A 30% Risk Reduction Sounds Big. The Actual Numbers Tell a Different Story.
A new study says GLP-1 drugs lower breast cancer risk. The real difference? Less than 1 percentage point.
The headline says 30%.
A study presented yesterday at the American Society of Clinical Oncology annual meeting made headlines: women taking GLP-1 drugs like Ozempic and Wegovy had a “30% lower risk” of breast cancer.
Here’s what that actually means.
2.3% versus 1.6%.
Researchers at Penn Medicine reviewed health records from over 100,000 overweight women who had mammograms or other breast imaging. About 15,000 had been prescribed a GLP-1 drug.
Among women not on the drug, 2.3% were diagnosed with breast cancer. Among women on the drug, 1.6% were. That’s a difference of 0.7%.
That’s where the “30%” comes from. It’s the relative risk reduction — how much smaller one number is compared to the other. The absolute risk reduction — the actual change in how many women got cancer — is less than 1%.
Here’s the math: (2.3% - 1.6%) / (2.3%) * 100% = 30%
This study can’t prove the drug did anything.
Nobody was randomly assigned to take a GLP-1 or not. The researchers didn’t account for income, exercise, alcohol use, family history, genetic risk, or other medications. Any of those could explain the difference. The authors themselves call this a starting point, not proof.
Metformin was supposed to prevent cancer, too.
About ten years ago, observational studies suggested metformin, a common diabetes pill, cut cancer risk by 20% to 40%. The numbers looked a lot like what we’re seeing now with GLP-1 drugs. But when researchers ran better-designed studies, the benefit disappeared. The problem was how the studies were set up, not a real drug effect.
This GLP-1 study has the same weaknesses.
Social media isn’t waiting for a clinical trial.
The story is spreading faster than the data supports. In a sea of social media hype, Stanford oncologist Dr. Shruti Patel put it plainly: “NONE of this data suggests people who don’t have weight/endocrine issues should be taking GLP1s.”
What this means for Medicare and for you.
Medicare is about to start covering GLP-1 drugs for weight loss. A study like this will probably be used to argue for broader coverage. It shouldn’t be. Spending billions based on a question we haven’t answered yet is the kind of decision that should wait for a clinical trial. And a trial is being planned.
In the meantime: do not take a GLP-1 drug to prevent breast cancer based on this study. If you’re already on one for weight loss or diabetes, nothing changes. Focus on maintaining a healthy weight, exercise, a diet rich in whole foods and plants, moderating alcohol, and quitting smoking.