 Keytruda may be used with certain chemotherapies before surgery when you have early stage lung cancer, which can be removed by surgery and then continued alone after surgery to help prevent your lung cancer from coming back. 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,