Patients receiving pembrolizumab, the immunotherapy drug sold as Keytruda, commonly report a range of side effects including fatigue, musculoskeletal pain, diarrhea, nausea, skin reactions, fever, cough, breathing difficulties, constipation, and abdominal pain. These symptoms, while unpleasant, accompany a drug that has become a standard treatment for multiple cancers since its U.S. approval in 2014. The medication works through a precise biological mechanism.
It targets the programmed cell death protein 1, or PD-1, receptor found on lymphocytes. Cancer cells typically use this receptor as a shield to evade immune detection.
By blocking PD-1, pembrolizumab allows the immune system to recognize and attack tumors. The drug is classified as a humanized monoclonal antibody, meaning it is a laboratory-engineered protein designed to bind to one specific target in the body. It belongs to the IgG4 isotype class of antibodies and is administered through slow intravenous injection, which remains the standard delivery method as of September 15, 2025.
Unlike chemotherapy, which kills dividing cells without discrimination, pembrolizumab releases the immune system’s natural brakes. This approach can lead to inflammation in healthy tissues, explaining the side effect profile.
Fatigue is the most frequently reported complaint, followed by musculoskeletal pain. Skin issues such as itching and rash are common, along with gastrointestinal problems including diarrhea, nausea, constipation, and abdominal pain. Some patients also experience decreased appetite.
Healthcare providers are instructed to monitor patients closely and manage these effects as essential for safe treatment. The World Health Organization has included pembrolizumab on its Model List of Essential Medicines, a designation reserved for drugs considered indispensable for basic health systems.
Fewer than 500 medications worldwide hold this status. The therapy has been tested across numerous cancer types, including lung cancer, melanoma, head and neck cancers, Hodgkin lymphoma, and urothelial carcinoma. A fixed-dose combination exists that pairs pembrolizumab with berahyaluronidase alfa, marketed under a separate brand name. Berahyaluronidase alfa breaks down hyaluronic acid in tissues, potentially altering drug absorption, though the report does not specify which cancers this combination targets or its performance outcomes.
As an IgG4 antibody, pembrolizumab belongs to a specific subclass of immunoglobulin G with unique properties that may reduce certain immune responses compared to other IgG subclasses. The drug requires careful oversight during infusion, as the body needs time to adjust to the antibody entering the bloodstream.
Medical staff are trained to watch for infusion-related symptoms during the procedure. With more than a decade of clinical use since its 2014 U.S. approval, pembrolizumab has become a mainstay in oncology—not a cure for every patient, but a tool that has changed how doctors approach cancer treatment.
