See also Warning section.
Diarrhea, dizziness, stomach/abdominal pain, constipation, nausea, vomiting, mouth sores, dry mouth, loss of appetite, tiredness, hair loss, nose bleeds, or headache may occur. Nausea and vomiting can be severe. In some cases, your doctor may prescribe medication to prevent or relieve nausea and vomiting. Eating several small meals, not eating before treatment, or limiting activity may help lessen some of these effects. If any of these effects last or get worse, tell your doctor or pharmacist promptly.
Remember that your doctor has prescribed this medication because he or she has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects.
Tell your doctor right away if you have any serious side effects, including: unusual tiredness, pale skin, easy bruising/bleeding.
Trastuzumab deruxtecan may lead to serious heart problems, including heart failure. Past use of certain other anti-cancer drugs (including anthracyclines such as doxorubicin) may increase your risk of heart problems. Tell your doctor right away if you have any symptoms of heart failure, including shortness of breath, swelling ankles/feet, unusual tiredness, unusual/sudden weight gain.
This medication may lower your ability to fight infections. This may make you more likely to get a serious (rarely fatal) infection or make any infection you have worse. Tell your doctor right away if you have any signs of infection (such as sore throat that doesn't go away, fever, chills, cough).
A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing.
This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist.
In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch.
In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.