GLP-1 Drugs and Breast Cancer: What the 2026 ASCO Studies Reveal About Risk, Spread, and Survival

A trio of major studies presented at the 2026 American Society of Clinical Oncology (ASCO) Annual Meeting has intensified scientific interest in whether GLP-1 receptor agonist drugs—widely prescribed for weight loss and type 2 diabetes under brand names such as Ozempic, Wegovy, and Zepbound—might offer protective effects against breast cancer. The findings are the strongest real-world signals to date, but researchers and clinicians are united around one critical point: the studies show associations, not causation.

The Penn Medicine study: a 30% reduction in breast cancer incidence

A large retrospective analysis led by Elizabeth McDonald, MD, PhD, a professor of radiology at the University of Pennsylvania Perelman School of Medicine, examined electronic health records and breast imaging data from more than 110,000 women between the ages of 45 and 80 who were overweight and eligible for mammogram screening . The study was managed by the American College of Radiology (ACR) and presented as Abstract 10506 at ASCO 2026.

The headline result: women who used GLP-1 medications were about 30% less likely to develop breast cancer compared with non-users . A news release from the ACR described the reduction as up to 35% . The analysis adjusted for age, race, ethnicity, BMI, breast density, and type 2 diabetes status .

What was not reported: The publicly available summaries from both Penn Medicine and the ACR did not include raw incidence rates per 1,000 person-years or a specific odds ratio with a 95% confidence interval for this study. The primary reported metric was the relative risk reduction, leaving a gap in the details needed for full independent interpretation .

The study authors also noted that the analysis did not stratify results by hormone receptor subtype—estrogen receptor, progesterone receptor, or HER2 status—which is a critical variable for understanding breast cancer risk and treatment response .

The Cleveland Clinic study: fewer patients progressed to stage IV

A separate real-world analysis from the Cleveland Clinic, presented as Abstract 3143 at the same ASCO meeting, focused on cancer progression rather than initial diagnosis. Researchers examined data from 12,112 patients with obesity-related cancers .

Across four cancer types—lung, breast, colorectal, and liver—patients taking GLP-1 drugs demonstrated a 38% to 50% lower likelihood of advancing to stage IV metastatic disease compared with patients taking DPP-4 inhibitors (gliptins), a different class of diabetes medication .

For breast cancer specifically, multiple reports indicate that metastasis occurred in roughly 10% of GLP-1 users versus 20% in the comparison group, and the risk reduction for breast cancer progression was around 43%, with a reported hazard ratio of 0.57 (95% CI 0.46 to 0.71) .

Mark David Orland, MD, of the Cleveland Clinic’s Taussig Cancer Institute, framed the results carefully: “Our study found that use of GLP-1 drugs, compared to DPP-4 inhibitors and other antidiabetic drugs, was associated with a meaningful reduction in cancer progression across four solid tumor types” .

The study also added a layer of biological plausibility: high GLP-1 receptor expression on tumors was associated with a 33% lower mortality risk across seven cancer types, suggesting that GLP-1 signaling might play a direct role in tumor behavior—something beyond weight loss alone .

Survival and recurrence: the evidence beyond ASCO

A separate large cohort study published in JAMA Network Open by Tatum et al. reinforces the mortality signal. Drawing on propensity score–matched data from 841,831 breast cancer patients, the analysis found that among patients with obesity, GLP-1 receptor agonist use was associated with a 64% lower hazard of all-cause mortality (HR 0.35; 95% CI 0.21–0.58; P < .001) and a 56% lower hazard of recurrence or death (HR 0.44; 95% CI 0.30–0.64; P < .001) over a 10-year follow-up period .

Additional research presented at the San Antonio Breast Cancer Symposium (SABCS) pointed in the same direction. BreastCancer.org reported that one SABCS study found a 46% lower risk of death from any cause among women taking GLP-1s over roughly 5.5 years of follow-up, while another reported a 74% lower risk of invasive or metastatic cancer progression . A separate SABCS analysis showed that GLP-1 use was associated with lower ctDNA positivity rates—25.8% versus 31.6%—potentially hinting at reduced circulating tumor activity .

Why experts are being cautious

Despite the consistency of the signal across multiple studies, every research group and independent commentator has emphasized major limitations.

Observational design. All the studies are retrospective and cannot establish causality. The Penn Medicine researchers described the findings as an “association,” not proof that GLP-1 drugs prevent breast cancer . Confounding factors—for example, healthier lifestyle behaviors among people who obtain and tolerate GLP-1 medications—cannot be eliminated from these analyses.

No cancer-prevention indication. GLP-1 medications are not FDA-approved as cancer prevention drugs, and experts explicitly advise against starting them solely to lower cancer risk, particularly in people at a healthy weight .

Mixed evidence across studies. Not all research aligns perfectly. Some meta-analyses have found no significant reduction in postmenopausal breast cancer risk with GLP-1 use, and a systematic review reported that GLP-1 receptor agonists probably have little or no effect on breast cancer risk (OR 0.95; 95% CI 0.60–1.49) . A 2025 cohort study in adults with overweight or obesity found a non-significant HR of 0.86 for breast cancer (95% CI 0.71–1.03) . The evidence base varies by population, comparator drug, follow-up duration, and study methodology.

The need for randomized trials. Researchers across all three major presentations called for randomized controlled trials to confirm whether GLP-1 receptor agonists directly reduce breast cancer incidence, metastatic progression, and mortality . Mechanistic studies looking at GLP-1 receptor expression, anti-inflammatory pathways, and immune-modulating effects are also considered essential next steps .

Where the research stands now

The 2026 ASCO findings add meaningful weight to the hypothesis that GLP-1 drugs could have a role in modifying cancer risk and progression, but they are not a clinical green light. As Orland told ASCO attendees, “This is an association, not a cause. It doesn’t apply to all patients and all cancers. But the results are provocative and provide early evidence that future studies are worth pursuing” .

For now, patients and clinicians should read these results as encouraging but preliminary. The potential for GLP-1 medications to influence breast cancer outcomes is real enough to justify large prospective trials, but not yet strong enough to change clinical practice.