Study assessments take place at study-entry (baseline), end of treatment, and 2 and 4 years post-baseline, and will be conducted by trained, academic level, research assistants, who are independent from the treatment-staff. Youth who apply for treatment at the participating treatment centers will be approached by treatment-staff with the request to participate in the study. Following screening for eligibility, participants are asked to provide informed consent, after which the baseline assessment and subsequent two- and four-years follow-up will be conducted.

For the purpose of the present study, we developed the Youth in Transition Questionnaire (YIT-Q [24]), which contains both widely used instruments like the Depression, Anxiety and Stress Scale (DASS-21 [25]) and the World Health Organization Disability Assessment Schedule (WHODAS-2.0 [26]) as well as domains with item-sets that were developed by our research group (see Table 1). Both the DASS-21 and the WHODAS-2.0 have shown a stable factor structure, and their scales have excellent internal consistency reliability and good convergent en discriminant validity (e.g., [27, 28]).

At baseline, we collect sociodemographic information and administer the substance use section of the Measurements in the Addictions for Triage and Evaluation – Youth version (MATE-Y [29]), which is the standard intake instrument in most youth addiction treatment organizations in the Netherlands. Both lifetime (onset, duration) and past-month (days of use) substance use data are collected, and the criteria of the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5 [30]) are administered to obtain a lifetime and past-year diagnosis of SUD for the primary and – if present – secondary substance for which the youths seek help. To assess the presence of comorbid non-SUD mental disorders, we developed a structured DSM-5 diagnostic interview which covers ten of the most common disorders: depression; panic disorder; agoraphobia; social phobia; post-traumatic stress disorder; attention-deficit/hyperactivity disorder; conduct disorder; anorexia nervosa and bulimia nervosa; generalized anxiety disorder. For each disorder, the main diagnostic criteria are used as screening questions, and only if these main criteria are met in a period that the person was not intoxicated and did not experience alcohol or drug withdrawal symptoms – hence, do not reflect a substance-induced disorder only – the full section of that disorder is administered. In addition to DSM-5 classification, the DASS-21 (depression, anxiety, stress), Prodromal Questionnaire (PQ-16 [31]) (subclinical psychotic symptoms), Substance Use Risk Profile Scale (SURPS [32]) (personality risk for substance abuse) and Screener for Intelligence and Learning disability (SCIL [33]) are administered to obtain dimensional measures of mental health, and the WHODAS-2.0 is used to assess general functioning, disability and quality of life. The PQ-16 was found to accurately identify ultra high risk (UHR) and full psychosis, with area under the curve (AUC) values ranging from 0.71 to 0.95 [34], and the SURPS showed adequate to high internal consistency (Cronbach’s alpha = 0.61–0.86), a stable factor structure that reflects the four scales of the instrument, and good test-retest reliability, with intra-class correlation coefficients (ICC) ranging from 0.68 to 0.88 [35]. The SCIL has shown excellent test-retest reliability (Pearson’s r = 0.92) and good predictive validity for detecting mild or borderline intellectual disabilities in both adults (AUC = 0.93) and youths (AUC = 0.91) [33].

The YIT-Q assessment domains pertaining to family history/environment, social functioning and treatment history/motivation all contain structured questions which inform about the youth’s self-reported perception of his/her own functioning or problems in these areas, or – in the case of family history – his/her perception of his family members’ functioning/problems (see Table 1). Questions pertaining to suicidal ideation/attempts were derived from the Mini International Neuropsychiatric Interview (MINI, version 5.0.0 [36]). For assessing family history of SUD, non-SUD mental disorders and criminality we used the Family History Method (FHM [37]) as described by Schuckit et al. (2020). Given that the FHM tends to underestimate family diagnosis, we aimed to increase its sensitivity by asking the youth whether his/her (grand)parent(s) or sibling(s) had ever received treatment for alcohol, drug or mental health problems, or whether they had ever been incarcerated. We derived and adapted a set of questions about family history of violence, abuse and neglect towards the youth from the International Society for the Prevention of Child Abuse & Neglect (ISPCAN) Child Abuse Screening Tools (ICAST), version for young adults (ICAST-R [38]). For assessing important ‘turning points’ that had a major impact on the course of the youth’s substance use career, we will use a qualitative approach with open-ended questions [39]. Questions on the youth’s social functioning, including employment, living arrangements, social support and delinquency were derived and adapted from item-sets in the Substance Use Recovery Evaluator (SURE [40]) and the Treatment Outcomes Profile (TOP [41]). Both instruments are widely used and validated, and are included in the recent International Consortium for Health Outcomes Measurement (ICHOM [42]) standard set for addiction. For example, the five subscales of the SURE showed good internal consistency (alpha = 0.68–0.87), and reflect a stable factor structure of the instrument [40]. In addition, all items of the TOP in the areas of employment, living arrangements and delinquency showed ‘substantial’ or ‘good’ inter-rater reliability, with Cohen’s kappa> 0.61, and ICC > 0.75, respectively [41]. We limited our assessment of intention/motivation for treatment to a single-item question pertaining to the youth’s self-reported primary goal of treatment – total abstinence or strong/small/no reduction of substance use.

In addition to the self-reported measures described above, we administer several computerized tasks pertaining to impulsive or risky choice and value-based decision-making (VBDM). Specifically, we include a delay discounting task, in which four versions are administered: a) delay discounting; b) probability discounting of gains; c) probability discounting of losses; d) mixed gambles. The task is administered using MATLAB software; further details on task description can be found in [43]. In addition, a computerized self-ordered pointing task is administered using Inquisit software, based on Petrides and Milner (1982) from the Inquisit test library [44]. Lastly, we administer an Inquisit version of the Risky Gains Task developed by Paulus et al. (2003) [45, 46]. Concerning discounting in adolescents, studies have shown that individual differences in decision-making are stable over time, as indicatated by moderate to good test-retest reliability even over a 2-year interval, with ICC-values ranging from 0.65 to 0.78 [47]. Lastly, we collect a scalp hair sample of (if possible) at least 3 cm and analyze hair cortisol and testosterone to obtain a biological measure of chronic stress. Hair cortisol analysis is increasingly used in mental health research, and is a reliable method to determine cortisol levels over a prolonged period of time [48, 49]. In addition, it has recently been shown that psychosocial stress increases testosterone levels in both healthy controls and patients with posttraumatic stress-disorder and borderline personality disorder [50]. We therefore also assess hair testosterone levels, in order to have an additional read-out for physiological effects of chronic stress and explore a potential link between elevated testosterone and impulse regulation issues among youth with substance use disorders.

The end-of-treatment assessment will be limited to documenting the judgement of the youth’s primary clinician about the youth’s short-term treatment response (partial or full remission; persistence; deterioration of problematic substance use) and documenting the treatment received (type; intensity; duration) and type of treatment termination (premature; planned).

With the exception of most demographic background variables and questions pertaining to pre-baseline history, the two- and four-year follow-up interviews will focus on the same domains and include the same instruments as the baseline assessment, with time-frames adapted to the follow-up period. In addition, we will inquire whether the youth experienced important ‘turning points’ or major life events that, in his or her view, had a major impact on the course of his/her substance use in two- and four-years follow-up period.

Time to administer the baseline and follow-up assessments is estimated at 2 and 1.5 h each, respectively. The assessments will preferably be conducted at the treatment center. Youth will receive a remuneration of 20, 30 and 50 euro for participating in the baseline and two- and four-year follow-up, respectively. In addition, they have the possibility to win a gift certificate of 200 euro (6 certificates available) for follow-up attendance, as has been done successfully in previous studies [51].