Researchers, whose findings were published in the Journal of Clinical Oncology, found that women with diabetes who took the medication had a 25% lower risk of developing breast cancer over more than a decade of follow-up.
“Metformin use in postmenopausal women with diabetes was associated with lower incidence of invasive breast cancer,” wrote lead researcher Rowan Chlebowski, at the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Centre.
“These results can inform future studies evaluating metformin use in breast cancer management and prevention.”
Metformin, also sold under the brand name Glucophage, has been on the market for many years and is generally considered safe, although five to 10% of patients experience side effects like nausea and bloating.
It is used by millions of type 2 diabetics every day to help control their blood sugar, and studies have shown it also shrinks lung and breast tumours in mice. Several reports show people taking it for diabetes appear to develop cancer less often.
“This is an area of great excitement,” said Pamela Goodwin, a breast cancer expert at Mount Sinai Hospital in Toronto, Canada.
“The evidence is coming together that metformin may actually have a clinically-relevant effect, but none of this is good enough to chance clinical practice just yet.”