Study senior author Leslie Bernstein said, “Use of low-dose aspirin at least three times a week was linked to a 20 percent risk reduction for cancers known as hormone-receptor positive, HER2 negative — the most common breast cancer subtype.”

“That’s a ‘moderate’ reduction in risk. It’s maybe not as good as exercise. More people might adhere to an aspirin regimen than an exercise routine.”

However, the study doesn’t establish a direct cause-and-effect relationship, and Bernstein said it’s too early to recommend taking daily aspirin for breast-cancer risk reduction.

Many adults already take low-dose aspirin — 81 milligrams — daily to lower their risk of heart attack.

This study — led by Christina Clarke, who was with the Cancer Prevention Institute of California — looked at the medication use of women enrolled in the ongoing California Teachers Study.

That trial, begun in 1995, recruited more than 133,000 active and retired women teachers, administrators and other public school professionals.

In 2005, more than 57,000 participants answered questions about their use of aspirin and other medications, family history of cancer, use of hormone therapy, exercise, height and weight. By 2013, nearly 1,500 had developed invasive breast cancer.

Overall, regular baby aspirin use reduced the risk of breast cancer 16 percent, the study said.

But the more significant finding was the risk reduction for developing HR-positive/HER2-negative cancer.

The researchers found a protective link with use of low-dose aspirin, but not with regular-dose aspirin or other non-steroidal anti-inflammatory drugs such as ibuprofen or acetaminophen.

Why wouldn’t higher-dose aspirin also be protective?

Regular-dose aspirin is more likely to be taken sporadically for pain such as headaches, the authors believe, while they think that women taking baby aspirin were doing so on a regular basis for heart protection.

The researchers can only speculate as to why the baby aspirin appears to reduce risk of breast cancer.

Bernstein said, “Aspirin not only reduces inflammation, but it’s a mild aromatase inhibitor.”

Aromatase inhibitor drugs are used to treat hormone-receptor positive breast cancer in women past menopause, since they reduce the amount of estrogen circulating in the blood, and the estrogen fuels the tumor.

Another researcher praised the study.

Sushanta Banerjee, a professor of hematology and oncology at the University of Kansas Medical Center, said, “This is really very exciting work.

“In his research, confined to the lab and animals, we found that aspirin has the capability to destroy the tumor-initiating cells that can lead to breast cancer.”

In a study presented at a recent cancer meeting, his team reported that aspirin may prevent new blood vessels from forming and ‘feeding’ the cancer.

Yet, he agreed it’s too soon to suggest taking baby aspirin to reduce breast cancer risk.

If more study bears out the link between baby aspirin and breast cancer prevention, Bernstein said low-dose aspirin may also help prevent recurrence.

The study was published in the Breast Cancer Research journal. It was funded by the US National Cancer Institute and the California Breast Cancer Research Fund.