A new study has found that a drug used for treating obesity and type 2 diabetes may also help slow the growth of breast cancer caused by obesity.
The medication, tirzepatide, is sold under the name Mounjaro for diabetes and Zepbound for obesity. Researchers shared their findings at ENDO 2025, a major medical conference held by the Endocrine Society in San Francisco.
The study was led by Amanda Kucinskas, a Ph.D. student at the University of Michigan. She and her team wanted to explore whether weight loss from this new drug could also reduce the growth of breast cancer in people with obesity.
“Obesity is a big risk factor for breast cancer,” Kucinskas said. “Our results from mice suggest these new weight-loss drugs could help lower the risk or lead to better outcomes for those with obesity-related breast cancer.”
Obesity is a major health concern around the world. It can increase the risk of many chronic diseases, including heart disease, diabetes, and several types of cancer. For women, obesity is known to raise the risk of breast cancer, especially after menopause. It can also make breast cancer harder to treat and lead to worse results.
Losing weight has been shown to improve health and cancer outcomes. But many people struggle to lose weight and keep it off using traditional methods like diet and exercise. That’s why scientists are studying new medications like tirzepatide, which help people lose weight by acting on certain hormones in the body.
Tirzepatide works by targeting two hormones that control appetite and blood sugar—GLP-1 and GIP. These hormones help people feel full and reduce hunger. The drug has already helped many people lose a significant amount of weight.
In this study, the researchers tested the drug in a group of 16 female mice. The mice were given a high-fat diet—40% of their calories came from fat—and kept in warm conditions to encourage weight gain.
By the time they were 32 weeks old, the mice had developed obesity. Then, the researchers gave half of the mice tirzepatide injections every other day for 16 weeks. The other half received a placebo, which had no effect. The researchers measured the size of tumors in all the mice twice a week.
At the end of the study, the mice that received tirzepatide lost about 20% of their body weight and body fat. This is similar to the weight loss seen in women who take the drug. The weight loss was mostly due to a drop in fat tissue. Importantly, the mice that received the drug also had smaller breast tumors than the mice that got the placebo.
The scientists found that the size of the tumors was closely linked to how much fat the mice had. Mice with more body fat and more fat stored in their liver had larger tumors. This suggests that reducing fat in the body may help slow cancer growth.
“These are early results, but they’re exciting,” Kucinskas said. “They show that a drug meant for weight loss might also help treat breast cancer.”
More research is underway. The team is now working with another research group at the University of North Carolina to find out whether the drug itself is affecting tumors or if the results are only due to weight loss. Understanding this could help doctors know how to best use tirzepatide in the future.
This study adds to the growing evidence that obesity and cancer are deeply connected, and that treating obesity may have far-reaching benefits—even in cancer care.
If you care about breast cancer, please read studies about a major cause of deadly breast cancer, and this daily vitamin is critical to cancer prevention.
For more information about cancer, please see recent studies that new cancer treatment could reawaken the immune system, and results showing vitamin D can cut cancer death risk.
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