ERBB2 and breast cancer: Analyses based on metformin use alone showed a heterogeneous effect of this drug over different tumor subtypes: While the number of years under metformin treatment was not associated with the risk of HER2 or TN tumors, a negative association with the risk of HR+/HER2− breast cancer was observed (ORper year 0.89; 95 % CI 0.81–0.99).