Dr. Elizabeth McDonald spends her days reading mammograms. As a breast radiologist at the University of Pennsylvania’s Abramson Cancer Center, she has looked at more breast imaging than most people will see in a lifetime. When she and her colleagues set out to study what happens to breast cancer rates in women taking GLP-1 medications, they were not expecting to find one of the most striking signals breast cancer prevention research has produced in years.
The result, presented at the 2026 American Society of Clinical Oncology Annual Meeting and published the same day in JCO Oncology Practice, was this: among more than 111,000 women, those taking GLP-1 medications were approximately 30 percent less likely to develop breast cancer than those who were not.
“While our study was observational and does not definitively confirm an association between GLP-1 medications and reduced breast cancer incidence, it does add to the growing body of evidence suggesting that it’s worth investigating these weight-loss drugs as potential cancer prevention tools,” McDonald said.
That sentence is doing careful, important work. It is also the reason this finding matters as much as it does.
The Study Behind the Number
McDonald’s team analyzed electronic health records from the University of Pennsylvania Health System, identifying 111,646 women between the ages of 45 and 80, all with a body mass index of 25 or above, who had undergone breast imaging with a documented outcome between January 2022 and June 2025. Of those women, 15,264, about 13.7 percent, had documented prescriptions for GLP-1 medications, the drug class that includes semaglutide, sold as Ozempic and Wegovy, and tirzepatide, sold as Mounjaro and Zepbound. The remaining 96,382 women, roughly 86.3 percent, had no documented exposure to these drugs.
The researchers looked at breast cancer diagnoses across two different analyses. In the full cohort of 111,646 women, those on GLP-1 medications had 35.1 percent lower odds of developing breast cancer. To address the obvious question, maybe women prescribed these drugs were simply different in ways that affected their cancer risk independent of the medication. The researchers built a second, more rigorous comparison: a matched cohort of 30,528 women, pairing each of the 15,264 GLP-1 users one-to-one with a control patient matched on age, race, ethnicity, BMI, breast density, and diabetes status. In that matched analysis, the protective association held at 30.5 percent lower odds.
The fact that the effect persisted, and barely moved, after this level of statistical matching is what makes the finding significant. The most common objection to observational drug studies is that the people taking the drug were healthier, wealthier, or more health-conscious to begin with, and that’s what’s really driving the result, which was directly tested here. The number held.
What GLP-1 Drugs Actually Do Inside the Body
GLP-1 receptor agonists were not designed with cancer in mind. Glucagon-like peptide-1 is a hormone the body produces naturally that regulates blood sugar and appetite. Drugs that mimic this hormone were developed first for type 2 diabetes, then approved for weight loss as their effects on appetite and metabolism became clear. The breast cancer finding emerged from researchers asking a different question: what else might a drug that changes a person’s metabolism so substantially be doing?
The mechanistic story McDonald’s team points to runs through several interconnected pathways. Excess weight is itself a well-established modifiable risk factor for breast cancer, particularly in postmenopausal women, because fat tissue produces estrogen, and estrogen exposure is a known driver of hormone-receptor-positive breast cancers, the most common subtype. A drug that produces substantial, sustained weight loss reduces the estrogen-producing tissue directly.
But weight loss alone may not be the whole story. GLP-1 medications also reduce chronic low-grade inflammation, measurable through markers like C-reactive protein. Chronic inflammation has been implicated in creating what researchers describe as a tumor-friendly environment, conditions that make it easier for cancerous cells to establish and grow. Separately, these drugs improve insulin resistance, lowering circulating levels of insulin and insulin-like growth factor 1, both of which have documented roles in directly promoting the growth of breast cancer cells in laboratory studies. A drug that simultaneously reduces estrogen-producing fat tissue, lowers systemic inflammation, and reduces the circulating growth factors that cancer cells can exploit is hitting multiple points in the biological pathway from “no cancer” to “cancer” at once.
“GLP-1 medications are intriguing from a cancer research perspective because they weren’t designed for cancer therapy, but they do affect many different targets and pathways associated with cancer development, so we’re eager to study them in this context,” McDonald said.
This Is Not the Only Cancer Signal These Drugs Have Produced
The Penn breast cancer finding did not arrive in isolation. It is part of a pattern that has been accumulating across multiple research groups studying different cancer types.
A separate study from the University of California San Diego, published in Cancer Investigation in November 2025, analyzed more than 6,800 colon cancer patients across University of California Health sites and found that those taking GLP-1 medications had a five-year mortality rate of 15.5 percent, compared to 37.1 percent for those not on the drugs less than half the mortality rate, after adjusting for age, BMI, disease severity, and other health factors. Led by Dr. Raphael Cuomo, an associate professor in the Department of Anesthesiology at UC San Diego School of Medicine, the study found the benefit was most pronounced in patients with a BMI over 35, consistent with the inflammation and insulin-pathway mechanisms McDonald’s team described for breast cancer.
