5 New Findings About GLP-1s and Breast Cancer

GLP-1 medicines like Ozempic, Trulicity, Victoza, Mounjaro, Wegovy, and Zepbound have been called miracle drugs — not only for how effective they are for weight loss, but for the benefits they provide for people with diabetes, at risk of cardiovascular disease, and more.

Early evidence suggests that GLP-1s can also help people with breast cancer and obesity lose weight, which may improve their prognosis. This is because obesity is linked to worse outcomes in people with breast cancer, particularly in postmenopausal women. Obesity can promote the growth of breast cancer. And it can make hormonal therapies like aromatase inhibitors less effective.

At a poster spotlight session at the 2025 San Antonio Breast Cancer Symposium, researchers presented their recent findings on the effects of GLP-1s on people with breast cancer. Here are five highlights from the session.

Many people with hormone receptor-positive breast cancer take hormonal therapy to block estrogen from feeding cancer cells. But these medicines can cause weight gain. A small study suggests taking weight loss medicines can help people with hormone receptor-positive breast cancer who are taking hormonal therapy lose weight.

Researchers at Valley Health System in New Jersey looked at the medical charts of 38 people with hormone receptor-positive breast cancer taking both hormonal therapy and weight loss drugs. Hormonal therapies included Arimidex (anastrozole), tamoxifen, Aromasin (exemestane), and Femara (letrozole).

The researchers found that the drugs tended to cause weight loss for about nine months, but then weight loss plateaued. By six months, the participants had lost 12.7% of their body weight on average. By 12 months, they had lost an average of 17.4% of their body weight. This is comparable to the amount of weight lost by people who don’t have cancer who use these drugs. The amount of weight lost was the same for the participants taking GLP-1s or other weight loss drugs. Larger studies are needed to confirm the results.

Researchers also looked at how GLP-1s affect death in people with breast cancer and obesity who are on hormonal therapy.

Colton Jones, MD, the lead researcher and a hematology and medical oncology fellow at the University of Texas at San Antonio, and colleagues analyzed the health records of nearly 8,800 people with breast cancer and obesity taking both hormonal therapy and GLP-1s. They compared the health records from these people with an equal number of people with breast cancer and obesity who were not taking GLP-1s.

They found that those who took GLP-1s had a 46% lower risk of death from any cause during the follow-up period of about 5.5 years. These results are promising, but more research is also needed to find out why GLP-1s may reduce death from all causes in this group.

In a separate study, researchers compared the health records of more than 5,600 people with breast cancer who were getting chemotherapy and using GLP-1s with people with breast cancer who were getting chemo and not taking GLP-1s. 

They found that people taking GLP-1s were less likely to have anemia, blood clots in veins, low levels of white blood cells called neutrophils, low blood platelet count, sepsis, nausea and vomiting, fatigue, cardiomyopathy, and neuropathy after chemotherapy.

People with ductal carcinoma in situ (DCIS) are at higher risk of the cancer coming back if they also have obesity and/or diabetes. 

Studies in cells and animals suggest that GLP-1s could improve outcomes in this group. So researchers looked at the medical records of more than 3,000 people with hormone receptor-positive DCIS on hormonal therapy and GLP-1s, as well as an equal number of people with hormone receptor-positive DCIS who were not taking GLP-1s.

The rates of invasive recurrence and metastatic disease were high in both groups, but the researchers found that people using GLP-1s had a 74% lower risk of invasive or metastatic cancer progression. Also, 93.5% of the people who took GLP-1s survived during the five-year follow-up, versus 85.7% of those who didn’t take GLP-1s. This is the first data in people showing that GLP-1s may affect DCIS survivorship, said poster presenter Danish Safi, MD, a medical oncologist at West Virginia University.

Some people stop hormonal therapy for breast cancer early due to the side effects of the medicines. In the study led by Jones, GLP-1 use was linked to a higher risk of side effects from hormonal therapy. These side effects included hot flashes, joint pain, depression, osteoporosis, and endometrial cancer. 

While more research is needed, the study suggests that there may be some risks in addition to benefits of using GLP-1s in people with breast cancer and obesity who are taking hormonal therapy.

Research presented at SABCS makes it clear that GLP-1s may affect people with breast cancer in many ways. But it will take years before researchers get a full picture of these effects. “We are just starting to shed light on this,” said session moderator Jasmine Sukumar, MD, a medical breast oncologist at MD Anderson Cancer Center.