Indicationused in her2-positive, metastatic breast cancer patients who have already used taxane and/or trastuzumab for metastatic disease or had their cancer recur within 6 months of adjuvant treatment.the fda label has two precautions. first that trastuzumab emtansine and trastuzumab cannot be interchanged. second that there is a black box warning of serious side effects such as hepatotoxicity, embryo-fetal toxicity, and cardiac toxicity.