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Study: Does aspirin lower a woman’s breast cancer risk?

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This study is about:

Whether use of aspirin and other non-steroidal anti-inflammatory medications (NSAIDS) can lower breast cancer risk.

Why is this study important?

There have been many previous studies examining whether taking aspirin can reduce breast cancer risk. The results of these prior studies have been mixed—some studies claim regular use of aspirin can lower breast cancer risk while others do not report risk reduction. Experts have also speculated about the dose of aspirin needed to decrease breast cancer risk. This study is the first to report that regular use of low-dose aspirin is associated with a reduction in risk of  and breast cancer. This study supports the need for an additional, larger clinical trial to prove that low-dose aspirin indeed lowers the risk for certain types of breast cancer.

Study findings:

  • Women who reported taking three or more tablets of low-dose aspirin per week were:
    • 16% less likely to develop breast cancer compared to women who did not take NSAIDS.
    • 20% less likely to develop ER/PR-positive/HER2-negative subtype breast cancer compared to women who did not take NSAIDS.
  • Women taking three or more tablets of “other” NSAIDS had marginally reduced risk of breast cancer that was not statistically significant. 

What does this mean for me?

This research study suggests that taking low-dose aspirin at least three times per week modestly lowers a woman’s risk of developing breast cancer, specifically ER/PR-positive/HER2-negative subtype breast cancer. However, it is important to note that this is an observational study and not a clinical trial. While aspirin use was associated with a lower risk of breast cancer, it does not prove that the aspirin caused the lower risk. There may be other reasons why the women in this study who took aspirin had lower risk.

Results of this study are not consistent with those of the Women’s Health Study, the only trial to prospectively look at whether low-dose aspirin reduces breast cancer risk. The Women’s Health Study found no decrease in overall breast cancer for women who took low-dose aspirin every other day regardless of hormone receptor status. More research needs to be done in much larger groups of women to better understand the benefit of regular use of low-dose aspirin on risk for different subtypes of breast cancer. In the meantime, women should talk to their healthcare providers to determine the best way to manage their breast cancer risk.

Posted 8/29/17

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Article reviewed

Clarke CA, Canchola AJ, Moy LM, Neuhausen, SL, Chung, NT, Lacey, JV and Bernstein, L. “Regular and low-dose aspirin, other non-steroidal anti-inflammatory medications and risk of HER2-defined breast cancer: the California Teachers Study.Breast Cancer Research. 2017. 19:52

Prior studies referenced in this review

  1. Cook NR, Lee IM, Gaziano JM, Gordon D, Ridker , Manson JE, Hennekens CH, Buring JE. “Low-dose aspirin in the primary prevention of cancer: the Women's Health Study: a .JAMA. 2005. 294(1):47-55.

  2. Zhang X, Smith-Warner SA, Collins LC, Rosner B, Willett WC, Hankinson SE.  “Use of aspirin, other nonsteroidal anti-inflammatory drugs, and acetaminophen and postmenopausal breast cancer incidence.J Clin Oncol. 2012. (28):3468-77.

  3. Takkouche B, Regueira-Méndez C, Etminan MJ.“Breast cancer and use of nonsteroidal anti-inflammatory drugs: a .”  J. Natl Cancer Inst. 2008.  100(20):1439-47.

  4. Zhong S, Chen L, Zhang X, Yu D, Tang J, Zhao J.  “Aspirin use and risk of breast cancer: systematic review and of observational studies.”  Cancer Epidemiol Biomarkers Prev. 2015.  (11):1645-55.


FORCE receives funding from industry sponsors, including companies that manufacture cancer drugs, tests and devices. All XRAYS articles are written independently of any sponsor and are reviewed by members of our Scientific Advisory Board prior to publication to assure scientific integrity.

This article is relevant for:

Women at average risk for breast cancer

This article is also relevant for:

people with ER/PR + cancer

healthy people with average cancer risk

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Study background:

Daily use of low-dose (81 mg) aspirin is recommended by the United States Preventive Service Task Force to reduce risk of heart disease and colon cancer. There have been many studies evaluating whether aspirin can lower breast cancer risk; however, results have been mixed.

  • One study, conducted between September 1992 and March 2004, was designed to examine the effect of aspirin on the risk of cancer among healthy women. This study involved 39,876 women in the U.S. who were randomly assigned to receive either 100 mg of aspirin or every other day. No effect of aspirin was observed on total cancers in this group, on breast cancer alone or by breast cancer subtype (estrogen-receptor positive/progesterone-receptor positive; estrogen-receptor positive/progesterone-receptor negative; estrogen-receptor negative/progesterone-receptor positive) (1).
  • Another large study did show reduction of risk for ER/PR-positive/HER2-negative breast cancer (2).
  • Another large analysis of many studies () concluded that breast cancer risk overall was reduced with use of any aspirin or other non-steroidal anti-inflammatory drug ()(3).
  • In contrast, a more recent concluded that aspirin use was not associated with reduced risk of breast cancer. However, when breast cancers were subtyped by hormone receptor status, there was a significant reduction in hormone receptor positive breast cancer (4).

