What You Need to Know About GLP-1s and Breast Cancer Care 

Breast cancer researchers are taking a close look at these revolutionary medications

Medications called glucagon-like peptide-1 (GLP-1) receptor agonists are transforming the treatment of obesity, and the benefits go beyond weight loss. GLP-1 medications have also been approved to treat sleep apnea and metabolic dysfunction-associated steatohepatitis, a serious liver disease. They’re also being evaluated for many other conditions, including breast cancer.

While research into GLP-1 medications and breast cancer is in its early stages, there are indications that these drugs could be an important tool in reducing the risk of breast cancer recurrence in overweight and obese patients and thrivers. Read on to learn about the role of weight in breast cancer, how GLP-1 receptor agonists work, what BCRF researchers are discovering about them, and more.

The relationship between body weight and breast cancer risk

Breast cancer, specifically in post-menopausal women, is one of 13 types of cancer linked to being overweight or obese. (Overweight is defined as having a BMI of 25 to 29.9; obesity is defined as having a BMI of 30 or higher.) The good news is that research shows that losing weight or maintaining a healthy weight can significantly reduce a woman’s risk. For example, a 2020 study of women over 50, conducted by BCRF investigator Dr. Walter Willett, found that those who lost 10 or more pounds and kept it off had a 32 percent reduction in their future breast cancer risk.

Losing weight or maintaining a healthy weight is also critical for breast cancer patients and thrivers. Obese patients are more likely to experience complications from breast cancer surgery, radiation, and chemotherapy. Treatments such as chemotherapy and endocrine therapy are less effective in obese patients. Obesity also increases the risk of breast cancer recurrence and is associated with poorer survival.

How does excess body weight promote breast cancer? While experts don’t fully understand the mechanisms, it’s believed that both inflammation and increased estrogen play a role. Being overweight or obese leads to a build-up of visceral fat — a type of fat that wraps around the organs and increases the risk of conditions such as heart disease and diabetes — which triggers inflammation, creating an environment for cells to multiply uncontrollably. As a result, there are more chances that uncontrolled cell growth and cancer will occur.

In addition, inflammation can prevent the body from responding to insulin, the hormone that regulates blood sugar, leading to insulin resistance. The body reacts by producing more insulin, which can cause cells to multiply more rapidly than normal.

Being overweight or obese also speeds cell multiplication by increasing estrogen levels. Fat cells make estrogen, fueling the development and growth of estrogen receptor (ER)-positive breast cancer. ER-positive breast cancer is the most common type, making up about 80 percent of all breast cancers, and post-menopausal obese women have a significantly higher risk of developing it.

Finally, it’s possible that obesity weakens the body’s immune system. Preliminary research suggests that obesity suppresses certain immune cells, including natural killer cells and CD8 T cells, which contributes to tumor progression.

Why many patients with breast cancer and thrivers gain weight

Losing weight is never easy, but it can be exceptionally challenging for patients with breast cancer and thrivers, who are prone to weight gain. A study presented last year at the annual meeting of the Endocrine Society found that nearly 1 in 5 breast cancer thrivers experience weight gain of more than 10 percent of their body weight.

“We do not understand all of the reasons why women gain weight during breast cancer treatment, but it is probably multifactorial,” said BCRF researcher Dr. Carol Fabian, who has been investigating the effects of the GLP-1 receptor agonist tirzepatide on blood, imaging, and breast tissue biomarkers in women with obesity and other risk factors for breast cancer.

Potential causes of weight gain in breast cancer patients and thrivers include:

• Chemotherapy. Some chemotherapy drugs temporarily lower metabolism, making it harder to lose weight or maintain a healthy weight. They can also induce menopause in premenopausal women, which is associated with weight gain.
• Steroids. These medications are often given to women undergoing chemotherapy to reduce nausea and vomiting. Steroids can stimulate appetite, potentially leading to overeating and weight gain.
• Hormone therapy. This type of treatment is prescribed for hormone-positive breast cancer patients, typically for five to 10 years. Hormone therapy works by suppressing production of estrogen or estrogen and progesterone, and lower levels of these hormones can increase body fat.
• Lower activity levels. Treatment side effects and fatigue can make it difficult for patients and thrivers to be physically active.

Many of these causes cannot be modified, so losing weight and avoiding weight gain can be frustrating and exceptionally difficult for patients with breast cancer. Even if they do all the right things necessary to control their weight, they may not see results. Dr. Fabian noted that patients who have insulin resistance — and those who are already struggling with their weight — seem to have the most difficulty avoiding weight gain.

