up to 20 percent.

Low-dose aspirin is sometimes called “baby aspirin,” and defined here as 81 milligrams. This common drug is no stranger to research regarding its effects on breast cancer. The current study, out of the City of Hope Beckman Research Institute in Monrovia, CA, builds upon previous work that pointed towards a possible link between daily aspirin use and lower risk of breast cancer. This new research dived deeper into the data to not only determine if and how aspirin guards against breast cancer, but also its effects on breast cancer subtypes as defined by hormone receptor (HR) status and human epidermal growth factor receptor 2 (HER2) expression.

If breast cancer cells contain receptors for the hormones estrogen or progesterone, their HR status is said to be positive. The HER2 gene makes HER2 proteins, which are also breast cancer receptors; if breast cancer cells contain too many HER2 genes, they are said to be HER2 positive.

The new research used as its dataset 57,164 women who were part of the California Teachers Study. In 2005, the participants completed questionnaires detailing their use of aspirin and other non-steroidal anti-inflammatory drugs. By January 2013, 1,457 women had developed invasive breast cancer. Out of these, 998 were HR-positive/HER2-negative, 138 were HR-negative/HER2-negative, 120 were HR-positive/HER2-positive, and 44 were HR-negative/HER2-positive. Data on HR and HER2 status were missing for the remaining 157 women.

The scientists discovered that that women who reported using low-dose aspirin at least three times weekly were 16 percent less likely to develop breast cancer, compared with women who used low-dose aspirin less frequently. Digging deeper, they learned that the risk of developing HR-positive/HER2-negative breast cancer was 20 percent lower for women who took low-dose aspirin at least three times per week.

"Our data are intriguing as regards the role of low-dose aspirin in breast cancer prevention but this question should be revisited in cohorts with larger numbers of incident breast cancers, in which HR and HER2 status are also recorded," the authors concluded.

The research has been published in Breast Cancer Research.