Fatigue. Musculoskeletal pain. Diarrhea. Nausea. Rash. Fever. Cough. Difficulty breathing. Constipation. Abdominal pain. These are the common side effects listed for patients receiving pembrolizumab, the cancer immunotherapy drug known as Keytruda.
None of these are pleasant. But the drug’s broader medical profile is starkly different. Since its U.S. approval in 2014, pembrolizumab has become a standard weapon against multiple cancers. It works by a specific mechanism: it targets the programmed cell death protein 1, or PD-1, receptor on lymphocytes. That receptor is a kind of shield cancer cells use to hide from the immune system. Block it, and the immune system can see the tumor and attack.
Pembrolizumab is a humanized monoclonal antibody. That means it is a lab-made protein designed to bind to a single target in the body. It belongs to the IgG4 isotype class of antibodies. It is given by slow intravenous injection. As of September 15, 2025, that remains the standard route of administration.
The drug is not just a clinical tool. The World Health Organization lists it on its Model List of Essential Medicines. That designation signals a drug is considered indispensable for a basic health system. Fewer than 500 drugs hold that status globally.
Pembrolizumab is also being combined with other agents. One fixed-dose combination pairs it with berahyaluronidase alfa. That combination is marketed under a separate brand name. The report does not specify which cancers this combination targets, only that it exists.
The side effect profile matters because pembrolizumab is not chemotherapy. Chemotherapy kills dividing cells indiscriminately. Pembrolizumab works by releasing the immune system’s brakes. That can cause inflammation in healthy tissue. Fatigue is the most common complaint. Musculoskeletal pain follows. Skin reactions, including itching and rash, are frequent. Gastrointestinal issues like diarrhea, nausea, constipation, and abdominal pain appear regularly. Fever, cough, and difficulty breathing can occur. Decreased appetite is also reported.
Healthcare professionals are instructed to monitor patients closely. Management of these side effects is considered essential for safe and effective use. The drug is not a casual prescription. It requires careful oversight.
The therapy has been tested in dozens of cancer types. Lung cancer. Melanoma. Head and neck cancers. Hodgkin lymphoma. Urothelial carcinoma. The list is long. The report does not list every approved indication, but the breadth is implied by its inclusion on the WHO essential list. That designation is not given lightly. It requires evidence of efficacy, safety, and public health value across diverse settings.
Pembrolizumab is an IgG4 antibody. That is a specific subclass of immunoglobulin G. IgG4 antibodies have unique properties. They are less likely to trigger certain immune responses than other IgG subclasses. That may reduce some types of infusion reactions, though the report does not say that directly. The drug’s mechanism is clear: it inhibits the PD-1 receptor on lymphocytes. That inhibition is what allows T cells to recognize and destroy cancer cells.
The drug was approved in the United States in 2014. That is more than a decade of clinical use. It has become a mainstay in oncology. Not a cure for every patient. Not without risks. But a tool that has shifted how doctors think about treating cancer.
Patients get the drug through slow IV infusion. The infusion time matters. Too fast can cause reactions. The body needs time to adjust to the antibody entering the bloodstream. Nurses and doctors are trained to watch for infusion-related symptoms during the procedure.
The combination with berahyaluronidase alfa is notable. That enzyme breaks down hyaluronic acid in tissue, allowing the drug to be absorbed more readily. It changes how the drug is delivered. But the report only notes its existence, not its performance.
Pembrolizumab is one drug. It has a known side effect list. It has a precise mechanism. It has a global health designation. It has a decade of U.S. use. Those facts are not in dispute. They are the foundation of how this immunotherapy is understood and used today.
