A 57-year-old male patient with advanced lung squamous cell carcinoma developed severe diarrhea, abdominal pain and bloody stool after receiving 5 cycles of pembrolizumab combined chemotherapy and 1 cycle of single drug pembrolizumab combined with the results of colonoscopy and histopathology, it was diagnosed as colitis, which was considered as severe immune related ulcerative colitis caused by pembrolizumab. After suspending immunotherapy and receiving sufficient glucocorticoid treatment, the patient's abdominal pain and diarrhea significantly improved 2 days later, and relieved 15 days later. In order to improve the safety of the clinical application of pembrolizumab, risk factors, clinical characteristics and treatment of immune associated colitis induced by pembrolizumab were further analyzed by reviewing the literature of the adverse reactions related to it.

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