Objective: This study was set out to determine whether metformin use influences survival in breast cancer
patients treated with antidiabetic drugs as compared to non-users.
Research Design and Methods: We used data from the Danish national registries (1996-2008) to identify adult female
patients diagnosed with breast cancer who were prescribed antidiabetic medication. We performed multivariate Coxproportional
hazard regression to assess all-cause and breast cancer-specific mortality risks associated with metformin
exposure. In a secondary analysis, we stratified use of metformin according to the cumulative number of prescriptions.
Results: Of the 1058 breast cancer patients 349 died during follow-up, with breast cancer listed as the primary cause of
death for 152 cases. Compared to non-use, current metformin treatment was associated with a significant reduction in
overall mortality (adjusted HR 0.74, 95% CI, 0.58-0.96). For breast cancer-specific mortality, a non-significant risk
reduction (adjusted HR 0.88, 95% CI, 0.59-1.29) was observed, which became significant after stratification according to
cumulative number of prescriptions. An increased risk of both overall and breast cancer-specific mortality was observed
in the first 12 months after discontinuation of metformin.
Conclusions: We observed a nonsignificant reduction in breast cancer-specific mortality associated with metformin
exposure among breast cancer patients treated with antidiabetic drugs. However, our findings suggest that long-term
metformin use may have a beneficial effect on survival in patients with breast cancer. Further confirmation of these
findings is needed.