Breast cancer is the most common cancer in women worldwide. One of the biggest factors that can raise the risk, but also be changed - is obesity. Doctors call this a “modifiable risk factor”, meaning it’s something you can influence through lifestyle changes or medical treatment.
Weight loss medications like Mounjaro are already transforming diabetes and obesity care. Now, early research suggests they may also play a role in breast cancer. But how strong is the evidence? Let’s take a look.
Key Takeaways
- Obesity increases the risk of both developing breast cancer and it coming back.
- A 2025 mouse study found Mounjaro (tirzepatide) slowed breast cancer growth linked to obesity.
- Human evidence is still pending. A trial called TRIM-EBC is testing whether tirzepatide-supported weight loss can reduce the risk of breast cancer returning.
- GLP-1 medicines are not licensed to treat or prevent cancer.
Breast cancer and obesity: What’s the connection?
About 1 in 7 women in the UK will develop breast cancer in their lifetime. Obesity is linked to higher oestrogen levels, insulin resistance, and inflammation, all of which can fuel breast cancer growth.
Studies show women with obesity are not only more likely to develop breast cancer after menopause, but also face higher recurrence rates and worse outcomes.
Because we know excess body fat can fuel breast cancer biology, scientists are exploring whether weight-loss medications like Wegovy and Mounjaro might reduce the risk.
Breast cancer and Mounjaro: New research
At ENDO 2025, the Endocrine Society’s annual scientific meeting in San Francisco, researchers presented early findings from a mouse study on tirzepatide (the active ingredient in Mounjaro).
In the study, mice with obesity-related breast cancer—developed by feeding them a high-fat diet and implanting breast cancer cells—were treated with tirzepatide. Their tumours grew more slowly than in mice that didn’t receive the GLP-1 treatment. These early findings suggest that tirzepatide might help slow the growth of obesity-linked breast cancer.
Treatment with tirzepatide led to about 20% weight loss, similar to what’s seen in women taking the medicine.
What are the limitations?
- The study was conducted in mice, and animal findings do not always translate to people.
- The mechanism is unclear: tumour slowing may be driven by fat loss and metabolic change rather than a direct drug effect. Follow-on work with Dr Steve Hursting’s lab (UNC Chapel Hill) aims to tease this apart.
- There are no completed human outcomes showing tirzepatide reduces breast-cancer risk or recurrence; trials such as TRIM-EBC are still in progress.
- Intentional weight loss is linked with lower risk of several obesity-related cancers, including postmenopausal breast cancer. Any future reduction in risk seen with tirzepatide could reflect the effects of losing body fat and improving metabolic health, rather than a drug-specific anticancer action.
Bottom line: this is early animal research; only well-designed human studies will show whether tirzepatide changes breast-cancer outcomes.
Will Mounjaro slow breast cancer growth in humans?
This study doesn’t necessarily mean that Mounjaro will slow the growth of breast cancer in humans. No completed human studies yet show that tirzepatide reduces breast cancer risk or recurrence.
But clinical trials looking into the link are underway. The TRIM-EBC trial at Baylor University Medical Center in Dallas is the first human study testing whether tirzepatide can lower breast cancer recurrence risk.
It’s enrolling people who have completed treatment for early breast cancer, live with overweight or obesity, and have detectable circulating tumour DNA (ctDNA)—tiny fragments of cancer DNA that can linger after treatment and predict recurrence.
Participants receive weekly tirzepatide and are followed for up to 24 months. If ctDNA levels drop or clear, and participants stay cancer-free, it might suggest tirzepatide could help reduce the risk of breast cancer returning.
Does Mounjaro prevent or treat breast cancer?
The idea that tirzepatide could slow breast cancer growth is exciting - but it’s very early-stage research.
If future human studies confirm these findings, medicines like Mounjaro might one day support cancer prevention in people with obesity.
For now, it remains a research question, not a clinical recommendation.
Currently, the best understood benefits of Mounjaro and other GLP-1 medicines are weight management and improved metabolic health, which can indirectly lower the risk of several diseases - including some cancers over time.
Should you take Mounjaro if you have breast cancer?
No. Tirzepatide is approved for type 2 diabetes and chronic weight management. Any cancer-specific benefit is still being studied. GLP-1 medicines are not licensed to treat or prevent breast cancer and won’t be prescribed if you have active cancer or are undergoing breast cancer treatment.
If you have a history of breast cancer and want help managing weight, speak with your oncology team and a clinician experienced in GLP-1 prescribing so your plan is safe and coordinated.
Can Mounjaro cause breast cancer?
Current evidence says no. In clinical trials lasting up to 72 weeks, overall cancer rates, including breast cancer, were no higher with tirzepatide than with comparison treatments. Large real-world studies of GLP-1 medicines also don’t show an increased breast-cancer risk; some even suggest neutral or lower risk as weight and metabolic health improve.
Evidence is still limited in people with a history of breast cancer (many trials excluded cancer survivors), so if you’re in remission and considering Mounjaro, decide with your oncology team.
Science-backed weight loss at Voy
At Voy, our focus is the safe, licensed use of GLP-1 medicines for weight management within a holistic, outcomes-led care model. Medication is just one tool. We pair it with nutrition guidance, resistance-training basics, sleep and stress support, and regular check-ins with expert clinicians so changes are sustainable. Improving weight and metabolic health can lower cardiovascular risk, reduce the likelihood of type 2 diabetes, and is associated with a lower risk of some cancers over time.
- We prescribe GLP-1s for weight management, not for cancer treatment.
- We do not prescribe if you have active cancer or are undergoing cancer treatment, for safety reasons.
- If you are in remission, we can consider Wegovy or Mounjaro once your oncology or specialist team confirms it is appropriate and you meet the usual eligibility criteria.
A Voy clinician will review your full medical history and medicines before any decision.
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