Although the antidiabetes agent metformin has been associated with a reduction in breast cancer risk and may improve survival after cancer by means of tumor-suppressing mechanisms, a recent evaluation showed no relationship between improved survival and increased cumulative metformin duration in older women with breast cancer and recent-onset diabetes.

Metformin, a biguanide, is an oral drug widely used in the treatment of type 2 diabetes. It has been linked with as much as a 30% reduction in new cancers and a reduction in tumor growth among persons with breast cancer but without diabetes, wrote Iliana C. Lega, MD, a research fellow at Women’s College Research Institute in Toronto, Ontario, Canada, and coauthors of the new study, which appears in Diabetes Care

However, Lega and colleagues found no significant association between cumulative duration of past metformin use and all-cause mortality or breast-cancer-specific mortality in their evaluation of 2,361 women. The women, aged 66 years and older, had received diagnoses of diabetes and breast cancer between April 1997 and March 2008 (mean age at cancer diagnosis was 77.4 years; mean follow-up was 4.5 years).

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Of the 1,101 deaths that occurred (46.6%), 386 (16.3%) were breast-cancer-specific. Lega’s group found no significant statistical correlation between cumulative use of metformin and death from all causes nor a significant reduction in deaths due to breast cancer.

According to Lega, her group’s study was unique in that previous research had not examined the cumulative effects of metformin on patients, particularly in persons with breast cancer and diabetes. “This is important given that diabetic patients may switch drugs over the course of their treatment,” she explained in a statement from Women’s College Hospital.

Lega’s team concluded that more research is needed to clarify the relationship between metformin use and survival after breast cancer.