A drug used to treat squamous cell cancer of the head and neck that has recurred (come back) or spread to other parts of the body. It is used in patients whose disease got worse during or after treatment. It is also used to treat certain types of non-small cell lung cancer that have spread to other parts of the body and got worse during or after treatment. Pembrolizumab is also used to treat melanoma that cannot be removed by surgery or has spread to other parts of the body. It is also being studied in the treatment of other types of cancer. Pembrolizumab binds to a protein called PD-1, which is found on T cells (a type of white blood cell). Pembrolizumab may block PD-1 and help the immune system kill cancer cells. It is a type of monoclonal antibody and a type of immune checkpoint inhibitor.