Penn Medicine study links GLP-1 drugs to lower breast cancer risk

A large Penn Medicine analysis found that women prescribed GLP-1 drugs were about 30 percent less likely to be diagnosed with breast cancer than women who were not, a signal that is promising but far from proof. The study reviewed electronic health records for 111,646 women ages 45 to 80 with a body mass index of 25 or higher, using data collected from January 2022 through June 2025.

Led by Elizabeth McDonald, MD, PhD, the researchers identified 15,264 women with documented GLP-1 prescriptions and 96,382 with no recorded exposure. They also built a matched cohort of 30,528 women, pairing patients one to one on age, race, ethnicity, BMI, breast density and diabetes status to reduce bias. Even with those controls, the work remained retrospective, so it can show an association but cannot prove the drugs caused the difference.

That caution matters because obesity is already known to raise breast cancer risk, especially after menopause. Excess body weight is believed to drive risk through inflammation, insulin resistance and higher estrogen levels, which makes weight loss a plausible part of the explanation. The Breast Cancer Research Foundation has cited a 2020 study in women over 50 showing that losing 10 or more pounds and keeping it off was associated with a 32 percent reduction in future breast cancer risk.

Penn presented the findings June 2 at the American Society of Clinical Oncology annual meeting as Abstract 10506 and published them the same day in JCO Oncology Practice. The study adds to a fast-growing body of research around GLP-1 medicines, which were first used for type 2 diabetes and are now widely used for weight management. Penn said the results set the stage for a multi-site clinical trial in women at high risk for breast cancer and in patients with a history of the disease.

The next phase of research will need to sort out whether the benefit comes mainly from weight loss, changes in hormone signaling and metabolism, or other factors such as screening behavior and closer medical follow-up. That question is especially important because a separate study presented at ASCO suggested GLP-1 receptor agonists may reduce metastatic progression in several obesity-related cancers, including breast cancer, and ASCO expert Marcin Chwistek said data this large and consistent warrant a prospective randomized trial. For now, the Penn result is a strong reason to study GLP-1 drugs further, not a basis to call them breast cancer prevention therapy.