Trastuzumab can treat gastric cancer, and the specific use of the drug should strictly follow the doctor’s instructions. The indication of this drug for gastric cancer is that for patients with advanced gastric or gastroesophageal adenocarcinoma with HER2 overexpression (immunohistochemical staining of ++++, or immunohistochemical staining of ++ and positive FISH test), it is recommended to use trastuzumab, a molecularly targeted therapeutic agent, in combination with chemotherapy. The indications are patients who have not received prior first-line therapy against metastatic disease or patients who have not received prior second-line or higher therapy with anti-HER2 therapy. Contraindications are a prior history of congestive heart failure, high-risk uncontrolled arrhythmias, angina requiring pharmacologic therapy, clinically significant valvular disease, electrocardiogram demonstrating transmural myocardial infarction, and poorly controlled hypertension.