Effectiveness of inpatient psychosomatic rehabilitation in Germany: protocol of a longitudinal study with a waitlist control group

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Background In Germany, psychosomatic rehabilitation is widely used to improve mental well-being and promote social and occupational participation among individuals with chronic mental and psychosomatic disorders. Although previous studies have reported clinically meaningful improvements following rehabilitation, most have relied on uncontrolled pre-post designs, limiting the strength of the current evidence. The present study exploits the current long waiting times for psychosomatic rehabilitation to form a waitlist control group without influencing the timing of rehabilitation. Overall, the primary objective of our study is to evaluate the short- and medium-term effects of psychosomatic rehabilitation on mental well-being (primary outcome), return to work, and work ability (secondary outcomes). Additional objectives include examining treatment effects in relevant patient subgroups (e.g., diagnostic groups), investigating moderator effects, and analyzing determinants of waiting time until rehabilitation. Methods We will conduct a prospective longitudinal study with a waitlist control group among individuals whose application for inpatient psychosomatic rehabilitation has been approved by the German Pension Insurance agency (Deutsche Rentenversicherung [DRV]). Participants will complete up to 12 online questionnaires at six-week intervals over a period of 16 months. Questionnaire data will be linked with routine data from the DRV rehabilitation statistics database to obtain objective information on rehabilitation and return to work. The effectiveness of psychosomatic rehabilitation will be examined using two complementary analytical approaches: a propensity score-matched waitlist control group comparison to examine the short-term effects of rehabilitation and a discontinuous growth model to examine the medium-term effects of psychosomatic rehabilitation. Both models will also be applied in the subgroup analyses and extended by the relevant interaction terms for the moderator analyses. Determinants of waiting time until rehabilitation will be analyzed using time-to-event models. Discussion The study will provide comparatively robust evidence on the effectiveness of psychosomatic rehabilitation in routine care. It may identify patient groups that benefit differently from rehabilitation and factors associated with waiting time until rehabilitation. These findings may inform future efforts to optimize rehabilitation interventions, improve equity in treatment outcomes, and reduce waiting times for psychosomatic rehabilitation. Trial registration German Clinical Trials Register (DRKS): DRKS00038670 (9th December 2025; https://drks.de/search/en/trial/DRKS00038670/details ); WHO UTN: U1111-1332-6419.