GLP-1s and Breast Cancer: Weight Loss, Safety and Timing Explained

For many individuals diagnosed with breast cancer, the journey involves more than just treating a tumor; it requires managing complex changes in metabolic health. Obesity at the time of diagnosis is linked to higher rates of disease recurrence and mortality. Paradoxically, standard treatments like chemotherapy and endocrine therapy, such as aromatase inhibitors or tamoxifen, often trigger significant weight gain, with 50% to 90% of patients experiencing an increase in body mass.

The emergence of GLP-1 receptor agonists — including Wegovy (semaglutide) and Zepbound (tirzepatide) — has offered a new tool for weight management. However, experts who recently spoke with CURE, including Dr. Maryam Lustberg, Professor of Medical Oncology at Yale University Comprehensive Cancer Center, and Dr. Neil Iyengar, director of Survivorship Services at Winship Cancer Institute of Emory University, a clinical member of Winship’s Glenn Family Breast Center, an associate professor and co-director of the Breast Medical Oncology Program in the Department of Hematology and Medical Oncology at Emory University School of Medicine, emphasize that while these drugs are promising, their use in breast cancer patients requires careful timing and holistic oversight.

A "no-blame" approach to metabolic health

Weight gain after a breast cancer diagnosis is often characterized by an increase in fat mass, while muscle mass remains the same or declines, a condition known as sarcopenic obesity. Lustberg emphasized that this shift is a biological side effect of treatment, not a lack of willpower.

"I really want to make sure that the article emphasizes that this is in no way patient's fault," Lustberg said. "I think this is not about body shaming. ... I have patients who were in optimal health before they got breast cancer, and our breast cancer treatments are the things that led them to note changes in body composition." She views GLP-1s as "one set of tools" that must be used as part of a "big picture of holistic health."

Timing is everything: The risks of active treatment

A critical consideration for patients is the timing of GLP-1 therapy. Iyengar advised extreme caution regarding the use of these drugs during active cancer therapy, such as chemotherapy or radiation.

One primary concern is the loss of lean muscle mass. "Lean mass, or muscle mass, is incredibly important for insulin metabolism and effective energy metabolism, particularly in people who are undergoing therapies that disrupt insulin signaling," Iyengar explained.

Furthermore, preliminary research suggests these drugs might alter the tumor's environment. "There's been some recent, very preliminary data to suggest that the GLP-1 receptor agonists might change the immune cells surrounding a tumor, and this could impact how effective immune therapy is for cancer treatment," Iyengar said. Because of these unknowns, his current recommendation is avoiding starting a GLP-1 receptor agonist for a patient who's on active cancer therapy.

Managing expectations: The impact of endocrine therapy

Patients should be aware that weight loss results may be more modest in breast cancer survivors than in the general population. While the general public may see weight reductions of 15% to 20% on these medications, retrospective studies in breast cancer patients show a mean weight loss of roughly 2% to 5%.

Lustberg noted that this "blunted" effect is likely due to the interaction with endocrine therapies. "It is biologically plausible that things may be more modest," she noted, adding that endocrine therapy and early menopause are inherently associated with weight gain.

Iyengar added that "something about the interaction between the GLP-1 and the hormone therapy is making the GLP-1 receptor agonist less effective for weight loss," noting that increasing the dose to compensate is often limited by side effects like nausea.

Potential benefits and side effects

Beyond weight loss, GLP-1s may offer a "two-for-one" benefit by protecting heart health, which is the leading cause of non-cancer-related death in breast cancer survivors. Some preclinical studies even suggest that GLP-1s could inhibit the growth of certain breast cancer cells by inhibiting "pro-oncogenic" pathways like NF-κB.

However, the side effect profile requires medical oversight. Common issues include gastrointestinal distress, but there is also the risk of losing vital muscle. Lustberg stressed that GLP-1 use "has to be coupled with adequate protein intake [and] resistance training" to mitigate the risk of sarcopenia.

The path forward: The "dream endpoint"

The ultimate goal for oncology researchers is to determine if these medications can improve survival. "The dream endpoint would be that we actually see a reduction in recurrences," Lustberg said. "If we are reducing body weights and reducing the fat composition, could we actually reduce breast cancer recurrences in some of our most high-risk cancers?"

As research continues, GLP-1s may become a standard part of survivorship, provided they are used with proper medical guidance and a focus on maintaining muscle mass.

References

1. “GLP-1 receptor agonists and breast cancer: metabolic insights and clinical implications” by Dr. Neil M. Iyengar, presented at 43rd Miami Breast Cancer Conference; March 5-8, 2026.
2. “GLP-1s for Patients With Breast Cancer: 5 Crucial Things to Know,” CURE.
3. “The Impact and Safety of GLP-1 Agents and Breast Cancer” by Dr. Maryam Lustberg et al., Cancer Medicine.

For more news on cancer updates, research and education, don’t forget to subscribe to CURE®’s newsletters here.