The results of the two meta-analyses suggest that aspirin may reduce risk of specific subtypes of breast cancer.

Christina Clarke and her colleagues from the Cancer Prevention Institute of California and other institutions published research in Breast Cancer Research in May 2017 that adds to the body of literature regarding aspirin’s effect on breast cancer risk. Because few studies have looked at aspirin use and risk reduction of specific breast cancer subtypes based on status, Clarke and her colleagues prospectively studied whether aspirin can reduce risk of breast cancer overall and by subtype.

Researchers of this study wanted to:

Determine whether low-dose aspirin and other NSAIDS lowered breast cancer risk.

Population(s) looked at in the study:

The women involved in this study were from the California Teachers Study cohort (CTS), established in 1995-1996. These women were asked to complete a questionnaire that included information on their family history of cancer, other medical conditions, menstrual and reproductive history, self-reported weight and height, living environment, diet, alcohol and tobacco use, physical activity history, and the frequency and duration of prior use of certain medications, including aspirin.

Ten years later, in 2005-2006, the women were asked to complete a follow-up questionnaire about how frequently they use/used aspirin, low-dose aspirin, and other pain-relieving medications. Among the 96,179 participants, 57,164 completed the follow-up questionnaire. During the follow-up period (from the date a woman completed the 2005-2006 questionnaire through December 2012), 1,457 women were diagnosed with invasive breast cancer.

Study findings:

  • Women who reported taking three or more tablets of low-dose aspirin per week were:
    • 16% less likely to develop breast cancer compared to women who did not take NSAIDS.
    • 20% less likely to develop ER/PR-positive/HER2-negative subtype breast cancer compared to women who did not take NSAIDS.
  • The use of other NSAIDS was marginally associated with lower breast cancer risk that was not significant. The authors speculate that low-dose aspirin may have been more effective because women took it more regularly than regular-dose aspirin or other NSAIDS


The major limitation of this study is the study design. It is observational, meaning it only observed whether women who took aspirin had a lower incidence of developing breast cancer. Thus, low-dose aspirin use was associated with a lower risk of developing breast cancer, but may not be the cause of that lower risk. The researchers note that there may be some factors affecting the results of their research particularly among women who took three or more tablets of low-dose aspirin each week. For example, women who do this may be generally more health conscious – they may eat healthier and exercise more regularly than women who did not take low-dose aspirin. If this is the case, these women may have lowered their breast cancer risk by diet and exercise versus aspirin. It may also be that the reasons women in this study took aspirin at all were the actual reasons they had lower risk. For example, some women may have taken aspirin to reduce symptoms of arthritis which may be due to lower levels. 

Additionally, this research did not study a representative sample of women. Specifically:

  • only women from one state, California were studied
  • the majority of the women involved in this study (88%) were white
  • women born outside the United States were not represented

Therefore the results of the study may not apply to all women throughout the United States.      


This study suggests that taking low-dose aspirin three or more times per week may lower a woman’s risk for developing breast cancer. These results are consistent with some existing studies but contrary to others. This is the first study to suggest that risk reduction occurs when women use low-dose aspirin and not regular-dose aspirin. This suggestion may reflect frequent use of low-dose aspirin for prevention versus periodic use of regular-strength aspirin taken for pain relief. This study also indicates that low-dose aspirin reduces risk particularly for ER/PR-positive/HER2-negative, the most common type of breast cancer. These results will need to be validated in a larger , trial before they can be universally accepted, especially since they contradict those of a previous trial, the Women’s Health Study. However, if low-dose aspirin, a widely available, inexpensive medication, was confirmed to be an effective risk reducing strategy for the most common type of breast cancer, the benefit could be extremely significant.

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Posted 8/29/17

Questions To Ask Your Doctor
Questions To Ask Your Doctor

  • How can I lower my breast cancer risk?
  • Should I regularly take low-dose aspirin to reduce my risk of breast cancer and other diseases?

Who covered this study?


Low-dose aspirin linked to lower breast cancer risk, study says This article rates 4.0 out of 5 stars


Low-dose aspirin may lower breast cancer risk This article rates 4.0 out of 5 stars

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Regular use of low-dose aspirin may lower risk of women’s breast cancer, study reports This article rates 2.5 out of 5 stars

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