The benefits of adding metformin to tamoxifen to protect the endometrium-A randomized placebo-controlled trial.

Background: We investigated whether metformin prevents tamoxifen-induced endometrial changes and insulin resistance (IR) after a diagnosis of breast cancer.

Methods: This was a single-centre, randomized, double-blind, placebo-controlled, parallel group trial. Postmenopausal women with hormone receptor-positive breast cancer taking tamoxifen were randomly allocated to metformin 850 mg or identical placebo, twice daily, for 52 weeks.