Emergency Contraceptive May Also Lower Breast Cancer Risk, Study Finds

• Researchers say that a drug used as an emergency contraceptive or to treat fibroids may help lower breast cancer risk.
• Ulipristal acetate, the “morning after pill,” was found to reduce the activity of breast cells where cancers start.
• The drug also altered the structure and stiffness of breast tissue.
• The authors say this discovery could potentially help prevent aggressive breast cancers in younger females.

A medication used as an emergency contraceptive or to treat fibroids may also help lower the risk of some of the most aggressive breast cancers.

In a new study, women who were more likely to develop breast cancer because of their family history took the drug ulipristal acetate, also known as the “morning after pill,” for 12 weeks.

Afterward, the researchers found a reduction in certain features linked to cancer risk, including the activity of certain breast cells believed to be the starting point for hard‑to‑treat cancers. The drug also changed the structure and “stiffness” of breast tissue.

The researchers speculate that this could open up a new way to prevent breast cancer in younger, premenopausal women. The findings were published on November 5 in Nature.

The research team conducted the Breast Cancer–Anti-Progestin Prevention Study 1 (BC-APPS1) at a single center in Manchester, United Kingdom.

Their goal was to see if blocking the hormone progesterone could reduce the risk of biological signs of breast cancer.

All participants were women ages 25 to 45, still having regular periods, and with a moderately to highly increased chance of getting breast cancer due to their family history. On average, their lifetime risk was about 1 in 4.

Thirty‑two women joined between 2016 and 2019. After screening, 26 began treatment and 24 finished the study.

Each took a 5-milligram tablet of ulipristal acetate every day for 12 weeks, starting on the first day of their period. This drug blocks progesterone’s effects.

Before starting and after finishing the 12‑week course, doctors collected small samples of breast tissue using a needle biopsy, with the beginning and ending samples being taken from a different breast.

The first biopsy was timed for the part of the cycle when progesterone is normally highest, so the team could see the maximum impact of the drug.

They then examined the tissue in a lab to:

• count different types of breast cells
• test how active certain cell types were
• examine changes in genes and proteins, especially those involved in tissue framework (the “extracellular matrix”)
• measure how fibers, such as collagen, were arranged and the stiffness of the tissue

They also reviewed MRI scans and mammograms for changes in breast density, a known risk factor.

The primary result was a significant decrease in breast cell growth. On average, 8.2% of cells were growing before treatment, compared with only 2.9% afterward.

One particular cell group, called luminal progenitor cells, decreased from 43% of breast cells to 30% after the drug was administered. These cells are thought to be the starting point for triple‑negative breast cancers, which are more aggressive and difficult to treat. Other cell types remained relatively unchanged.

Tests of cell activity showed similar results. The number of “mixed” colonies (cells that can develop into different types) dropped from 70% to 55%, and the ability to form mammospheres (small cell clusters grown in lab dishes) fell by about half.

Gene and protein studies showed that the drug altered the behavior of hormone-sensing cells — the ones that respond to progesterone — and reduced the production of key collagen proteins that help give breast tissue its structure, especially collagen VI.

This matters because the way collagen is organized affects tissue stiffness and breast density, both of which are linked to cancer risk.

The team also found fewer “collagen signals” being sent to other cells like fibroblasts and basal cells, probably because the hormone‑sensing cells were making fewer signalling molecules (such as WNT5A). In turn, fibroblasts made less collagen.

Physical measurements confirmed the tissue changes: collagen fibers were less tightly aligned, and the breast tissue was softer after treatment.

The drop in high‑risk progenitor cell activity was greatest in women who had denser breasts to start with, suggesting they may benefit most from this approach.

Lab experiments supported these findings. When human breast cells were grown in stiff environments, progesterone’s effects on progenitor cells were stronger.

Both ulipristal acetate and another anti‑progesterone drug reversed this, breaking the cycle between stiffness, progesterone activity, and progenitor cell growth.

Overall, taking ulipristal acetate for 12 weeks reduced cell growth, decreased the number of high-risk breast cells, and altered tissue structure in ways associated with a lower cancer risk, without serious side effects.

While she said more data is needed before uliprital acetate can be recommended as a preventive measure, Sagun Shrestha, MD, Chief of Medical Oncology at City of Hope Cancer Center in Phoenix, AZ, noted the importance of visiting your primary care doctor and gynecologist for regular exams and following mammogram guidelines. Shrestha wasn’t involved in the study.

“Be aware of your body and if there is anything abnormal to report and get checked at the earliest for early diagnosis of breast cancer,” she told Healthline.

Shrestha said that at-risk individuals should consider minimizing hormone replacement therapy.

Metin Çakmakçi, a general and breast surgical oncologist at Acıbadem Health Group Ataşehir Hospital in Istanbul, Turkey, who was not involved in the study, added that nursing can help lower the risk of breast cancer for those who become pregnant.

“If you have children, breastfeeding for several months can slightly reduce your breast cancer risk,” he told Healthline.

According to Çakmakçi, females with BRCA gene mutations or a strong family history of breast cancer may consider preventive medications like tamoxifen (for premenopausal women) or raloxifene and aromatase inhibitors (for postmenopausal women). These help reduce the risk of developing breast cancer by blocking or lowering estrogen in the body.

“These medications serve as risk-reduction options for high risk patients, but they come with their own set of side effects, which need discussion with a specialist,” he said.

Çakmakçi added that women who are at very high risk, such as those who are carriers of the BRCA1 or BRCA2 mutation, might choose to have prophylactic surgery.

“This might involve mastectomy (removal of healthy breasts to prevent cancer), and/or removal of the ovaries (which produce most of the body’s estrogen),” he said.

While this choice can be life-altering, it also comes with significant risk reduction, as high as 90%, with a mastectomy.

“People need to evaluate their own needs before getting the surgery since it involves assessing both the benefits and the potential dangers and emotional consequences,” said Çakmakçi.

Finally, a tailored screening plan combined with these preventive measures is ideal for breast cancer prevention.

“Every woman’s situation is unique,” Çakmakçi said, “so I encourage discussing your individual risk and prevention plan with your doctor — but these general measures are beneficial for virtually everyone.”