A cheap drug some women are given to stop breast milk after childbirth was linked to about 70% less breast cancer in the years that followed
After giving birth, some women are offered a single pill to stop their breast milk, usually after a pregnancy loss or when breastfeeding is not possible. The drug, cabergoline, has been used this way for decades. An international team led from Spain and Canada went through the health records of more than 150,000 women in Spain and Norway to see what happened to those who took it. Years later, one difference stood out.
Women who had taken cabergoline were about 70% less likely to develop breast cancer in the years after childbirth than women who had not. Postpartum breast cancer is rare in absolute terms, and the study had few cases among women who took the drug, so the true size of the effect is uncertain.
That matters because the years after pregnancy are a vulnerable time. “Breast cancer is the leading cause of cancer-related death among younger women, and the majority of these cancers occur during the years after pregnancy,” the researchers said in a statement.
The explanation they propose has to do with what the breast does once milk is no longer needed. In a process called involution, the gland clears out milk-producing cells and remodels itself, which brings inflammation and tissue changes that can favor tumors.
“We think of involution as the breast’s version of winter,” said Dr. Maxwell of the University of British Columbia, one of the study’s leaders. Cabergoline suppresses prolactin, the hormone that drives milk production, and appears to help the breast get through that reset faster and more cleanly.
The team tested the idea in mice carrying mutations in BRCA1 and TP53, genes linked to aggressive breast cancers. A single dose of cabergoline after lactation cut tumor rates by about half. Treated mice also had less potentially precancerous tissue, less cell growth and immune cells that looked more active against damaged cells.
The second finding is about timing, and it is where the mouse and human evidence part ways. The mice got the drug after weaning, while the women took it right after birth to stop milk from coming in. So it is not yet known whether giving cabergoline after a woman finishes breastfeeding, which is how it would most likely be used for prevention, would protect her.
That is the question a planned clinical trial, funded by the Gray Foundation, will try to answer. Women will receive either cabergoline or a placebo after weaning while researchers track biological markers tied to future breast cancer risk. Because cabergoline is already approved in many countries, a positive result could move faster than a brand-new drug. Women’s health has many such open questions, from how endometriosis really presents to which form of hormone therapy is safest.
For now, the findings are not a reason to take cabergoline to prevent cancer, and the researchers stress it is not approved for that use. Women with a high inherited risk, such as BRCA1 carriers, who are planning a pregnancy may want to ask their doctor about prevention options and ongoing trials.
There are limits to what this shows. The human data come from health records, so the study shows a link, not proof. Women who received cabergoline may have differed from others in health history, breastfeeding or other ways that affect breast cancer risk. The mouse experiments used a specific high-risk genetic model, and results in mice often do not carry over to people.
Source
García-Sancha N, Corchado-Cobos R, Ghorayeb R, et al. “Cabergoline Suppresses Postpartum Breast Cancer by Enhancing Mammary Gland Involution in Brca1-Mutant Mice and Is Associated with Reduced Risk in Younger Women.” Cancer Discovery, 2026.
DOI: 10.1158/2159-8290.CD-26-0187
