BACKGROUND: The present study aimed at answering three research questions: (a) Does shared decision making (SDM) yield similar effects for patients with involuntary admission or incidents of aggression compared to patients with voluntary admission or without incidents of aggression? (b) Does SDM reduce the number of patients with incidents of aggression and the use of coercive measures? (c) Does the use of coercion have a negative impact on patients' perceived involvement in decision making? METHODS: We used data from the cluster-randomized SDM-PLUS trial in which patients with schizophrenia or schizoaffective disorder in 12 acute psychiatric wards of 4 German psychiatric hospitals either received an SDM-intervention or treatment as usual. In addition, data on aggression and coercive measures were retrospectively obtained from patients' records. RESULTS: The analysis included n = 305 inpatients. Patient aggression as well as coercive measures mostly took place in the first days of the inpatient stay and were seldom during the study phase of the SDM-PLUS trial.Patients who had been admitted involuntarily or showed incidents of aggression profited similarly from the intervention with regard to perceived involvement, adherence, and treatment satisfaction compared to patients admitted voluntarily or without incidents of aggression. The intervention showed no effect on patient aggression and coercive measures. Having previously experienced coercive measures did not predict patients' rating of perceived involvement. CONCLUSION: Further research should focus on SDM-interventions taking place in the very first days of inpatients treatment and potential beneficial long effects of participatory approaches that may not be measurable during the current inpatient stay.