About two thirds of the New Zealand female population are either overweight (Body Mass Index, BMI 25 – 29) or obese (BMI 30 or more). The figures for women with breast cancer are similar. At diagnosis, 31.1% of New Zealand women are overweight and 34.9% are obese. According to the NZ Medical Journal, evidence is strengthening that obesity is not only associated with increased risk of developing breast cancer, but also with a poorer outcome once diagnosed.  You can read more about the complex interactions between obesity and breast cancer risk here.

GLP-1 weight loss medications are becoming increasingly popular worldwide as a way to manage body weight, in combination with eating well and regular physical activity. Over 65,000 New Zealanders received at least one dose of a GLP-1 weight loss injectable medication between July 2025 and May 2026 (HealthNZ data cited by NZ Herald).

GLP-1 medications such as semaglutide (Wegovy) and tirzepatide (Mounjaro) are available on prescription if you meet the criteria. You can also get them from some pharmacies, after talking with the pharmacist. They are not funded in Aotearoa New Zealand, and you will need to pay for them. People with a BMI of 30 or more are generally considered suitable candidates for these medications, although those with a BMI of 27 or more and a weight-related health condition such as high blood pressure or type 2 diabetes may also qualify. Read more about this here.

Researchers and clinicians at Breast Cancer Trials’ July 2026 meeting in Brisbane described recent data about the impacts of GLP-1 medication use on breast cancer as ‘very provocative’, with many questions about their use remaining.

All studies so far are retrospective or ‘observational’ i.e. based on data collected in the past. There have not yet been any trials set up specifically to assess the effects of GLP-1 medications on the risk of developing breast cancer or the risk of recurrence after treatment, or any impacts on the effectiveness of breast cancer therapies.

Obesity after menopause is a known risk factor for breast cancer (probably because excess fat tissue can produce oestrogen and also cause inflammation), so it would not be surprising if the weight loss brought about by GLP-1 medication use had some effect on that risk. GLP-1 medications themselves might also have direct effects on breast cancer tumour tissue.

Australian medical oncologist and researcher Dr Belinda Yeo summarised the latest research at the Breast Cancer Trials July 2026 meeting in Brisbane.

Prevention

UPenn study: Health records of 111,646 women aged 45 to 80 who were overweight or obese (BMI > 25) and had undergone breast imaging were examined by researchers at the University of Pennsylvania. They found that women who had received GLP-1 treatment were less likely to be diagnosed with breast cancer (1.65%)  than those who had not (2.6%). Click here to read this research abstract from the 2026 ASCO conference.

Roswell Park study: Data from TriNetX, a global web-based real-world data platform which allows researchers to access de-identified patient data from medical centres around the world, were used to evaluate 148,709 non-diabetic women with BMI 25 - 35. Over a 36-month median follow-up, those who had received GLP-1 medication were less likely to be diagnosed with HR+/HER2- breast cancer than those who had not (0.29% vs 0.33%). Overall survival was also better for those who had received GLP-1 medications. There were no differences for those diagnosed with other breast cancer subtypes. Click here to read this research abstract from the 2026 ASCO conference.

Metastatic progression

Cleveland Clinic study: Data from TriNetX were used to identify 10,225 patients with one of seven obesity-related cancers, including breast cancer, at stage I-III, who had also begun taking GLP-1 medication after diagnosis. They were ‘matched’ with a similar number of patients who had taken a different medication for diabetes. Comparisons showed that those taking GLP-1 had a reduced risk of progression to Stage IV in four of the cancers: breast (43% reduction in risk), lung (50%), liver (38%) and colorectal (31%).

Researchers also used the Cancer Genome Atlas to look at gene expression levels of GLP-1R in tumours. GLP-1R (glucagon-like peptide-1 receptor) is the receptor for GLP-1, a natural hormone which regulates digestion; it is also the target molecule for GLP-1 medications. They found that high GLP-1R expression levels in breast tumours correlated with improved overall survival, raising the possibility that GLP-1 medications could have anti-tumour effects in addition to weight-loss mediated effects. Click here to read this research abstract from the 2026 ASCO conference.

With systemic therapy

Italian study: TriNetX data were used to identify two matched groups of 604 patients each with metastatic breast cancer and receiving endocrine therapy plus a CDK 4/6 inhibitor. The only difference between the groups was that one received a GLP-1 medication as well as their cancer therapy and the other group did not. Median overall survival was 67.9 months for the GLP-1 group compared with 49 months for the other group, corresponding to a possible 30% reduction in the risk of death. The study demonstrated only an association and not a causative link, so these results should be treated with caution. However, they do suggest that further, more thorough investigations are warranted. Click here to read this research abstract from the 2026 ASCO conference.

Given the rapid global uptake of weight loss medications, there is clearly an urgent need for well-designed prospective clinical trials to establish their effects on breast cancer risks and outcomes.

You can read an excellent recent overview of this subject here.

8 October 2026