Effect of Metformin on Endometrial Thickness in Postmenopausal Breast Cancer Patients Receiving Tamoxifen

Background: Adjuvant hormonal therapy with tamoxifen reduces recurrence rate, morbidity, and mortality. Tamoxifen increases endometrial thickness, resulting in relevant gynecological complications. Several studies revealed the benefits of co-administering metformin with tamoxifen to avoid endometrial hyperplasia, but the available clinical data are still limited. Objective: To evaluate the effect of metformin on endometrial thickness in a sample of postmenopausal Iraqi ER+ breast cancer patients on adjuvant tamoxifen therapy. Methods: A total of 63 postmenopausal breast cancer patients on adjuvant tamoxifen therapy were enrolled in a double-blinded randomized control trial (RCT). Patients were randomly allocated to two groups in a 1:1 ratio, with one group receiving metformin along with tamoxifen and the other group receiving a placebo along with tamoxifen for a 12-month duration. Baseline endometrial thickness was obtained using ultrasound, and follow-up visits were scheduled in 3, 6, 9, and 12-month periods to evaluate the ultrasound changes in endometrial thickness as well as related adverse events. Results: Over 12 months, the placebo group exhibited a significant ET increase (6.32mm), while the metformin group showed minimal change (4.13mm) (p