Can Mounjaro Cause Breast Cancer? What the Evidence Actually Shows

If you're taking Mounjaro, or thinking about starting it, you might have seen headlines linking weight loss drugs to breast cancer. It's a worrying discussion to come across, especially if you're already managing health concerns. We want to help you understand what the science actually says, so you can feel more informed about your treatment.

Article summary:

• There's no evidence that Mounjaro (tirzepatide) increases the risk of breast cancer. Early research points the other way, though it's too soon to draw firm conclusions.
• Obesity is a known risk factor for breast cancer, and losing weight has been linked to a lower risk in some studies.
• GLP-1 medicines like Mounjaro aren't licensed to prevent or treat cancer. Research into this is still ongoing.

Can Mounjaro cause breast cancer?

In short, none of the current evidence shows a link between Mounjaro and a higher risk of breast cancer. If anything, early research points in the opposite direction, towards a possible lower risk linked to weight loss achieved as a result of taking this medication. But it's important to be clear that Mounjaro isn't licensed to prevent or treat cancer of any kind, including breast cancer. [1] Research into this area is still at an early stage.

So why are people asking this question?

Obesity itself is a well-established risk factor for breast cancer. In the UK, around 1 in 7 women will develop breast cancer at some point in their life. [2] Roughly 8% of UK breast cancer cases are linked to being overweight or living with obesity. [2]

One reason for this is hormonal. Fat tissue produces oestrogen, so carrying more body fat means higher oestrogen levels circulating in your body. Since breast cancer is often sensitive to oestrogen, this is thought to be part of why extra body weight raises risk, particularly after menopause. [3]

Because Mounjaro is a weight loss medicine, and because obesity and breast cancer are linked, it's a reasonable question to ask whether the medicine itself changes that risk in either direction.

What the research on Mounjaro and cancer shows

Mounjaro contains tirzepatide, which is a GLP-1 medicine. GLP-1 stands for glucagon-like peptide-1, a natural gut hormone that Mounjaro copies to help you feel full and manage your blood sugar.

A 2025 review looked at data from 13 clinical trials involving nearly 13,800 people taking tirzepatide. [4] It compared people taking tirzepatide against those taking a placebo (a dummy treatment), insulin, or another GLP-1 medicine. Across the board, tirzepatide wasn't linked to a higher risk of any cancer, including breast cancer specifically. [4]

It's worth being honest about the limits of this evidence too. These trials followed people for between six months and a year and a half. They weren't designed specifically to study cancer, so larger, longer-term studies are still needed before anyone can say for certain how tirzepatide affects cancer risk over many years. [4]

Could losing weight actually lower the risk?

A large US study published in 2020 followed women aged 50 and over for around 10 years. [5] Women who lost 9kg or more, and kept it off, had a 32% lower risk of breast cancer compared with women whose weight stayed stable. This applied to women who weren't using hormone replacement therapy. [5] This study didn't involve Mounjaro. It looked at weight loss in general, from any cause.

More recently, a 2026 study looked at health records from over 100,000 women using GLP-1 medicines, including Mounjaro. [6] Women who'd been prescribed a GLP-1 medicine had roughly 30% fewer cases of breast cancer than women who hadn't, even after accounting for factors like age, weight and family history [6]

This is a promising early signal, but it's important to understand what this type of study can and can't tell you. It looked back at existing health records rather than following people forward in a controlled trial. It can show a pattern, but it can't prove that GLP-1 medicines directly caused the lower cancer rate. The researchers behind the study said the same thing themselves, calling for proper clinical trials to test this directly. [6]

Is there a link with inflammation?

Ongoing inflammation in the body is thought to play a role in several types of cancer, including breast cancer. Obesity is linked to this kind of low-level, long-term inflammation.

In July 2026, researchers presented early findings at the International Congress on Obesity showing that GLP-1 medicines reduced a marker of inflammation in the blood, called hsCRP, by around 45% compared with a placebo. [7] This is an interesting finding, but it's worth flagging that this was presented at a medical conference and hasn't yet gone through full peer review and publication in a medical journal, so it should be treated as early, preliminary evidence rather than an established fact.

Is Mounjaro licensed to prevent or treat cancer?

No. Mounjaro is licensed for weight management and as a treatment for type 2 diabetes. [1] It isn't approved by the MHRA (Medicines and Healthcare products Regulatory Agency) or any other regulator to prevent, treat, or reduce the risk of breast cancer or any other cancer.

That said, researchers are actively studying this question. One example is a clinical trial called TRIM-EBC, which is testing whether tirzepatide-supported weight loss can help lower the risk of breast cancer returning in people who've already been treated for it. [8] Trials like this take time, and results aren't expected for a few years yet.

