This multicenter study in participants with HER2-positive eBC will investigate participants' pain and discomfort of SC trastuzumab (Herceptin) administered either via a single-use injection device (SID) or via vial for manual administration using a hand-held syringe (SC vial). In total, participants will obtain at least 18 cycles/1 year of trastuzumab (4 cycles of intravenous [IV] and 14 cycles of SC trastuzumab).