Beyond Weight-Loss Injections, Clues Emerge on GLP-1 Drugs and Breast Cancer Prevention

Breast cancer prevention has long relied on screening, risk assessment, and a small number of drug options; now, GLP-1 drugs originally used for diabetes and weight loss have unexpectedly appeared on this path. A team from the University of Pennsylvania presented a study at the 2026 American Society of Clinical Oncology annual meeting showing that women who used these drugs were about 30% less likely to be subsequently diagnosed with breast cancer than non-users.

The analysis covered 111,646 women aged 45 to 80, all of whom underwent breast imaging in the University of Pennsylvania Health System and had a body mass index of at least 25. The study period ran from January 2022 to June 2025. Among them, 15,264 had prescription records for GLP-1 drugs, while 96,382 had no records of related medication use. The team first examined the entire population, then created a matched cohort of 30,528 people, comparing users and non-users according to factors including age, race, ethnicity, BMI, breast density, and diabetes status.

The results pointed in the same direction in both analyses: in the overall population, the odds of developing breast cancer were 35.1% lower among GLP-1 drug users; in the matched analysis, the reduction was 30.5%. The findings have been published in JCO Oncology Practice. The Guardian’s report on the same ASCO study also noted that this was a retrospective analysis of electronic medical records from about 110,000 middle-aged and older women, presented by Elizabeth McDonald, a breast radiologist at the University of Pennsylvania and Abramson Cancer Center.

GLP-1 drugs mimic the body’s glucagon-like peptide-1 signal, affecting appetite, blood sugar, and metabolic regulation; semaglutide, used in Ozempic and Wegovy, and tirzepatide, used in Mounjaro and Zepbound, are among the widely used representative drugs in recent years. These drugs were not designed for cancer, but obesity, especially postmenopausal obesity, is already one of the risk factors for breast cancer, so weight loss may explain part of the association.

The issue is that this is not yet proof of causation. The study could not distinguish among different brands or molecules, and it did not include key information such as duration of medication use, genetic risk, tumor subtype, or cancer stage; electronic medical record studies may also be affected by differences in health care-seeking behavior, metabolic health, prescription access, and lifestyle. In other words, the roughly 30% reduction is an epidemiological signal worth pursuing, not equivalent to saying that “taking a weight-loss injection can prevent breast cancer.”

The biological hypotheses proposed by the researchers include weight loss, reduced chronic low-grade inflammation, improved metabolic environment, and epigenetic pathways that may affect gene regulation. However, these mechanisms still need to be connected through experimental and clinical data. McDonald’s team is planning a multicenter clinical trial intended to test whether GLP-1 drugs can truly reduce the risk of new breast cancer in women at high risk of breast cancer, including some with a prior history of breast cancer.

Other observational studies at the same ASCO annual meeting also linked GLP-1 drugs with lower cancer mortality or less progression to stage IV disease, but these results likewise cannot replace randomized clinical trials. For clinical practice and public health, the truly critical questions are not only whether the drugs have a protective effect, but also who might benefit, how long treatment would be needed, what the long-term side effects and costs would be, and whether they can be used alongside existing breast cancer prevention strategies without causing overtreatment.