Darzalex 400mg Injection is used in the treatment of multiple myeloma, which is a type of cancer of plasma cells. Darzalex 400mg Injection is given as an IV infusion into veins under the supervision of a doctor. Your doctor will decide what dose is necessary and how often you need to take it. This will depend on what you are being treated for and may change from time to time. You should take it exactly as your doctor has advised. Taking it in the wrong way or taking too much can cause very serious side effects. It may take several weeks or months for you to see or feel the benefits but do not stop taking it unless your doctor tells you to. Infusion reaction such as swollen face, lips, mouth, tongue or throat, difficulty swallowing or breathing are very common side effects. This medicine may reduce the number of blood cells (decrease white blood cells) in your blood, thereby, increasing the susceptibility to infections. Regular blood tests are required to check your blood cells while taking this medicine. Many other medicines can affect, or be affected by, this medicine so let your healthcare team know all medications you are using. This medicine is not recommended during pregnancy or while breastfeeding. Use of effective contraception by both males and females during treatment is important to avoid pregnancy.