Targeted drugs for breast cancer have very few toxic side effects compared to regular chemotherapy drugs. However, they have their own more common side effects, such as major allergic reactions, skin rashes, and milder gastrointestinal reactions. Trastuzumab has a landmark role in targeted drugs, although in the treatment of breast cancer, trastuzumab reduces the risk of recurrence of breast cancer, but the drug has cardiotoxicity. Therefore, cardiac ultrasound is required every three cycles to evaluate the ejection fraction of the heart during the course of trastuzumab. In patients whose ejection fraction is greatly affected by trastuzumab, the drug may need to be discontinued or even not applied for life. Another relatively common toxic side effect of trastuzumab is the development of flu-like symptoms, such as fever and chills, when the drug is first administered. However, the reaction is usually mild and does not require special treatment, or the patient will get better if only ordinary analgesics and antipyretics are given.