Pembrolizumab (formerly lambrolizumab, brand name Keytruda) is a humanized antibody used in cancer immunotherapy. This includes to treat melanoma, lung cancer, head and neck cancer, Hodgkin lymphoma, and stomach cancer. It is given by slow injection into a vein.Common side effects include fatigue, musculoskeletal pain, decreased appetite, itchy skin (pruritus), diarrhea, nausea, rash, fever (pyrexia), cough, difficulty breathing (dyspnea), constipation, pain, and abdominal pain. It is an IgG4 isotype antibody that blocks a protective mechanism of cancer cells and thereby, allows the immune system to destroy them. It targets the programmed cell death protein 1 (PD-1) receptor of lymphocytes. It works by targeting the cellular pathway of proteins found on the body's immune cells and some cancer cells, known as PD-1/PD-L1.Pembrolizumab was approved for medical use in the United States in 2014. In 2017, the US Food and Drug Administration (FDA) approved it for any unresectable or metastatic solid tumor with certain genetic anomalies (mismatch repair deficiency or microsatellite instability). It is on the World Health Organization's List of Essential Medicines.