Association between Metformin Therapy and Breast Cancer Incidence and Mortality: Evidence from a Meta-Analysis

PURPOSE: Metformin may be associated with a decreased risk of breast cancer. We performed a meta-analysis to assess the effect of metformin intake on breast cancer risk and mortality. METHODS: We performed a PubMed and EMbase search for all available studies that described the risk of breast cancer and all-cause mortality in relation to the use of metformin among patients with type 2 diabetes mellitus. Pooled relative risks (RRs) were determined using a random effects model to assess the strength of association between metformin and the risk of breast cancer. RESULTS: Fifteen articles from PubMed satisfied the inclusion criteria, including a total of 838,333 participants. Compared with the control group, metformin use was not related to a reduced incidence of breast cancer (RR, 0.964; 95% confidence interval [CI], 0.761-1.221; p=0.761). However, metformin therapy was associated with decreased all-cause mortality (RR, 0.652; 95% CI, 0.488-0.873; p=0.004). No obvious publication bias was detected (incidence: p Begg=0.755, p Egger=0.008; mortality: p Begg=0.072, p Egger=0.185). CONCLUSION: The present study suggested that metformin therapy may decrease the all-cause mortality of patients affected by breast cancer. However, this finding should be considered carefully and confirmed with further studies.