Two large studies, two different cancer types, two different research institutions, and a converging mechanistic story involving inflammation, insulin signaling, and metabolic health. That kind of convergence across independent research groups is one of the things that makes a signal worth taking seriously, even before any single study reaches the level of proof.
Research into the gut microbiome has separately documented how chronic inflammation driven by diet and metabolic dysfunction creates the kind of internal environment these cancer studies describe, and GLP-1 drugs, by improving metabolic health broadly, may be intervening on that same underlying biology from a different angle.
The Steelman: Why “30 Percent Less Likely” Does Not Mean What It Might Sound Like
This is the section that matters most, and McDonald’s own words point directly to it: this is an observational study, and observational studies cannot establish that a drug causes an outcome, no matter how careful the statistical matching.
Here is the honest version of what “30 percent less likely” means and does not mean. It means that within this specific population, women aged 45 to 80, with a BMI of 25 or above, who had breast imaging at Penn Medicine between 2022 and 2025, those with a GLP-1 prescription on record had a lower rate of breast cancer diagnosis than those without, even after matching on five specific characteristics. It does not mean that if a woman starts taking Ozempic today, her personal risk of breast cancer drops by 30 percent. It does not mean the drug was proven to prevent cancer in any individual case. And it does not account for every possible difference between the two groups. Matching on five variables narrows the gap but cannot eliminate every form of confounding.
There is also a timing question that observational studies of this kind cannot fully resolve. Breast cancer can take years to develop and be detected. A woman who started a GLP-1 medication in 2023 and was not diagnosed with breast cancer by 2025 might simply not have had enough time for a cancer that was already developing to become detectable, or she might genuinely have a lower underlying risk because of the drug’s effects. The study’s window, January 2022 to June 2025, is not long enough to fully disentangle these possibilities for a disease with the latency profile of breast cancer.
This is precisely why McDonald described the findings as something that “adds to the growing body of evidence suggesting that it’s worth investigating these weight-loss drugs as potential cancer prevention tools” rather than as a recommendation. The next step, the step that would move this from a signal to something closer to evidence, is a randomized controlled trial, in which women are randomly assigned to receive the drug or not, removing the possibility that the people taking GLP-1 medications differ from those who don’t in ways that matching cannot capture. No such trial for breast cancer prevention currently exists. ScienceDaily’s coverage noted that clinical trials are now being planned specifically to test whether these drugs could help prevent breast cancer, which is itself a meaningful response to a finding of this magnitude, but it underscores that the proof does not yet exist.
What This Means for the Tens of Millions of Women Already Taking These Drugs
GLP-1 medications are not a niche prescription. Tens of millions of people in the United States alone are currently taking semaglutide or tirzepatide for diabetes or weight management, and a substantial proportion of them are women in the age range of 45 to 80, which this study examined. For those women, this finding lands differently than it would for a drug nobody is using.
The economics of these medications are themselves a significant story. GLP-1 drugs cost more than $1,000 a month without insurance, and access remains uneven even as evidence for benefits beyond diabetes and weight loss continues to accumulate. A breast cancer risk reduction signal, even an unproven one, adds another dimension to an already complicated calculus that millions of women and their doctors are navigating: cost, access, side effects, and now a growing list of potential benefits that extend well beyond the drug’s original indication.
McDonald was careful to frame the goal in terms of prevention infrastructure rather than individual decision-making. “Ultimately, we want to find better options to prevent breast cancer,” she said. “It’s been encouraging to see the survival rates for breast cancer improve over recent decades, and we’d love to see the same gains in prevention.” Survival rates for breast cancer have improved substantially because of advances in detection and treatment. Prevention, actually reducing how many women develop the disease in the first place, has been a much harder problem, and the tools available for primary prevention have been limited.
The Signal and the Sentence That Contains It
Elizabeth McDonald read through health records for 111,646 women and found that the ones taking GLP-1 medications developed breast cancer at meaningfully lower rates, 35.1 percent lower in the full group, 30.5 percent lower after the most rigorous matching her team could construct. The biological story behind that number, less estrogen-producing fat tissue, lower inflammation, improved insulin signaling, fits with a separate, independent study from UC San Diego finding that the same drug class was associated with less than half the five-year mortality rate in colon cancer patients.
None of this proves that Ozempic prevents breast cancer. McDonald said so herself, in the same sentence that described the finding as significant enough to study further. Both things are true at once: the number is real, it was produced by one of the largest studies of its kind, it survived rigorous statistical matching, and it is not proof. It is the kind of finding that generates the next study, the randomized trial that could eventually turn a 30 percent association into something doctors can act on with confidence.
For now, it is a signal. A strong one. Sitting inside the health records of over a hundred thousand women, waiting for the trial that will tell us what it actually means.
