Making sense of associations between type 2 diabetes, metformin, and breast cancer risk.

The relationship between type 2 diabetes (T2D), metformin, and breast cancer is complex. T2D may increase risk, but metformin used as first-line treatment of T2D may decrease breast cancer risk. This comment explores efforts to disentangle effects of T2D and metformin use on breast cancer risk in a prospective study.