GLP-1 receptor agonists (GLP-1 RAs) are associated with reduced incidence of breast cancer among women with overweight or obesity, according to two observational studies.

One found that GLP-1 RA users had a roughly 30% lower incidence of breast cancer than was seen in matched nonusers. The smaller of the two, which followed more than 80,000 women at high risk for breast cancer, found a more modest 16% reduction.

Using GLP-1 drugs to prevent cancer is “an exciting new frontier that will likely revolutionize the field of oncology, but like any new exciting therapy we need [randomized] clinical trials to understand how to optimally” use them, said Neil M. Iyengar, MD, a breast medical oncologist who was not involved in either study.

The larger study was presented as an oral abstract at the 2026 ASCO Annual Meeting. The other study was presented as a poster at the same meeting.

These new data add to a rapidly growing body of retrospective evidence linking the popular weight-loss drugs with reduced cancer risk.

Weight-Loss Drugs Already Linked With Reduced Cancer Risk

Obesity is one of the most important modifiable risk factors for breast cancer, particularly after menopause, and there’s now a wave of oncology research involving GLP-1s, said Jennifer Ligibel, MD, who was also not involved in either study.

There are now “a few dozen, at least, publications looking at the relationship between the use of these drugs and primary cancer risk,” she told Medscape Medical News, including several GLP-1 RA abstracts presented at this year’s ASCO meeting alone.

Most studies have focused on patients with diabetes, among whom GLP-1 RA use appears to reduce the risk for breast and other cancers, said Ligibel, who is a breast oncologist at Dana-Farber Cancer Institute in Boston. It remains unclear whether these protective benefits translate to women without diabetes, she added.

Both new studies help address this uncertainty, as they examined women with overweight or obesity, rather than a population selected for diabetes.

GLP-1 Use Linked With Lower Breast Cancer Incidence in Screened Women

The larger study, led by Elizabeth S. McDonald, MD, PhD, a radiologist at the University of Pennsylvania in Philadelphia, was restricted to women undergoing breast cancer screening.

Ligibel praised the investigators for restricting the population to screened individuals because it overcomes a criticism of prior studies on the same topic: women who take GLP-1 RAs tend to be more engaged with the health system than nonusers, and may therefore be screened more often, potentially skewing cancer rates between groups.

Drawing on University of Pennsylvania Health System health records from January 2022 to June 2025, McDonald and colleagues identified 111,646 women aged 45-80 years with a BMI of ≥ 25 who had undergone breast cancer screening.

Within this total population, 15,264 (14%) women had been prescribed a GLP-1 RA. Among GLP-1 RA users, 1.62% developed breast cancer, compared with 2.31% of matched nonusers, an absolute difference of about 0.7 percentage points.

After propensity score matching on age, race, ethnicity, breast density, BMI, and diabetes, GLP-1 RA exposure was associated with 30% lower odds of breast cancer (P < .0001).

When asked about the lack of adjustment for family history data, McDonald acknowledged the limits of observational data.

“There will always be unmeasured confounders in observational data,” she told Medscape Medical News. “This is why a clinical trial is important to show causality.”

McDonald argued that, on the whole, imbalances between groups were more likely to weaken the apparent risk reduction than exaggerate it.

For one, the women taking GLP-1 RAs were actually the sicker group, she noted, with higher Charlson comorbidity scores and more heart, kidney, and liver disease. Because those users started out less healthy, the imbalance would hide a protective effect, not create one.

Similarly, if women counted as nonusers were losing weight through undetected means — obesity medications, bariatric surgery, or GLP-1 RAs obtained outside the health system — the resulting data would blur the difference between the groups, diluting any observed benefit.

McDonald also pointed to a newer, not-yet-published analysis that reinforced the association. In a time- and dose-dependent study restricted to longer follow-up, “we saw HRs [hazard ratios] that were actually even lower,” she said during the session’s question-and-answer period. 

A More Modest Reduction in High-Risk Women

The smaller ASCO study was conducted by senior author Saba Shaikh, MD, of UT Health San Antonio in San Antonio, and colleagues.

Using the TriNetX global database, the investigators focused on women at high risk for breast cancer, including those with a genetic predisposition, a family history, dense breasts, or high-risk breast lesions.

Among 80,480 such women with obesity, GLP-1 RA users had a 16% lower incidence of breast cancer than matched nonusers (HR, 0.84; 95% CI, 0.78-0.92; P < .001).

The study “was retrospective and as a result hypothesis-generating but not practice changing,” Shaikh told Medscape Medical News, adding that “a prospective study would be needed” before GLP-1 RAs could be considered a viable prevention strategy for breast cancer.

Whether the potential benefit comes from weight loss or a more direct effect, she said, is “an area of active investigation” that her data cannot address.

Weight Loss Likely Driving Risk Reduction, but Other Factors Could Be Involved

Outside experts shared differing perspectives on the mechanisms driving the apparent reduction in breast cancer risk among GLP-1 RA users.

Any protective effect is “most likely mediated primarily through weight loss,” said Iyengar, of Winship Cancer Institute of Emory University in Atlanta, who studies metabolism and cancer.

Bariatric surgery has long been tied to lower breast cancer risk in a dose-dependent fashion, he noted.

“The more weight loss, the greater reduction in breast cancer risk,” Iyengar told Medscape Medical News. By reducing fat mass, “the GLP-1 receptor agonists have global effects on multiple pathways linking obesity to cancer,” including those related to insulin resistance and chronic inflammation.

Ligibel agreed that weight loss is probably “a big piece of it,” pointing to preclinical work in mice, in which a very-low-calorie diet shrank tumors even more than a GLP-1 RA did.

Even so, she speculated that GLP-1 RAs may also reduce risk independently of weight loss, citing emerging evidence that the drugs directly reduce inflammation, including anecdotal reports of improvements in autoimmune conditions uncorrelated with weight loss.

McDonald suggested that endocrine pathways could be playing a role, noting that GLP-1 RAs can directly reduce estrogen production, particularly after menopause. In addition, they increase adiponectin, which dampens obesity-associated inflammation, another potential driver of breast cancer development.

Randomized Trial Needed to Change Practice

Both Ligibel and Iyengar emphasized that the growing body of observational data, however consistent, cannot prove causation, and neither one endorsed GLP-1 RAs for breast cancer prevention.

Iyengar said he would prescribe the medications for patients who already meet standard indications such as obesity or diabetes, expecting that the drugs would “also likely reduce cancer risk”; however, he “would not yet use these weight-loss therapies solely for cancer risk reduction.”

Asked whether GLP-1 RAs might someday be used like tamoxifen to prevent breast cancer, Ligibel was direct: “We are absolutely nowhere near that.”

McDonald, Shaikh, and Ligibel reported no relevant financial relationships. Iyengar disclosed consulting or advisory roles with Pfizer, Novartis, and AstraZeneca, among others, and institutional research support from the National Cancer Institute and others; none involve makers of GLP-1 RAs.