Trastuzumab is primarily used in the treatment of HER2-positive breast cancer. It works by targeting the HER2 protein on cancer cells, inhibiting their growth and promoting their destruction. This monoclonal antibody is often administered in combination with chemotherapy to enhance its effectiveness. It is also used in HER2-positive metastatic stomach or gastroesophageal junction cancer. Trastuzumab can be given intravenously or subcutaneously, depending on the patient's condition and treatment plan. Regular monitoring is essential to manage potential side effects, such as heart complications. Its targeted approach has significantly improved outcomes for patients with HER2-positive cancers.