GLP-1 Drugs Linked to 30% Lower Breast Cancer Risk in Study of 110,000 Women

The finding

Researchers at the University of Pennsylvania have found that women taking GLP-1 receptor agonists — the drug class behind Ozempic, Wegovy, Mounjaro, and Zepbound — had significantly lower odds of developing breast cancer than women not taking the medications. The study, presented at the 2026 American Society of Clinical Oncology (ASCO) Annual Meeting and published in JCO Oncology Practice on June 6, is among the largest observational analyses on this question to date.

The findings add to a growing body of cancer-related signals around GLP-1 drugs — though researchers are careful to distinguish what the data can and cannot show.

What they looked at

The Penn Medicine team reviewed electronic health records from 111,646 women between ages 45 and 80 with a BMI of 25 or higher who had undergone breast imaging within the Penn Medicine health system between January 2022 and June 2025.

Of those women, 15,264 (13.7%) had documented GLP-1 prescriptions. The remaining 96,382 had no documented exposure. To reduce the risk of confounding, the researchers also analyzed a matched cohort of 30,528 women — pairing each GLP-1 user with a non-user who shared similar age, race, ethnicity, BMI, breast density, and diabetes status.

What they found

The results were consistent across both analyses:

• In the full population of 111,646 women, GLP-1 users had 35.1% lower odds of developing breast cancer.
• In the matched cohort, the reduction was 30.5% lower odds.
• In absolute terms: 1.62% of GLP-1 users were diagnosed during the study period, versus 2.31% of non-users — roughly seven fewer diagnoses per 1,000 women.

A separate real-world analysis presented at the same ASCO meeting found GLP-1 use was associated with a 45% reduction in metastatic progression of breast cancer.

Lead author Elizabeth McDonald, MD, PhD, a professor of Radiology at the Penn Perelman School of Medicine and practicing breast radiologist at Penn's Abramson Cancer Center, cautioned that observational data cannot establish causation — but said the findings "add to the growing body of evidence suggesting that it's worth investigating these weight-loss drugs as potential cancer prevention tools."

What this study cannot tell us

The study also did not distinguish between specific drugs (semaglutide vs. tirzepatide), did not track treatment duration, and did not account for genetic risk factors, cancer stage, or tumor subtype. The researchers acknowledge these limitations and say additional analyses are planned.

Why this matters for women

Breast cancer is the most commonly diagnosed cancer among women in the United States. Current prevention options — chemoprevention with tamoxifen, prophylactic surgery — carry meaningful downsides, and uptake among eligible women is low.

GLP-1 drugs are already prescribed to millions of women for weight management and metabolic health. The possibility that they may also reduce breast cancer risk — particularly in women with obesity, a known risk factor for postmenopausal breast cancer — would be clinically significant if confirmed in a trial. Excess adipose tissue increases circulating estrogen levels after menopause, and GLP-1s are known to reduce systemic inflammation and influence adipose metabolism, which may partially explain the observed signal.

The Penn team is now planning a multisite clinical trial to test whether GLP-1s can reduce breast cancer incidence in high-risk women, including those with a prior breast cancer history.

What's next

The Phase 3 ORCA trial is currently evaluating semaglutide in high-risk cancer prevention populations and will provide randomized evidence in coming years. Until those results are available, the Penn findings — while notable — remain hypothesis-generating. They are a reason to study GLP-1s in this context, not yet a reason to use them for breast cancer prevention.