ERBB2 and breast carcinoma: Accordingly, a subpopulation analysis of the Women’s Health Initiative found that metformin use was related to lower risk of ER-positive tumors and fewer human epidermal growth factor receptor 2 (HER2) negative tumors [58]; similarly, a case-control study from 23 Spanish hospitals observed a protective effect of metformin towards ER-positive/HER2-negative breast cancer [62].