Other things to know about Mounjaro

If you're taking Mounjaro or considering it, it helps to have the full picture of how it works and what to expect. Our complete guide to Mounjaro covers how the medicine works, who it's suitable for, and what results people typically see.

It's also worth understanding the common and more serious side effects of Mounjaro, and how to manage them if they come up.

If you're pregnant, breastfeeding, or planning a pregnancy, Mounjaro isn't recommended. You can read more about Mounjaro and pregnancy for detailed guidance on this.

For a closer look at Mounjaro's overall safety profile, head to our guide on whether Mounjaro is safe.

Weight management, breast awareness and when to get in touch

Mounjaro is one part of a wider approach to weight management. On the NHS, weight management usually combines lifestyle changes, such as changes to your diet and how active you are, with medicines like tirzepatide, and in some cases, weight loss surgery. [9] Your MedExpress Clinical team will help you get the balance right.

It's also important to be aware of your breast health, whatever your weight and whether or not you're taking Mounjaro. Get to know what's normal for you, and contact your GP straight away if you notice any new breast lumps, skin changes, or anything else that doesn't feel right. You don't need to wait for your next check-up. [10]

If you're aged 50 to 70, you'll usually be invited for routine mammography (a low-dose x-ray of the breast) every 3 years through the NHS Breast Screening Programme. [10]

Finally, if you notice digestive problems like nausea, diarrhoea, or stomach pain while taking Mounjaro, this is one of the most common and expected side effects of the medicine. [1] Still, do contact your Clinical team if these symptoms are severe, don't improve, or are worrying you. We're here to support you throughout your weight management journey.

In summary, there’s no evidence that Mounjaro raises your risk of breast cancer. Some early research even points towards a possible lower risk linked to weight loss, though this isn't proven yet and Mounjaro isn't licensed for this use. If you're worried about your personal risk of breast cancer, whether or not you're taking Mounjaro, the best next step is to speak with your GP as soon as possible.

At MedExpress, we offer online consultations with regular clinical check-ins, and a range of weight loss treatments including Mounjaro Pen, Wegovy Pen, Wegovy Pill and Foundayo. Our team can help you find the right treatment for you.

References

1. Mounjaro KwikPen solution for injection in pre-filled pen. Summary of Product Characteristics (SmPC). electronic medicines compendium (emc). Available from: https://www.medicines.org.uk/emc/product/15481/smpc
2. Cancer Research UK. Breast cancer risk. Health professional statistics [Internet]. Last reviewed 14 December 2023 / 17 April 2019. Available from: https://www.cancerresearchuk.org/health-professional/cancer-statistics/statistics-by-cancer-type/breast-cancer/risk-factors
3. Al-Shami K, Awadi S, Khamees A, Alsheikh AM, Al-Sharif S, Ala’ Bereshy R, et al. Estrogens and the risk of breast cancer: A narrative review of literature. Heliyon [Internet]. 2023 Sept 1 [cited 2026 Aug 26];9(9):e20224. Available from: https://www.sciencedirect.com/science/article/pii/S2405844023074327
4. Kamrul-Hasan ABM, Alam MS, Dutta D, Sasikanth T, Aalpona FTZ, Nagendra L. Tirzepatide and cancer risk in individuals with and without diabetes: a systematic review and meta-analysis. Endocrinol Metab (Seoul). 2025;40(1):112-124. https://pubmed.ncbi.nlm.nih.gov/39814031/
5. Teras LR, Patel AV, Wang M, et al. Sustained weight loss and risk of breast cancer in women 50 years and older: a pooled analysis of prospective data. J Natl Cancer Inst. 2020;112(9):929-937. https://pubmed.ncbi.nlm.nih.gov/31845728/
6. McDonald ES, Gillis LB, Gabriel PE, Xapakdy KM, Young AJ, Doucette AG, et al. GLP-1 Agonists Are Associated With a Significant Reduction in Breast Cancer Incidence in Women. JCO oncology practice [Internet]. 2026 July [cited 2026 Aug 25];22(7):1341–8. Available from: https://pubmed.ncbi.nlm.nih.gov/42227662/
7. Torres-Chavez MC, et al. GLP-1 receptor agonists and systemic inflammation: a systematic review and meta-analysis. Presented at the International Congress on Obesity (ICO 2026), Mexico City, 15-17 July 2026.
8. ClinicalTrials.gov. Tirzepatide weight loss for MRD+ early breast cancer (TRIM-EBC). NCT06517212. Available from: https://clinicaltrials.gov/study/NCT06517212
9. NHS. Obesity [Internet]. nhs.uk. Available from: https://www.nhs.uk/conditions/obesity/
10. NHS. Who breast screening is for [Internet]. nhs.uk. Available from: https://www.nhs.uk/tests-and-treatments/breast-screening-mammogram/who-breast-screening-is-for/