Effects of Metformin on Breast Cancer Risk and Mortality in Type 2 Diabetes Mellitus: A Systematic Review and Meta-analysis

ABSTRACT

BACKGROUND: The protective effect of metformin against breast cancer is inconclusive. OBJECTIVE: To evaluate the effect of metformin on breast cancer risk and mortality in patients with type 2 diabetes. METHOD: A comprehensive literature search was performed for pertinent articles published prior to June 30, 2014, using PubMed and EMBASE. Study heterogeneity was estimated with I2 statistic. The data from the included studies were pooled and weighted by random-effects model. The quality of each included study was assessed on the basis of the 9-star Newcastle-Ottawa Scale and publication bias was evaluated by visual inspection of a funnel plot. RESULTS: Ten studies were included in the meta-analysis of the association of metformin and breast cancer risk. By synthesizing the data from the studies, the pooled odds ratio (OR) was 0.72 (95% CI 0.59, 0.87) (p = 0.0005). Three cohort studies were included for meta-analysis of the association between metformin and breast cancer-related mortality. Metformin was associated with a significant decrease in mortality (Risk ratio 0.68; 95% CI 0.51, 0.90, p = 0.007). CONCLUSION: The present meta-analysis suggests that metformin appears to be associated with a lower risk of breast cancer incidence and mortality in patients with type 2 diabetes.