non-small cell lung cancer, where keytruda is approved to be used with certain chemotherapies as your first treatment if you do not have an abnormal "egfr" or "alk" gene. keytruda can cause your immune system to attack healthy parts of your body during or after treatment. this may be severe and lead to death. see your doctor right away if you have cough, shortness of breath, chest pain, diarrhea, severe stomach pain, severe nausea or vomiting, headache, light sensitivity, eye problems, irregular heartbeat, extreme tiredness, constipation, dizziness or fainting, changes in appetite, thirst, or urine, confusion, memory problems, muscle pain or weakness, fever, rash, itching, or flushing. there may be other side effects. tell your doctor about all your medical conditions, including immune system problems, if you've had or plan to have an organ or stem cell transplant, received chest radiation or have a nervous system problem. depending on the type of cancer, keytruda may be used alone or in combination with other treatments, and is also being studied in hundreds of clinical trials, exploring ways to treat even more types of cancer.