The effectiveness of GLP-1 agonists for weight loss sparked interest among many breast cancer researchers, who are investigating the role these medications might play in breast cancer care and prevention in women who are overweight or obese. Investigators are also interested in the role these medications play in reducing the risk of heart disease and diabetes in patients with breast cancer, as they are more susceptible to developing these conditions.

How GLP-1s cause weight loss

There are three GLP-1 agonists approved in the last few years for the treatment of obesity: tirzepatide (Zepbound), semaglutide (Wegovy), and liraglutide (Saxenda). These drugs aren’t technically new; they were previously prescribed under the brand names Mounjaro, Ozempic, and Victoza (respectively) for the treatment of Type 2 diabetes. Zepbound, Wegovy, and Saxenda are also approved for people who are overweight and have at least one weight-related condition (high blood pressure, high cholesterol, etc.) that could be improved with weight loss.

GLP-1 agonists support weight loss in several ways. They slow stomach emptying, which prolongs feelings of fullness, and they send signals to the brain that increase sensations of fullness. This helps patients eat less, which can lead to weight loss.

These medications also impact levels of insulin and the hormone glucagon in the body. GLP-1 agonists trigger the pancreas to release insulin, lowering the amount of glucose (sugar) in the blood. They also block the secretion of glucagon, which prevents more glucose from entering the bloodstream.

What the research on breast cancer and GLP-1s shows so far

Research exploring the potential effects of GLP-1 receptor agonists on patients with breast cancer and thrivers is in the early stages, so further investigation is necessary to confirm an impact on breast cancer prevention and outcomes. Results thus far have been mixed, and several studies — including one conducted by BCRF researcher Dr. Neil Iyengar and his team published in the journal Oncology — suggest that these medications may be less effective at causing weight loss in breast cancer thrivers compared to those who do not have cancer. This may be due to the cancer treatments themselves or issues with compliance, Dr. Fabian said.

Nonetheless, there are reasons to be optimistic about the roles GLP-1s may play in breast cancer prevention, treatment, and survivorship. At the 2025 San Antonio Breast Cancer Symposium (SABCS), Dr. Fabian reported progress on her study of the effects of 6-month treatment with tirzepatide on risk biomarkers for the development of breast cancer, including absolute fibroglandular volume (FGV), visceral fat, and others. Results so far are encouraging, showing a median relative change in BMI of -15 percent, visceral fat -19 percent, and FGV -22 percent, among other findings. Dr. Fabian said the results of this study will inform the design of a larger trial. 

Earlier this year, results of the largest study to date that analyzed patterns of GLP-1 use in breast cancer survival were published in Clinical Cancer Research. Researchers found that while there were no significant differences in disease-free survival between those who received GLP-1s and those who did not, there was a significant improvement in overall survival for patients on GLP-1s compared to the control group.

Results of a study led by Dr. Cleo Ryals of the University of North Carolina at Chapel Hill — also presented at the 2025 SABCS — indicated there may be a connection between the use of GLP-1 agonists and reduced circulating tumor activity, suggesting that these medications may have benefits for patients with breast cancer beyond weight loss.

And yet another GLP-1 investigation presented at SABCS showed this class of drugs may enhance the treatment of triple-negative breast cancer (TNBC), a type of the disease with few targeted therapies and poor prognosis. Laboratory models of TNBC that were treated with both radiation and a GLP-1 experienced significantly delayed tumor growth and improved overall survival compared to those treated with radiation alone.

On the horizon, GLP-1s offer a promising new pathway to reduce breast cancer risk, one that BCRF-investigator Dr. Steven Hursting is exploring in triple-negative and ER-positive breast cancers. And others are looking at their role in risk of recurrence and potentially as a mechanism to slow tumor growth.

GLP-1 medication safety concerns in patients with breast cancer

There is limited data on the safety of GLP-1 agonists in cancer patients, especially those who are undergoing treatment. Dr. Iyengar has stated that he does not recommend prescribing GLP-1s to women who have just been diagnosed with breast cancer or are undergoing chemotherapy, citing a lack of data as well as concerns that these drugs may worsen side effects of chemotherapy such as nausea.

For thrivers, however, studies suggest that GLP-1s do not pose any risk beyond those that have already been identified in the general population. These medications should always be avoided if a patient has:

• a personal or family history of medullary thyroid cancer or multiple endocrine neoplasia syndrome type 2
• a history of pancreatitis
• an allergic reaction or hypersensitivity to the drug

“It’s also probably best to avoid GLP-1 agonists if an individual has a history of gastroparesis or inflammatory bowel disease or is on chemotherapy,” Dr. Fabian said.