RCT: The addition of Metformin to standard therapy did not improve outcomes in patients with breast cancer.

Effect of Metformin vs Placebo on Invasive Disease–Free Survival in Patients With Breast Cancer: The MA.32 Randomized Clinical Trial – JAMA (free for a limited period)

Editorial: Who Will Benefit From Metformin? – JAMA Oncology

News Release: Common diabetes drug not effective against early-stage breast cancer, landmark trial reveals – Lunenfeld-Tanenbaum Research Institute