Metformin
Also called: Glucophage.
Last reviewed 2026-09-16. How this works.
Treating breast cancer Grade D
No human evidence of benefit, or none that survived testing.
A very large trial gave this diabetes drug to women with breast cancer and found it made no difference.
A trial randomised 3,649 patients and found nothing.
The claim
The claim is that metformin, a cheap and very widely used diabetes drug, treats cancer. It came from observing that people with diabetes taking metformin seemed to get less cancer.
What it does
Several mechanisms were proposed, and the observational signal was large and consistent across a great many studies. This entry is in the book because of what happened when that signal was tested.
The evidence
MA.32 randomly assigned 3,649 people without diabetes, with high-risk non-metastatic breast cancer, to metformin or placebo alongside standard therapy, double-blind, across four countries, enrolled from 2010 and followed to 2020.1
It found no effect on invasive disease-free survival and no effect on overall survival. The event rates in the hormone-receptor-positive group were 2.78 per 100 patient-years on metformin against 2.74 on placebo.1
A later analysis of the same trial records that this negative result is consistent with a large group of smaller randomised trials across other settings. The observational signal did not survive being tested, anywhere.3
The trial's lead investigator said so publicly: that the results tell us metformin is not effective against the most common types of breast cancer, and that off-label use for those types should be stopped. The same statement notes that the negative result underscores the need for randomised trials before observational findings are put into practice.2
One thing survives, narrowly. Exploratory analyses suggested benefit in HER2-positive disease, specifically in carriers of a particular genetic variant, and the investigators state that these require replication. That is a hypothesis, not a finding, and the public statement above explicitly does not cover HER2-positive disease.12
The studies this rests on
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MA.32, metformin versus placebo in high-risk non-metastatic breast cancer (NCT01101438)1
A randomised trial, double blind, phase 3, measured survival, 3649 participants.
no effect on invasive disease-free survival and no effect on overall survival.
What is absent
The one surviving signal is an exploratory finding in a genetic subgroup, and the investigators themselves state it requires replication. Nobody has replicated it.
Scope: the HER2-positive genetic subgroup.
Established by trial scope, checked 2026-09-16. Result: the finding is an exploratory subgroup analysis within a single trial, flagged by its own investigators as requiring replication.1
What to ask
Is there any reason to take metformin for my cancer if I do not have diabetes?
The dish-to-person gap
The register holds no human pharmacokinetic measurement for metformin against a laboratory concentration. This entry rests on a large randomised trial rather than on a concentration argument.
When to call your team — no warning signs are recorded
Whatever you are taking or thinking of taking, the people treating you can only weigh it if they know about it. That is true when nothing has been written down here, and it is most true then.
The register records no warning signs for metformin. That is not a statement that nothing can go wrong, and anyone taking a prescription drug outside its licence should tell their oncology team.
Interactions
The register holds no measured interaction between metformin and a cancer treatment class.
Nobody having looked is not the same as nothing being there. That is what this line says and all it says.
The whole interaction list, and how to turn it into a pharmacy review request.
Find a trial instead
The open question after MA.32 is a narrow one about a genetic subgroup, and a trial is where it would be answered.
Search ClinicalTrials.gov for recruiting studies of metformin
A trial is the same decision with monitoring, a known preparation and a doctor who knows what else you are taking.
References
Goodwin PJ, Chen BE, Gelmon KA, et al. Effect of metformin vs placebo on invasive disease-free survival in patients with breast cancer: the MA.32 randomized clinical trial. JAMA 2022;327(20):1963-1973.
doi:10.1001/jama.2022.6147 · NCT01101438
How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 14 note 1. The author string is abbreviated in the source that was read and has not been expanded.
Statement by Pamela J. Goodwin issued by the Canadian Cancer Trials Group when the MA.32 results were presented at the 2021 San Antonio Breast Cancer Symposium, reproduced by the Breast International Group and in subsequent reporting.
No persistent identifier recorded.
How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 14 note 2.
Goodwin PJ, Ennis M, Chen BE, et al. Exploration of body mass index and circulating metabolic factors as predictors of metformin benefit in the Canadian Cancer Trials Group MA.32. JNCI Cancer Spectrum 2026;10(4):pkag045.
How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 14 note 3. The author string is abbreviated in the source that was read and has not been expanded.