New Study: Weight Loss Drug May Reduce Breast Cancer Tumor

July 31, 2025 / Editor

New Study: Weight Loss Drug May Reduce Breast Cancer Tumor Size

New evidence suggests that tirzepatide, a medication approved for type 2 diabetes and chronic weight management, may offer additional benefits beyond weight loss.

In a recent study presented at the ENDO 2025 Annual Meeting of the Endocrine Society, researchers reported that obese mice with breast cancer experienced both significant weight loss and tumor shrinkage after receiving tirzepatide.

The study was led by Amanda Kucinskas, a Ph.D. candidate at the University of Michigan, and is part of a growing body of research exploring the intersection between obesity, metabolic health, and cancer outcomes.

“Obesity is a significant risk factor for breast cancer,” Kucinskas explained. “And while it is very preliminary data, our studies in mice suggest that these new weight loss drugs may be a way to reduce obesity-associated breast cancer risk or improve outcomes.”

The study focused on 16 young mice with breast cancer. They were kept warm and well-fed to encourage weight gain and replicate obesity-associated conditions. Once obesity was established, the mice were divided into two groups:

One group received tirzepatide injections every other day for 16 weeks
The other group received a placebo

Throughout the study, tumor volume was measured twice weekly. Researchers found that mice receiving tirzepatide lost about 20% of their body weight, and their breast cancer tumors decreased in size. These mice also demonstrated reduced liver fat, which further supports the systemic benefits of the drug in improving metabolic health.

Tirzepatide is a dual-action medication that mimics two naturally occurring gut hormones:

GLP-1 (glucagon-like peptide-1): Regulates appetite and insulin secretion
GIP (glucose-dependent insulinotropic polypeptide): Enhances insulin response and influences fat metabolism

Together, these mechanisms help reduce appetite, promote weight loss, and regulate blood glucose. Tirzepatide is currently marketed under the brand names Mounjaro (for type 2 diabetes) and Zepbound (for chronic obesity).

While its primary uses are metabolic, this new study suggests it may also have oncologic implications, particularly for breast cancer influenced by obesity.

Why Obesity Matters in Breast Cancer

Obesity has long been identified as a key risk factor for breast cancer, especially in postmenopausal women. According to Dr. Sue Decotiis, a medical weight loss specialist based in New York City, several physiological mechanisms explain this connection:

Chronic inflammation: Obesity contributes to systemic inflammation, which can disrupt normal cellular growth and immune regulation.
Hormonal imbalance: Excess fat, particularly in the abdominal area, raises estrogen levels, which can drive the development and progression of estrogen-receptor-positive (ER+) breast cancer.
Insulin resistance: Obesity is often accompanied by elevated insulin levels, which may further stimulate estrogen production in adipose tissue.

“Even a modest reduction in fat can have an important anti-inflammatory effect,” said Dr. Decotiis. “In my clinical weight loss practice, I’ve observed that the most meaningful secondary benefits of GLP-1 medications occur in patients who experience a significant decrease in fat, not just weight.”

Potential Clinical Implications

Although these findings are based on preclinical animal studies, they open a promising avenue for future research into GLP-1/GIP receptor agonists as part of a multi-faceted approach to treating breast cancer in patients with obesity.

It’s important to note that while GLP-1 medications like tirzepatide, semaglutide (Ozempic/Wegovy), and others have shown substantial benefits in metabolic health, they also come with potential side effects, including:

Gastrointestinal symptoms (nausea, vomiting, diarrhea)
Less commonly, hair loss, skin discomfort
Rare but serious effects such as behavioral changes or vision issues

Tirzepatide, however, may be better tolerated in some patients, with fewer side effects and greater weight reduction than other medications in its class.

The scope of GLP-1 receptor agonists continues to expand. Ongoing research is investigating their potential to slow Alzheimer’s disease progression, improve obstructive sleep apnea, treat non-alcoholic fatty liver disease (NAFLD), reduce alcohol cravings.

“Hold on to your hats and buckle up,” said Dr. Decotiis. “As this category of drug grows, we’ll likely see an even wider range of applications and a deeper understanding of the mechanisms behind their effects.”

Conclusion

While these are early-stage results from a mouse model, they highlight an exciting possibility: that anti-obesity medications like tirzepatide may one day play a role in reducing breast cancer risk or improving cancer outcomes.

Further clinical studies in humans will be essential to determine the effectiveness, safety, and long-term implications of using these therapies in oncology settings. Until then, this research serves as a compelling reminder of the complex links between metabolic health and cancer risk, and how addressing one may benefit the other.