An international team of scientists has found that a lactation drug could reduce the risk of breast cancer in mothers after childbirth.
Cabergoline, a medication used to stop milk production after pregnancy loss or when breastfeeding is not possible, is generally well tolerated by patients with few side effects. But it may also offer unexpected benefits.
The study, published today in Cancer Discovery , analyzed health records from Spain and Norway, including more than 150,000 women with information on whether they had used cabergoline.
Women who had the medication to suppress milk production were around 70 per cent less likely to develop breast cancer in the years that followed, compared with those who did not receive cabergoline.
This finding mirrored what the researchers found in mice. They tested a single dose of cabergoline in mice carrying BRCA1 and TP53 mutations, which are associated with aggressive forms of breast cancer, and tumour incidence rates fell by about half.
"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,” said study co-lead Dr. Christopher Maxwell, professor in UBC's faculty of medicine and investigator at BC Children's Hospital Research Institute. “This is particularly true for women at high risk of breast cancer. But our findings suggest there may be an opportunity to reduce that risk with an existing drug by helping the breast complete a natural process that happens after breastfeeding ends."
Like a fruit tree preparing for winter
After a woman stops breastfeeding, the breast undergoes a major clean-up process called “involution”.
Dr. Maxwell likened it to a fruit tree. In spring and summer, the tree grows branches and produces blossoms and eventually fruit. Once the fruit is gone, the tree sheds what it no longer needs and prepares for the next season.
Similarly, during pregnancy, hormone levels rise and the milk-producing structures within the breast branch out, creating a ‘ductal tree’. But once breastfeeding ends, those cells are no longer needed and the breast undergoes involution to remove excess tissue.
"We think of involution as the breast's version of winter," said Dr. Maxwell. "It's a period when structures that were needed for pregnancy and breastfeeding reset for the future."
But the researchers found that this clean-up process can be incomplete in the experimental mice, leaving behind cells which may become cancerous.
Cabergoline appears to help complete that transition by accelerating the breast's natural post-weaning process. In the mouse studies, it reduced potentially precancerous tissue, shrunk the ductal area, reduced cell growth, and boosted immune cells responsible for clearing damaged cells.
Clinical trial in the works
With one in eight women in Canada developing breast cancer in their lifetime, the researchers say new prevention strategies are urgently needed.
"We think the period after breastfeeding is an important window for prevention,” said study co-lead Dr. Jesús Perez-Losada, a researcher at the Cancer Research Center, a joint institute of Spanish National Research Council and the University of Salamanca. “There is a unique opportunity to target involution, the natural process that begins after weaning. This is not about changing a woman’s breastfeeding plans; any preventive intervention would be considered only after breastfeeding has ended."
The next step for the researchers is to test cabergoline in clinical trials and determine whether the benefits observed in mice can be confirmed in people. The multinational research team has already secured funding from the Gray Foundation for a clinical trial spanning multiple countries.
Women with a BRCA1 mutation will be recruited through high-risk clinics and will choose how long they want to breastfeed. After weaning, they will receive either cabergoline or a placebo while researchers monitor biological markers linked to future breast cancer risk.
It’s a promising avenue because current preventative measures for women at high risk include hormone therapy or surgery, both of which can have significant physical and emotional side effects.
If successful in clinical trials, the drug’s existing approval by regulatory agencies including Health Canada and the European Medicines Agency could accelerate its path to use as a breast cancer prevention strategy — providing a kinder option to the women who need it most.
This research was funded by the Canadian Institutes of Health Research, the Canadian Cancer Society, the Terry Fox Research Institute as well as Spain’s Ministerio de Ciencia e Innovación/Agencia Estatal de Investigación, with additional support from funding agencies in the United Kingdom and the United States.
Cancer Discovery
Experimental study
Animals
Cabergoline Suppresses Postpartum Breast Cancer by Enhancing Mammary Gland Involution in Brca1-Mutant Mice and Is Associated with Reduced Risk in Younger Women
15-Oct-2026