Do GLP-1 Medications Lower Cancer Risk? What the Penn Study Actually Found

I prescribe tirzepatide. I want to say that in the first paragraph, before anything else, because what follows is a piece about research suggesting these medications might lower cancer risk, and you should know from the outset that I have a commercial interest in you finding that interesting.

Here is what I am not going to tell you: that our weight loss program prevents cancer. It doesn’t. Nobody has shown that, and the studies I’m about to walk through do not show it either. What they show is more interesting than that, and more limited.

What did the Penn study actually find?

In June, researchers from the University of Pennsylvania presented findings at the American Society of Clinical Oncology annual meeting in Chicago, published the same day in JCO Oncology Practice. The study looked at 111,646 women between 45 and 80 with a BMI of 25 or above, all drawn from Penn Medicine’s own electronic health records between January 2022 and June 2025. Of those women, 15,264 had a documented GLP-1 prescription. The other 96,382 did not.

Women taking GLP-1 medications were diagnosed with breast cancer less often. In the full group, the odds were 35.1 percent lower. The researchers then built a tighter comparison — 30,528 women, each GLP-1 patient matched one-to-one against a control of similar age, race, ethnicity, BMI, breast density and diabetes status. In that matched cohort the effect held at 30.5 percent lower odds.

That is a large signal in a large population, and it came out of a Philadelphia health system. The lead author, Dr. Elizabeth McDonald, is a breast radiologist at Penn’s Abramson Cancer Center. If you get your mammograms in this city, there is a reasonable chance your records were part of the denominator.

Does this mean GLP-1s prevent breast cancer?

No. And the researchers are the ones saying so most clearly.

McDonald’s own framing was that the study is observational and cannot establish that the medications are the cause — that it adds to a body of evidence suggesting these drugs are worth investigating as prevention tools. That is a careful sentence, and the care in it is the whole point.

Observational means nobody was randomly assigned to anything. The researchers looked backward at who happened to be taking a GLP-1 and who happened to develop breast cancer. Women who get prescribed and stay on these medications differ from women who don’t, in ways that are hard to fully adjust for — they are engaged with the healthcare system, they show up for appointments, they are being screened. The matched cohort controls for a lot of that. It cannot control for all of it.

There are specific gaps the authors name themselves. The study did not account for which GLP-1 medication was used, or for how long. It did not account for genetic risk factors. It did not account for cancer stage or type at diagnosis. Those are not small omissions, and further analyses are planned to address them.

Fact vs. Fad: “Thirty percent lower breast cancer risk” is going to end up on a lot of medspa websites this year, stripped of every qualifier above. When you see it, ask whether the page mentions the word observational. If it doesn’t, the page is selling.

What about other cancers?

A separate group presented data at the same meeting on cancers that had already been diagnosed. Using the TriNetX database, researchers compared 12,112 patients with stage I, II or III obesity-related cancers who started either a GLP-1 medication or a DPP-4 inhibitor after diagnosis, and looked at who went on to develop stage IV disease.

They found reduced metastatic progression in four cancers: lung, breast, colorectal and liver. They also found that tumors expressing more GLP-1 receptors were associated with better overall survival — which is the part I find most scientifically intriguing, because it hints at something happening at the tumor itself rather than only downstream of weight loss.

Same caveat: retrospective, real-world, hypothesis-generating. Not a trial.

Why would a weight-loss drug affect cancer at all?

The link between excess weight and cancer risk is not new or controversial. Being overweight or obese after menopause is an established breast cancer risk factor, and maintaining a healthy weight has been part of prevention guidance for decades. A medication that produces substantial, sustained weight loss would be expected to move that risk somewhat, simply by acting on the underlying exposure.

But weight loss may not be the whole mechanism. GLP-1 medications reduce systemic inflammation through several pathways, and chronic low-grade inflammation has long been suspected in breast cancer development. They also have metabolic and epigenetic effects that could plausibly inhibit tumor growth. The Penn researchers’ working hypothesis is that these effects act together rather than any one of them explaining the result.

The honest position is that we have a plausible mechanism and a correlation, and the gap between those two and a proven causal benefit is exactly the gap a randomized trial exists to close.

Is anyone running that trial?

Yes — and this is the part of the story I’d actually watch. McDonald and collaborators are working to stand up a multi-site clinical trial to test whether GLP-1 medications reduce breast cancer incidence in high-risk women, including women with a personal history of breast cancer.

That matters because the current prevention toolkit is thin. Tamoxifen genuinely reduces incidence in high-risk patients, but uptake among eligible women is poor because of its side effect profile. Risk-reducing mastectomy is appropriate for some people carrying high-risk genetic mutations and is, by any measure, a serious intervention. Beyond screening, there isn’t much in between.

Against that backdrop, a medication that millions of Americans already tolerate well is worth studying seriously. That is a very different claim from saying it works.

Should you start a GLP-1 to reduce your cancer risk?

No. I would not prescribe one on that basis and I’d be skeptical of anyone who would.

If you are carrying excess weight with metabolic consequences, and a GLP-1 is clinically appropriate, then these findings are a reason for cautious optimism about a possible additional benefit. They are not the reason to start.

I’d also point out something specific to how we practice here. We use tirzepatide. The Penn study did not separate results by drug — it grouped GLP-1 prescriptions together and explicitly did not account for medication type. Tirzepatide is a dual GIP/GLP-1 receptor agonist, mechanistically distinct from semaglutide. I cannot tell you that the Penn finding extends to what I prescribe, because the study was not designed to answer that question. Anyone claiming otherwise is going beyond the data.

What we’re doing with this

Nothing, for now. Our approach to medical weight loss hasn’t changed on the basis of these abstracts, and it shouldn’t — retrospective data is where hypotheses come from, not where practice changes come from.

What has changed is what I say when a patient asks. Six months ago the honest answer to “does this affect my cancer risk?” was that we didn’t really know. Today the answer is that there’s a growing observational signal pointing in a favorable direction, from good groups including one twenty minutes from this office, and a trial being organized to test it properly. That is genuine progress. It is also still an open question, and I’d rather tell you it’s open than tell you what you’d like to hear.

If you’re on a GLP-1 already, none of this changes your screening. Keep your mammograms — and if nobody is currently keeping track of those intervals for you, that is one of the things concierge medicine is for.

References

McDonald ES, et al. Association of GLP-1 agonists with breast cancer incidence in women. Presented at the 2026 ASCO Annual Meeting, Abstract 10506; published in JCO Oncology Practice, 2 June 2026. doi:10.1200/OP-26-00485 (conference abstract with same-day journal publication)

GLP-1 receptor agonists and metastatic progression in obesity-related cancers. Presented at the 2026 ASCO Annual Meeting, Abstract 3143. (conference abstract)