Weight-loss jabs could cut breast cancer risk by nearly a third, major study suggests

Weight-loss jabs could significantly cut the risk of breast cancer – even among women at high risk of the disease, a landmark study suggests.

Obesity has been linked to at least 13 different types of cancer – including breast, bowel and pancreatic cancers.

While losing weight is known to reduce that risk, experts now believe blockbuster injections such as Ozempic, Wegovy and Mounjaro may offer additional protective effects beyond simply helping people shed pounds.

After analysing the health records of 94,827 women aged between 45 and 80, researchers found those taking the drugs were around 30 per cent less likely to develop breast cancer than those who had never used them.

A second study found adding weight-loss drugs to standard treatment for breast cancer reduced the risk of patients dying from the disease by almost a third.

Professor Elizabeth McDonald, the study's lead author, said: 'If the association proves causal, and GLP-1s prevent breast cancer, that is truly game-changing.'

However, experts stressed that more research is needed to establish cause and effect.

The study included women of screening age in the US who had a BMI of 25 or above – meaning they were clinically overweight or obese – and had undergone at least one breast screening.

To address potential confounding factors – such as age, race, diabetes status and breast density – women prescribed GLP-1 drugs were matched with non-users who had similar health profiles.

During the study, 2,314 women were diagnosed with breast cancer.

Overall, 15,107 women had used a GLP-1 drug prior to screening. Of these, around 1.7 per cent went on to develop breast cancer.

By contrast, 2.6 per cent of non-users were diagnosed with the disease.

The researchers concluded that even after accounting for known risk factors – including breast density, type 2 diabetes and obesity – GLP-1 drugs could help protect against breast cancer, and may even prevent it.

Presenting the findings at the American Society of Clinical Oncology conference in Chicago, Professor McDonald said: 'We know that breast cancer risk is reduced after weight-loss interventions such as bariatric surgery, so a reduction in risk from weight alone is biologically plausible.

'At the same time, there is growing scientific interest in whether GLP-1 receptor agonists may have more direct biological effects, such as reducing inflammation – but the evidence remains mixed.'

She added: 'If GLP-1s prevent breast cancer, that is truly game changing. However, establishing causality is essential so that women's healthcare is guided by high-quality evidence rather than observational associations alone.'

The team, from the University of Pennsylvania, are now calling on government agencies and cancer charities to come together to fund a large-scale clinical trial.

A separate study presented at ASCO found that patients who started using the jabs after being diagnosed with cancer appeared to slow the spread of the disease.

The effect was most noticeable in those with lung and liver cancers, although the drugs were also shown to slow the progression of breast and bowel tumours.

Experts at the conference cautioned that it is still unclear whether any potential benefits are simply due to weight loss – or whether the drugs themselves have additional, as yet unknown, anti-cancer effects.

Obesity is now overtaking smoking as the leading modifiable risk factor for several cancers.

It is also the only major behavioural risk factor that has been rising among younger adults over the past two decades, while more established risks such as smoking, alcohol, red meat consumption and physical inactivity have remained stable or declined in England.

Breast cancer is the most common cancer in women in the UK, with around 59,000 new cases each year.

In the US, it accounts for around one in three new cancer diagnoses among women, with about 322,000 expected to be diagnosed in 2026.