Vomiting in a leukemia patient 20 days after pembrolizumab is most likely related to a side effect reaction to the medication. However, further evaluation is also needed to see if it is related to the leukemia itself. 1. Pemantase is a commonly used drug in the treatment of leukemia and lymphoma, and common adverse reactions include allergic reactions, liver function impairment, abnormal coagulation function, increased bleeding tendency, thrombosis, acute pancreatitis, and electrolyte metabolism disorders. It can be accompanied by vomiting with rash, skin jaundice, skin petechiae, black stools, or abdominal pain. Pemantase is contraindicated in patients with severe hepatic or renal insufficiency and in patients who are allergic to the product. 2. In addition, anemia or central invasion caused by leukemia itself may be accompanied by nausea and vomiting, dizziness, headache and other uncomfortable symptoms. It is recommended to contact the supervising doctor in time and go to the hospital for further evaluation and identification and treatment under the guidance of the physician.