Background: Preclinical data suggest that metformin May reduce breast cancer incidence and improve cancer prognosis. However, the current evidence in observational studies is inconclusive. A systematic review and meta-analysis was conducted to assess the effect of metformin on the incidence of breast cancer and all-cause mortality in patients with type II diabetes (T2D). Methods: A literature search was performed on Medline, EMBASE, and the Cochrane library from inception to November 2016. Outcomes were incidence of breast cancer and all-cause mortality. Risk of bias and overall certainty of evidence was assessed using the Newcastle-Ottawa Scale and Grading of Recommendations Assessment, Development, and Evaluation (GRADE), respectively. Meta-analyses were performed using the most fully adjusted ORs or HRs and 95% confidence intervals (95% CI) as effect measures. Results: A total of 12 observational studies were included for breast cancer incidence and 11 studies for all-cause mortality. No significant association was found between metformin exposure and incidence of breast cancer (OR ¼ 0.93; 95% CI, 0.85–1.03; I2 ¼ 35%). A 45% risk reduction was observed for all-cause mortality (HR ¼ 0.55; 95% CI, 0.44–0.70; I2 ¼ 81%). Presence of publication bias is strongly suspected for both outcomes using Egger's funnel plots. Conclusions: The use of metformin May improve overall survival in patients with T2D and breast cancer. No effect of metformin on the incidence of breast cancer was observed. Interpretation of results is limited by the observational nature of the studies and resulting biases. Impact: Clinical trials are warranted to determine the role of metformin in breast cancer risk reduction and prognosis. Cancer Epidemiol Biomarkers Prev; 27(6); 627–35. 2018 AACR.
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