Daniel Flora, Medical Oncologist and Medical Director of Oncology Research at St. Elizabeth Healthcare, shared a post on LinkedIn:
“For oncologists/breast cancer specialists: Given the early signals, further prospective studies and trials are warranted to evaluate whether GLP-1 RAs might play a role in breast cancer prevention or as an adjunct in breast cancer survivors (especially those with obesity/metabolic risk).
An updated summary on weight-loss drugs and their role in breast cancer risk reduction. Here, Jasmin and I break down the data accumulated so far and describe how we are using these to support breast cancer survivors in the clinic.
Obesity and breast cancer are closely linked. The explosion of highly effective weight-loss drugs like semaglutide (Ozempic, Wegovy) and tirzepatide (Mounjaro, Zepbound) has created new hope and a few big questions for patients and clinicians.
Can these medications help breast cancer survivors by reversing harmful metabolic changes? Or could they influence tumor biology in ways we do not yet understand?
Here’s the short version: GLP-1 drugs are very effective for weight control. The safety data in cancer patients looks reassuring so far, but it is still incomplete. We need prospective trials to know their true impact.
Why This Matters
Since 1990, global obesity rates have more than doubled. In 2022, about 44% of women were overweight and 18% were obese. During that same time, breast cancer has remained the most common cancer among women worldwide, with roughly 2.2 million new cases each year.
Many women are overweight or obese at diagnosis, and we know that excess body fat predicts worse outcomes: higher recurrence rates, increased breast cancer–specific mortality, and higher overall mortality. Weight gain after diagnosis is also common, especially during chemotherapy. It often leads to ‘sarcopenic obesity,’ where muscle is lost and fat accumulates, which can reduce function and potentially affect long-term outcomes.
GLP-1 receptor agonists represent a real advance. For the first time in decades, we have medications that reliably produce and sustain meaningful weight loss. But because people with cancer were largely excluded from the studies that led to their approval, we do not yet know how they affect cancer survivors.
What These Drugs Actually Do
GLP-1 receptor agonists mimic a natural hormone that helps regulate blood sugar and appetite. They increase insulin when glucose levels are high, reduce glucagon, slow stomach emptying, and decrease hunger.
In clinical trials, semaglutide led to about 15% average weight loss, while tirzepatide, which also targets another hormone called GIP, achieved up to 21%. Other agents like liraglutide, exenatide, and oral formulations such as orforglipron produce smaller but still meaningful reductions.
What the Lab Data Show
Laboratory and animal studies have offered both encouraging and but also cautionary findings.
Several preclinical models show potential anti-tumor effects. GLP-1 drugs such as exendin-4 and liraglutide have been found to reduce cancer cell proliferation, increase apoptosis, and lower tumor volume. These effects appear to involve reduced NF-κB signaling and epigenetic changes that slow tumor growth.
Some research in aggressive triple-negative breast cancer (TNBC) cell lines has shown possible tumor-promoting effects, perhaps through oxidative stress and VEGF-related pathways. These findings occurred at higher-than-clinical doses and in specific laboratory conditions, so their real-world relevance is uncertain.
Adding to the discussion, a 2025 study from the University of Michigan, presented at the Endocrine Society’s annual meeting, found that obese mice treated with tirzepatide lost about 20% of their body weight and developed significantly smaller breast tumors than control mice. Tumor size closely tracked with body fat and liver fat, suggesting that metabolic improvements themselves may help slow tumor growth.
The takeaway is that preclinical data are not conclusive. Biology is complex, and these findings remind us not to over-interpret early signals, whether positive or negative.
What We Know from Human Studies
The real-world data are more reassuring.
A 15-year retrospective study of over 1.6 million people with type 2 diabetes found no increased risk of postmenopausal breast cancer among GLP-1 users. Meta-analyses and pooled data from randomized trials show the same.
Among survivors, the evidence is still limited but encouraging. Retrospective studies show modest weight loss, around 3% to 5% over 6 to 12 months, less than in the general population. Some researchers think endocrine therapy (tamoxifen or aromatase inhibitors) may blunt the effect.
A retrospective study from MD Anderson that included over 1,000 non-metastatic breast cancer patients treated with GLP-1 agents found no difference in disease-free survival compared to non-users, and a possible improvement in overall survival. As always, retrospective data cannot prove cause and effect, but it is a reassuring start.
Across large datasets, including a pooled analysis of about 46,000 GLP-1 users, there is still no signal of increased breast cancer risk.
Safety and Side Effects
GLP-1 receptor agonists are generally safe, with predictable side effects:
- Gastrointestinal symptoms such as nausea, vomiting, and diarrhea are common but usually improve with gradual dose titration.
- Pancreatitis has been a theoretical concern but has not been shown to increase meaningfully in large studies.
- Thyroid tumors were observed in rodents but not in humans. These drugs should not be used in people with a personal or family history of medullary thyroid carcinoma or MEN2 syndrome.
Theoretical tumor-promoting concerns from lab studies remain unproven and should be viewed in that context.
How We Use It in the Clinic
When a patient asks about GLP-1 drugs after breast cancer, we talk about the whole picture.
The benefits of weight loss are real: better metabolic health, lower inflammation, improved energy, and a likely reduction in recurrence risk. The unknowns are real too. We do not yet have long-term, cancer-specific data.
For now, we tell patients these medications can be considered when weight loss through lifestyle changes alone is not working. It is important to emphasize shared decision-making, realistic expectations, and close monitoring. And we always pair it with nutrition, resistance training, and strategies to preserve lean muscle.
What We Still Need
- Prospective, randomized trials in breast cancer survivors
- Mechanistic research to understand why some lab results show opposite effects
- More diversity in study populations by race, age, and socioeconomic status
- Long-term follow-up to track recurrence and survival outcomes
The Bottom Line
GLP-1 receptor agonists are transforming obesity care. For breast cancer survivors, they may also offer a chance to improve metabolic health, an often-overlooked piece of survivorship.
The data we have so far is cautiously optimistic. There is no sign of increased risk, and early evidence hints at possible benefit. But we still need rigorous, long-term studies before calling them completely safe for this population.
Until then, these medications should be used thoughtfully, balancing metabolic benefits against potential risks, and prioritizing shared decision-making and careful monitoring.”
More posts featuring Daniel Flora.