Taking low-dose aspirin at least three times per week may reduce women’s risk of breast cancer by up to 20 percent, a new study suggests.

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Researchers have found a link between low-dose aspirin and reduced risk of breast cancer.

Study co-author Leslie Bernstein, Ph.D., of the Division of Biomarkers of Early Detection and Prevention at the City of Hope Beckman Research Institute in Monrovia, CA, and colleagues recently reported their findings in the journal Breast Cancer Research.

After skin cancer, breast cancer is the most common cancer among women in the United States. This year, more than 252,000 new cases of invasive breast cancer will be diagnosed.

Previous research has suggested that there may be a link between daily aspirin use and lower risk of breast cancer.

However, according to Bernstein and colleagues, few studies have investigated the effects of aspirin use on the risk of certain breast cancer subtypes, and it has been unclear as to whether low-dose aspirin, or “baby” aspirin, protects against breast cancer.

With this in mind, the researchers set out to determine the effects of low-dose aspirin – defined as a dose of 81 milligrams – on the risk of breast cancer overall, as well as its effects on breast cancer subtypes defined by hormone receptor (HR) status and human epidermal growth factor receptor 2 (HER2) expression.

HR status is whether or not the breast cancer cells contain receptors for the hormones estrogen or progesterone. For example, breast cancer cells that possess receptors for estrogen would be deemed estrogen receptor-positive (ER-positive).

HER2 status is whether or not breast cancer cells contains too many HER2 receptors, which can fuel breast cancer growth.

The researchers came to their findings by analyzing the data of 57,164 women who were part of the California’s Teachers Study, which has monitored the health of more than 133,000 teachers and administrators in California since 1995.

In 2005, the participants completed questionnaires detailing their use of aspirin and other non-steroidal anti-inflammatory drugs (NSAIDs).

By January 2013, 1,457 women had developed invasive breast cancer. Of these cases, 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.

Overall, the researchers found 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.

Looking at breast cancer subtypes, the team found 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.

No link was found between the use of other NSAIDs and the risk of breast cancer, the team reports.

“We also did not find associations with regular aspirin since this type of medication is taken sporadically for headaches or other pain, and not daily for prevention of cardiovascular disease,” notes lead author Christina A. Clarke, Ph.D., from the Cancer Prevention Institute of California.

Their findings remained after accounting for a number of possible confounding factors, including the use of hormone therapy and a family history of breast cancer.

The study was not designed to pinpoint the mechanisms by which low-dose aspirin may lower the risk of breast cancer, but the researchers speculate that it may be down to the drug’s anti-inflammatory effects.

Additionally, the team notes that aromatase inhibitors are used to treat ER-positive breast cancers. Since aspirin is a weak aromatase inhibitor, this may partly explain its protective effect against HR-positive breast cancers.

Overall, the researchers believe that their findings suggest that low-dose aspirin could be effective for the prevention of breast cancer, but they stress that further studies are needed before recommendations can be made.

The authors conclude:

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.”

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