The human epidermal growth factor receptor 2 (HER2) is overexpressed in HER2-positive breast cancer. This confers characteristics associated with an overall poor prognosis, with early metastases to major visceral sites and a relative resistance to chemotherapy. Trastuzumab is a humanised monoclonal therapeutic antibody that specifically targets HER2 receptor overexpressing breast cancer cells, inhibiting their growth and proliferation, and inducing their regression. It is licensed for metastatic and early HER2-positive breast cancer, determined by slide-based testing techniques. Trastuzumab dosing regimens are aimed at increasing its efficacy, while minimising its potentially undesirable effects which include cardiotoxicity and the development of resistance.