Mounjaro boobs? Weight loss drug can shrink breast cancer tumors, says new study

Turns out that GLP-1 drugs aren’t just slimming waistlines — they’re shrinking tumors, too.

A new study reports that mice shed about 20% of their body weight and saw their breast cancer tumors decrease after getting injected with tirzepatide, which is sold as Mounjaro for diabetes and Zepbound for obesity.

“Obesity is a significant risk factor for breast cancer,” said study author Amanda Kucinskas, a Ph.D. candidate at the University of Michigan.

“And while it is very preliminary data, our studies in mice suggest that these new anti-obesity drugs may be a way to reduce obesity-associated breast cancer risk or improve outcomes.”

Tirzepatide is part of a class of GLP-1 drugs, like Ozempic and Wegovy, that reduce appetite by mimicking the GLP-1 hormone the body naturally produces after eating.

Tirzepatide also mimics the effects of GIP, a gut hormone that stimulates insulin release and influences fat storage and appetite.

For the new study, 16 young mice with breast cancer were fattened up and kept in a warm home to encourage obesity.

The mice were then given tirzepatide or a placebo every other day for 16 weeks.

The researchers measured their tumor volumes twice a week, finding that as the mice shed the pounds and the fat stored in their livers, their tumors shrank accordingly.

The findings were presented this month at ENDO 2025, the Endocrine Society’s annual meeting.

“While these are very preliminary results, they suggest that this new anti-obesity drug may also have a beneficial impact on breast cancer outcomes,” Kucinskas said.

Breast cancer is the most commonly diagnosed cancer in women worldwide, besides skin cancer.

One in eight American women will likely be diagnosed with it at some point.

There are several reasons why obesity is a major risk factor for breast cancer.

“Obesity causes widespread inflammation throughout the entire body, deregulating the balance of cell growth patterns, body defenses and the gut microbiome,” Dr. Sue Decotiis — an NYC-based medical weight loss doctor, who was not involved in the new study — told The Post.

“Even a modest reduction of fat could have an important anti-inflammatory effect.”

Excess abdominal fat can also lead to higher estrogen levels. Hormonal imbalances promote breast cancer development, Decotiis said.

Plus, obesity is often associated with higher-than-normal levels of insulin in the blood, which can stimulate the production of estrogen in fat tissue.

“In my clinical weight loss practice, all secondary benefits from GLP-1 drugs occurred in patients who lost not weight, but a significant reduction in fat,” Decotiis said.

Although GLP-1 meds have been hailed as “wonder drugs,” there are risks associated with them. Side effects can include gastrointestinal distress and, less often, hair loss and skin pain. Blindness and behavioral changes can occur in very rare cases.

Decotiis noted that tirzepatide is often better tolerated than the other meds because it can cause less nausea and vomiting while also spurring more weight loss.

Meanwhile, research continues on the potential for these drugs to slow the progression of Alzheimer’s disease, improve sleep apnea, reduce alcohol cravings and treat liver disease, among other conditions.

“Hold on to your hats and buckle up as this category of drug grows and we see more of their wide range of positive effects and learn more about their mechanisms,” Decotiis said.