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IntroductionThe decisions of withholding or withdrawing life-sustaining treatments are difficult to make in the context of emergency departments (EDs) because most patients are unable to communicate. Relatives are thus asked to participate in the decision‐making process, although they are unprepared to face such situations. We therefore aimed to develop a standardised intervention for announcing decisions of withholding or withdrawing life-sustaining treatments in EDs and assess the efficacy of the intervention on the stress of relatives.Methods and analysisThe DISCUSS trial is a multicentre stepped-wedge cluster randomised study and will be conducted at nine EDs in France. A standardised intervention based on human simulation will be codesigned with partner families and implemented at three levels: the relatives, the healthcare professionals (HCP) and the EDs. The intervention will be compared with a control based on treatment as usual. A total of 538 families are planned to be included: 269 in the intervention group and 269 in the control group. The primary endpoint will be the symptoms of post-traumatic stress disorder (PTSD) at 90 days. The secondary endpoints will be symptoms of PTSD at 7 and 30 days, diagnosis of PTSD at 90 days and anxiety and depression scores at 7, 30 and 90 days. Satisfaction regarding the training, the assertiveness in communication and real-life stress of HCPs will be measured at 90 days.Ethics and disseminationThis study was approved by the ethics committee Est III from Nancy and the French national data protection authority. All relatives and HCPs will be informed regarding the study objectives and data confidentiality. Written informed consent will be obtained from participants, as required by French law for this study type. The results from this study will be disseminated at conferences and in a peer-reviewed journal.Trial registration numberNCT06071078.