Despite the wealth of results from observational and preclinical studies, the only solid data from clinical, prospective studies confirm metformin-related oncological benefits for only a small subset of patients with HER2-positive breast cancer and early-stage colorectal cancer; metformin-related detrimental/lack of effect for lung, breast, prostate cancer, and glioblastoma. This evidence concerns the gene ERBB2 and breast cancer.