The Food and Drug Administration approved trastuzumab deruxtecan to treat metastatic, HER2-positive breast cancer that has grown after at least two past lines of HER2-targeted treatment for metastatic breast cancer. These include common therapies like trastuzumab (Herceptin or biosimilar) and ado-trastuzumab emtansine (Kadcyla).

In the clinical studies of trastuzumab deruxtecan, the people who took part reported these common side effects that you may experience

• nausea and vomiting
• fatigue
• low white blood cell counts
• hair loss
• constipation or diarrhea
• low red blood cell counts

In rare cases, some people developed a very serious lung condition called interstitial lung disease. On scans, this disease looks like scarring or inflammation of the lungs. This condition can be fatal. You should report any lung problems you have while taking this medicine to your doctor immediately, including

• cough
• trouble breathing
• shortness of breath
• fever

Like other HER2-targeting medicines, trastuzumab deruxtecan can also affect your heart function. Your doctor will watch for problems in your heart, but let them know if you have symptoms such as sudden weight gain, swelling, dizziness, or if you faint.

Trastuzumab deruxtecan may be dangerous to a fetus if taken during or before a pregnancy. You should not take it if you are pregnant or breastfeeding, and you should take steps to avoid getting pregnant while on trastuzumab deruxtecan. Men taking trastuzumab deruxtecan should also take steps to avoid getting their partner pregnant.