Pembrolizumab injection is used alone or in combination with other chemotherapy medications to treat certain types of melanoma (a type of skin cancer), Merkel cell carcinoma (MCC; a type of skin cancer), and cutaneous squamous cell carcinoma (CSCC; skin cancer). It is also used to prevent the return of melanoma after surgery. Pembrolizumab injection is also used alone and/or in combination with other chemotherapy medications to treat certain types of lung cancer (non-small-cell lung cancer; NSCLC), head and neck cancer, Hodgkin's lymphoma (Hodgkin's disease), primary mediastinal B-cell lymphoma (PMBCL; non-Hodgkin lymphoma), urothelial cancer (cancer of the lining of the bladder and other parts of the urinary tract), bladder cancer, colorectal cancer (cancer that begins in the large intestine), gastric cancer (cancer of the stomach), esophageal cancer (cancer of the tube that connects your throat to your stomach), cervical cancer (cancer that begins in the opening of the uterus [womb]), cancer of the endometrium (lining of the uterus), hepatocellular carcinoma (HCC; a type of liver cancer), renal cell carcinoma (RCC, a type of cancer that begins in the kidneys), breast cancer, and certain other solid tumors. Pembrolizumab injection is in a class of medications called monoclonal antibodies. It works by helping your immune system to slow or stop the growth of cancer cells.