ABSTRACT
Adults with ADHD show substantial variability in response to pharmacological treatment, and comorbid personality pathology may help explain why symptom improvement often fails to translate into meaningful functional improvement. This naturalistic longitudinal study examined whether dimensional personality pathology predicts symptomatic and functional outcomes during routine stimulant treatment. A total of 246 adults with ADHD (66% women; mean age = 33.5) contributed repeated assessments over irregular follow-up intervals, with substantial attrition across waves. Linear mixed-effects models were used to accommodate unbalanced repeated measures. Linear mixed-effects models indicated that higher personality dysfunction was strongly associated with less improvement in functioning (β = 0.44, p < 0.001) and more persistent ADHD symptoms (β = 0.20, p < 0.001), independent of stimulant dose and time in treatment. Negative affectivity, detachment, psychoticism, and disinhibition were the maladaptive trait domains most consistently linked to poorer functional outcomes, whereas disinhibition showed the strongest association with residual ADHD symptom burden. Medication-related effects were modest in comparison. These findings identify personality pathology as a clinically relevant source of heterogeneity in adult ADHD treatment response and support routine assessment of personality functioning to inform clinical decision-making.