Background and Objectives: Attention-deficit/hyperactivity disorder (ADHD) is recognized as a condition that can persist into adulthood, often with psychiatric comorbidities that worsen the overall prognosis. ADHD has been associated with substance use disorders (SUDs), especially through symptoms like hyperactivity and impulsivity, while the role of inattentive symptoms remains more difficult to judge. This study aimed to estimate the likelihood of ADHD in a sample of young adults with SUDs undergoing opioid agonist treatment, and to evaluate how inattentive and hyperactive/impulsive traits affect their clinical, psychological, and social functioning. Methods: 57 individuals were evaluated using comprehensive clinical interviews and self-administered questionnaires to assess characteristics of SUDs, current and childhood ADHD symptoms, levels of impulsivity, and psychiatric comorbidities. Results: Overall, 22.8% of participants were classified as possible ADHD cases (p-ADHD). Compared to the comparison group, p-ADHD patients exhibited more severe SUDs symptomatology and greater impairments in specific domains of adaptive functioning. They also had a higher lifetime prevalence of social anxiety disorder, unspecified Diagnostic and Statistical Manual of Mental Disorders fifth edition (DSM-5) diagnoses, and clinically relevant impulsivity. Further, p-ADHD individuals were more likely to experience limitations and craving, with the inattentive symptoms significantly mediating these associations. Discussion and Conclusions: ADHD frequently co-occurs in SUDs patients in OAT and can worsen the SUDs clinical presentation. Moreover, inattentive symptoms may play a prominent role in SUDs development. Scientific Significance: This study suggests that different ADHD presentations influence SUDs manifestations, challenging the idea that impulsivity is the main contributor in SUDs.
