Mental disorders are among the leading causes of the global burden of disease and are often associated with severe functional impairment and high societal costs. Psychotherapeutic, psychopharmacological and internet-based mental health interventions have proven to be helpful, but challenges remain, including only moderate response rates, high relapse rates and barriers to accessing mental healthcare. Much of the existing evidence stems from studies conducted in controlled, often standardised settings that only partially reflect real-world conditions, contributing to a ‘scientist-practitioner gap’. Moreover, the mechanisms of change, such as the interaction between treatment intensity, common factors (eg, the therapeutic relationship) and specific intervention techniques, have not been sufficiently investigated. In particular, the relationship of changes in personality functioning (PF) with mental and physical health has not yet been extensively researched.

The PSYMPACT (Psychological Impact Factors of Mental Health Treatments) study will use a longitudinal study design with a naturalistic sample (N 3000) to examine changes in psychopathology, PF and allostatic load in psychotherapeutic, psychopharmacological and internet-based treatments. The aim is to identify factors contributing to improvements and deteriorations in mental and physical health across different settings, including common and specific factors. Additionally, to provide patient perspectives, qualitative interviews will be conducted with individuals with varying levels of severity of mental health problems. Allostatic load will be assessed using repeated hair cortisol measurements. Furthermore, ecological momentary assessment will be used to examine the diurnal variability of PF as well as its more momentary correlates and longer-term outcomes. The central research questions and aims include (1) the assessment of common factors across different treatment settings, (2) associations of specific and common factors with improvements in mental health, including PF, (3 and 4) the importance of treatment intensity and interaction effects with common and specific factors, (5) the association of changes in psychopathology with changes in allostatic load, (6) the trait and state variability of PF, (7) the identification of patients who deteriorate under specific treatments and (8) patients’ perspectives on the effectiveness of different treatment modalities.

Approval was obtained from the Ethics Committee of the Department of Education and Psychology at the Freie Universität Berlin, Germany. Results will be submitted to peer-reviewed specialised journals and presented at national and international conferences.

Before data collection started in November 2024, the study was registered in the German Clinical Trials Register (https://www.drks.de/search/de/trial/DRKS00035560).