Impact of medication side effects on medication adherence and implications for treatment outcomes in patients with borderline personality disorder: a pilot study

Borderline Personality Disorder (BPD) is a severe mental illness characterized by unstable interpersonal relationships, affective instability, and impulsivity. While psychotherapy is the recommended first-line treatment, over 80% of BPD patients use psychotropic medication and often report side effects. This pilot study examines whether perceived medication side effects influence treatment outcomes in BPD, anxiety, and depression symptoms indirectly through their effect on medication adherence, with adherence conceptualized as a potential mediator of this relationship. This retrospective study included patients from the personality disorder unit at University Hospital Eggenburg. BPD diagnoses were confirmed using the Borderline Symptom List (BSL-23). Perceived side effects and medication adherence were assessed via self-report questionnaires at admission. Three mediation analyses examined the association between side effects and treatment outcomes (using the BSL-23, PHQ-9, and GAD-7 for BPD, depression, and anxiety symptoms, respectively), with adherence as a mediator and gender and baseline symptom severity as covariates. 84% of patients ( n = 48) reported side effects, most commonly depression, fatigue, dizziness, and tremor. The leading cause of nonadherence was forgetting to take medication. The mediation analyses showed no indirect effect of X on Y via M ( n = 31; BPD: indirect effect = 0.0011, 95% CI = [-0.0209, 0.0169], anxiety: indirect effect = 0.0038, 95% CI = [-0.0989, 0.1075], depression: indirect effect = 0.0002, 95% CI = [-0.0813, 0.0946]). Despite frequent side effects, adherence remained high—likely due to the structured inpatient setting and voluntary admission. The lack of significant associations suggests other factors may influence treatment outcomes. Future research should involve larger samples, outpatient settings, follow-ups, and objective adherence measures. Targeted interventions addressing specific side effects may help improve adherence and clinical outcomes. Not applicable (retrospective observational study, no healthcare intervention trial).