GLP-1 receptor agonists, the class of drugs that includes semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro), have already reshaped how we treat type 2 diabetes and obesity. Now, a large new study suggests they may do something no one originally designed them to do: lower the risk of breast cancer.

The findings, published in JCO Oncology Practice and presented at the 2026 American Society of Clinical Oncology (ASCO) Annual Meeting, come from a retrospective analysis of more than 111,000 women conducted by researchers at the University of Pennsylvania’s Perelman School of Medicine. Women who had been prescribed GLP-1 medications were approximately 30 percent less likely to develop breast cancer than those who had not, and that association held even after accounting for age, race, ethnicity, BMI, breast density, and diabetes status.

What Are GLP-1 Drugs?

GLP-1 stands for glucagon-like peptide-1, a hormone naturally produced in the gut that helps regulate blood sugar and appetite. GLP-1 receptor agonists mimic this hormone, prompting the pancreas to release insulin in response to meals, slowing stomach emptying, and signaling the brain to reduce hunger. Originally developed for type 2 diabetes, they are now among the most widely prescribed medications in the world, used by millions of people for weight management.

Their cancer-relevant effects, however, go well beyond the number on the scale.

Why Breast Cancer Researchers Are Paying Attention

Excess body weight, particularly fat tissue accumulated after menopause, is one of the most well-established modifiable risk factors for breast cancer. Adipose tissue produces estrogen, and higher estrogen levels over time are linked to increased risk of hormone receptor-positive breast cancers, which are the most common subtype. Obesity is also associated with chronic low-grade inflammation, insulin resistance, and altered levels of growth factors, all of which can create a biological environment more hospitable to tumor development.

GLP-1 drugs address several of these pathways simultaneously, not just by promoting weight loss, but through direct anti-inflammatory effects, improvements in insulin sensitivity, and metabolic and epigenetic changes that researchers believe may independently inhibit tumor development. As lead author Elizabeth McDonald, MD, PhD, put it: “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.”

What the Study Found

The Penn Medicine team examined electronic health records from 111,646 women between the ages of 45 and 80, all with a BMI of 25 or above (the clinical threshold for overweight), who had undergone breast imaging and had a documented outcome in the health system between January 2022 and June 2025. Of those women, 15,264 had documented GLP-1 prescriptions; the remaining 96,382 served as the comparison group.

The researchers looked for new breast cancer diagnoses across two separate cohorts: the full group of 111,646 women, and a matched cohort of 30,528 women, pairing each GLP-1 user one-to-one with a control matched on age, race, ethnicity, BMI, breast density, and diabetes status. This matched analysis is important because it helps rule out the possibility that the results simply reflect differences between the types of people who get GLP-1 prescriptions and those who don’t.

The reduction in breast cancer incidence appeared in both analyses: 35.1 percent lower odds in the full cohort, and 30.5 percent lower odds in the matched cohort. The consistency across both approaches strengthens the signal.

Important Caveats

The researchers are careful to note what this study cannot tell us. Because it is observational, looking back at what happened to real patients in a health system, rather than randomly assigning people to treatment, it cannot definitively establish that GLP-1 drugs caused the reduction in breast cancer risk. The analysis also did not account for which specific GLP-1 medication patients were taking, how long they had been using it, genetic risk factors such as BRCA mutations, or the stage and subtype of cancers diagnosed.

These are meaningful gaps, and the researchers acknowledge them. Further analyses are planned to address some of these variables.

What Comes Next

The study is best understood as a strong signal pointing toward the need for prospective clinical trials, the kind of rigorously controlled, forward-looking research that can actually confirm or refute causation. McDonald and collaborators are now working to launch a multisite clinical trial specifically designed to test whether GLP-1 medications can reduce breast cancer incidence in high-risk women, including those with a personal history of breast cancer.

The context here matters. Current options for breast cancer prevention beyond screening are limited and often carry significant downsides. Tamoxifen is effective at reducing risk in high-risk patients but sees low uptake because of its side effect profile. Prophylactic mastectomy is an option for some with high-risk genetic mutations, but it is a life-altering surgical decision. GLP-1 drugs, by contrast, are already taken by millions of people for other reasons, are generally well tolerated, and may be turning out to have benefits that extend well beyond metabolism.

“Ultimately, we want to find better options to prevent breast cancer,” McDonald 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.”

This study won’t be the last word, but it may be the opening of an important new chapter.

Source: McDonald ES, Gillis LB, Gabriel P, et al. GLP-1 Agonists Are Associated With a Significant Reduction in Breast Cancer Incidence in Women. JCO Oncol Pract. Published online June 2, 2026. doi:10.1200/OP-26-00485. Presented at ASCO 2026, Abstract 10506.