Aim of the study:
The main aim of this study was to examine the mutual effect of schizophrenia and substance use disorder on depression and stress level.

Subject or material and methods:
The sample comprised a total of 184 participants, including 51 patients with schizophrenia (S), 45 with dual diagnosis (DD), 44 with substance use disorder (SUD) and 44 healthy controls (HC). The Patient Health Questionnaire-9 and the Hamilton Depression Rating Scale were used to measure depressive symptom severity, while the Social Readjustment Rating Scale was used for the assessment of stressful life events.

Results:
We found a greater main effect of substance use disorder relative to schizophrenia on the severity of depression symptoms and stressful life events. In addition, the observed significant interaction effect for stressful life events suggested that SUD patients experienced their greatest number while HC reported the fewest such incidents. Interestingly, significant links between stressful life events and depressive symptoms emerged in both SUD patients and HC. What is more, disease duration and exacerbations were significantly associated with depressive symptoms in DD and S patients.

Discussion:
This study demonstrated the effect of schizophrenia and substance use disorders on depressive symptoms and the severity of stress. The relationship between stressful life events and depressive symptom severity was also investigated.

Conclusions:
Compared to schizophrenia, substance use disorder has a stronger effect on the presence of depression symptoms and the occurrence of stressful life events. The factors modifying depression symptom severity in both schizophrenia groups differ from those found in SUD patients and HC.