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Background Substance use disorders (SUD) often co-occur with attention deficit hyperactivity disorder (ADHD). Although the short-term effects of some specific interventions have been investigated in randomized clinical trials, little is known about the long-term clinical course of treatment-seeking SUD patients with comorbid ADHD. Aims This paper presents the protocol and baseline clinical characteristics of the International Naturalistic Cohort Study of ADHD and SUD (INCAS) designed and conducted by the International Collaboration on ADHD and Substance Abuse (ICASA) foundation. The overall aim of INCAS is to investigate the treatment modalities provided to treatment-seeking SUD patients with comorbid ADHD, and to describe the clinical course and identify predictors for treatment outcomes. This ongoing study employs a multicentre observational prospective cohort design. Treatment-seeking adult SUD patients with comorbid ADHD are recruited, at 12 study sites in nine different countries. During the follow-up period of nine months, data is collected through patient files, interviews, and self-rating scales, targeting a broad range of cognitive and clinical symptom domains, at baseline, four weeks, three months and nine months. Results A clinically representative sample of 578 patients (137 females, 441 males) was enrolled during the recruitment period (June 2017-May 2021). At baseline, the sample had a mean age (SD) of 36.7 years (11.0); 47.5% were inpatients and 52.5% outpatients; The most prevalent SUDs were with alcohol 54.2%, stimulants 43.6%, cannabis 33.1%, and opioids 14.5%. Patients reported previous treatments for SUD in 71.1% and for ADHD in 56.9%. Other comorbid mental disorders were present in 61.4% of the sample: major depression 31.5%, post-traumatic stress disorder 12.1%, borderline personality disorder 10.2%. Conclusions The first baseline results of this international cohort study speak to its feasibility. Data show that many SUD patients with comorbid ADHD had never received treatment for their ADHD prior to enrolment in the study. Future reports on this study will identify the course and potential predictors for successful pharmaceutical and psychological treatment outcomes. Trial registration ISRCTN15998989 20/12/2019.

Aims: This study addressed the age of onset of conduct disorder (CD) and oppositional defiant disorder (ODD) in treatment-seeking substance use disorder (SUD) patients with and without adult attention-deficit/hyperactivity disorder (ADHD) and its association with early onset of SUD. Methods: We examined data from the 2nd International ADHD in Substance Use Disorders Prevalence Study, including 400 adults in SUD treatment from Puerto Rico, Hungary, and Australia. ADHD, SUD, and CD/ODD were assessed with the Conners Adult ADHD Diagnostic Interview for DSM-IV, the MINI International Neuropsychiatric Interview, and the K-SADS, respectively. Cox regression analyses modeled time to emergence of CD/ODD separately for SUD patients with and without adult ADHD. Linear regression models examined associations between age of onset of SUD and presence of ADHD and adjusted for sex, age, and country. To assess the mediating role of CD/ODD on the association of ADHD with onset of SUD, adjusted regression models were estimated. Results: Treatment-seeking SUD patients with ADHD presented an earlier onset of CD/ODD compared with those without ADHD. CD/ODD symptom loads were higher among the SUD and ADHD group. Age of first substance use and SUD were significantly earlier in SUD patients with ADHD, and these findings remained significant after adjustment for demographics and coexisting CD/ODD. Conclusions: ADHD is associated with earlier onset of SUD as well as with an earlier onset of more frequent and more severe disruptive behavioral disorders. These findings may inform preventive interventions to mitigate adverse consequences of ADHD.