Women taking popular GLP-1 weight loss drugs such as Wegovy and Ozempic were found to be 30% less likely to develop breast cancer than those not on the medications, according to a study from the University of Pennsylvania Perelman School of Medicine.
The research, presented at the 2026 American Society of Clinical Oncology (ASCO) Annual Meeting and published in JCO Oncology Practice, examined the health records of more than 110,000 overweight American women aged 45 to 80 over a three-year period from 2022 to 2025. Of those women, 15,264 were initially taking a GLP-1 receptor agonist. The study did not account for the specific type of GLP-1 drug used, nor did it consider patients’ genetic history or the stage of cancer at diagnosis.
Dr. Elizabeth McDonald, a radiologist and professor at the university, stressed that the findings are observational and do not confirm a causal link. “While our study was observational and does not definitively confirm an association between GLP-1 medications and reduced breast cancer incidence, it does add to the growing body of evidence suggesting that it’s worth investigating these weight-loss drugs as potential cancer prevention tools,” she said in a statement.
The study builds on earlier work linking GLP-1 use to improved outcomes for several types of cancer, although past research has produced mixed results — some earlier studies found little effect on obesity-related cancers like breast cancer. A 2016 BMJ study on women with type 2 diabetes, for instance, found no overall increased risk of breast cancer with GLP-1 analogues, but did observe associations with two to three years of use.
Now, researchers are focusing on the mechanisms that could explain the apparent protective effect. Central to the picture is weight loss — and the way these drugs work in the body.
How GLP-1 drugs mimic hormones and drive weight loss
GLP-1 drugs work by mimicking a naturally occurring hormone called glucagon‑like peptide‑1, or GLP-1. This hormone plays a key role in regulating blood sugar and appetite. By stimulating GLP-1 receptors, the drugs help control cravings and reduce food intake, enabling users to lose significant amounts of weight — often dozens of pounds over several months.
The drugs were originally developed for the management of type 2 diabetes, but millions of people now take them primarily for weight loss. In the United States, obesity has become pervasive: more than 100 million Americans live with the condition, according to federal data, and women are disproportionately affected. Obesity rates are projected to rise dramatically by 2050, driven by sedentary lifestyles and ultraprocessed diets.
Obesity itself is a well-established risk factor for breast and other cancers. Excess body fat, particularly after menopause, leads to higher levels of oestrogen because fat cells produce the hormone. Elevated oestrogen is known to increase the risk of breast cancer. Additionally, obesity triggers low-grade chronic inflammation throughout the body — a condition researchers have long suspected creates a tumour-promoting environment.
GLP-1 drugs tackle both of these problems. By promoting substantial weight loss, they reduce the amount of fat tissue and lower oestrogen levels. At the same time, the drugs are known to reduce systemic inflammation through various biological pathways. It remains unclear, however, whether these effects are directly responsible for the reduced breast cancer incidence observed in the study, or whether additional mechanisms are at play.
The drugs also improve insulin sensitivity, which is significant because high levels of insulin and blood sugar can facilitate the multiplication of cancer cells. Better glucose control has been associated with lower cancer risk. Some research suggests GLP-1s may even enhance the immune system’s ability to detect and eliminate damaged cells before they become cancerous, through metabolic and epigenetic effects that inhibit tumour growth.
Broader implications and future research
The recent findings add to a growing body of evidence that GLP-1 drugs may have broader anti-cancer effects. A separate study presented at the ASCO meeting found that patients taking GLP-1 receptor agonists had a 38% to 50% lower risk of cancer spread (metastasis) across four solid tumour types — non-small cell lung cancer, breast cancer, colorectal cancer, and liver cancer — compared to those taking a different class of diabetes drugs called gliptins. Another study reported that GLP-1 use was associated with a meaningful reduction in cancer progression across the same four tumour types.
Researchers at the University of Pennsylvania are now working to set up a multi-site clinical trial to assess whether the drugs can lower breast cancer rates in women who are at higher risk and those with a history of the disease. Dr. McDonald noted that GLP-1 medications were not designed for cancer therapy, “but they do affect many different targets and pathways associated with cancer development, so we’re eager to study them in this context.”
Breast cancer remains a major health challenge. In the United States, it accounts for nearly a third of all cancer cases among women, and the American Cancer Society estimates about 42,140 women will die from the disease this year. In the UK, breast cancer is the most common cancer in women, representing around 30% of new female cancer cases annually — approximately 59,000 new cases each year — and is the second most common cause of cancer death among women.
In the UK, GLP-1 receptor agonists are prescription-only medicines. Licensed options include semaglutide (marketed as Wegovy for chronic weight management and as Ozempic and Rybelsus for type 2 diabetes), tirzepatide (Mounjaro for diabetes and weight management), and liraglutide. Wegovy is specifically licensed by the Medicines and Healthcare products Regulatory Agency (MHRA) for weight loss in adults with obesity or overweight with weight-related health problems, while Ozempic is licensed exclusively for diabetes — using it for weight loss is considered off-label. NICE has recommended semaglutide (Wegovy) for use within specialist weight management services, subject to specific BMI criteria. The MHRA has also issued warnings about the potential misuse of these medicines and highlighted risks of severe gastrointestinal side effects, including nausea, vomiting, diarrhoea, and constipation, which can lead to dehydration and hospitalisation. Less common but serious side effects include pancreatitis, gallstone disease, and severe allergic reactions.
Dr. McDonald expressed cautious optimism about the potential for these drugs in cancer prevention. “Ultimately, we want to find better options to prevent breast cancer. It’s been encouraging to see the survival rates for breast cancer improve over recent decades, and we’d love to see the same gains in prevention.”