Key Clinical Message The occurrence of a large pericardial effusion is not a commonly noted adverse event associated with pembrolizumab and our report demonstrates that a rapid development can be diagnosed with close monitoring and triage to acute medical settings. Abstract Pembrolizumab is an immune checkpoint inhibitor used in various types of cancers. Pericardial tamponade is a rare side effect reported in only very few case reports. Early recognition and therapeutic intervention is vital in all cases. We report a case of a 54‐year‐old male with Stage 3 lung adenocarcinoma who developed cardiac tamponade secondary to pembrolizumab and subsequently required pericardial window.

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