Research evidence shows that there is a high prevalence of substance use among young people in the UK. Early onset and high levels of use are associated with a range of negative outcomes, including increased risk of later problematic use and dependence. A growing body of research has identified family interventions to be effective in treating young people's substance use problems. However, despite this evidence, takeup of family based approaches, at least in the UK, has been low. A key factor appears to be the resource intensive nature of many family interventions, making them difficult to implement and deliver in many service settings, especially in the context of substantial cuts to drug and alcohol services for young people (1). Another potential barrier may be the cultural adaptation of approaches developed in the USA to a UK setting. There is growing awareness of the need to adapt evidence-based treatments to different cultural groups and settings in order to ensure successful implementation (2,3,4).

Following on from developmental and adaptation work, this study aims to demonstrate the feasibility of recruiting young people to specifically developed family and network based intervention. In addition the feasibility of training staff from existing young people addiction services to deliver this intervention will be explored and treatment retention will be assessed. Qualitative interviews will elicit the participants' views on the acceptability of the intervention and their experiences of both it and the study process.

Following the adaptation, we developed a manualised family-based intervention for young substance misusers up to the age of 18. This was an evidence-based approach, drawing on our previous research, a review of the literature and the views of young substance misusers with experience of treatment (Phase 1).

We aim to undertake a smallscale trial in two sites (Phase 2), to assess both the feasibility of the intervention and the scope for moving to a full trial of clinical and cost effectiveness. We also aim to evaluate the implementation of this new intervention using mixed methods research , monitoring randomisation procedures, treatment fidelity, staff time (and associated costs), engagement, retention and a range of outcome measures, reflecting the multiple vulnerabilities of this population. We will thereby address the brief's central aims of assessing the feasibility of a familybased intervention with this group and whether a trial could be successfully undertaken.

Phase 1: Intervention Development Using the original Intervention structure and content as a platform, the overall aim was to adapt this family and social intervention in collaboration with those who have experience of using services in order to ensure that it is acceptable to young people and their families, remains focused on the important aspects that promote and support behaviour change and yet is realistic and deliverable in a way that maximises treatment engagement, taking into account young people's views and preferences. The resulting intervention will therefore be one in which much of the original material is oriented to a youth population, whilst retaining the key principles of the original Social Behaviour and Network Therapy. At the time of completing this ethics application, two of the three key sources of information which informed this process were already carried out. First, a Systematic Review was conducted on the already existing evidencebase on the effectiveness and acceptability of family based interventions in treating young people's substance abuse. Second, Patient and Public Involvement (PPI) was carried out by actively engaging a sample of young people with a history of treatment for substance abuse in a number of consultation processes. Third, we are yet to explore the the point of view of those who will deliver the intervention by drawing on a group of therapists and service managers working in child and adolescent addiction services. This group will comment on each stage of the intervention and handbook design, being invited to 'walk through' the delivery of this new intervention.

Phase 2: Feasibility Randomised Controlled Trial (RCT) The purpose of this ethics application is to obtain ethical approval for this specific phase of our study. For phase 2 we propose a prospective pragmatic randomised controlled trial to assess the feasibility and impact of the adapted family and social network intervention for drug and alcohol misuse in young people. The proposed trial will use concealed randomisation, intention to treat analysis and characterisation of refusers and dropouts. It will be a pragmatic trial, delivered in two Young People services. It involves a parallel group design comparing two conditions: (i) Adapted youth social network intervention (YouthSBNT) and (ii) Treatment as usual.

For those that agree to participate, the researcher will:

1. Obtain written consent from them to participate in the trial;
2. Conduct a baseline assessment;
3. Telephone the York Trials Unit Freephone randomisation service or use the online system to randomise the patient (hereafter referred to as the participant);
4. Provide the participant with an appointment to see the therapist or clinician appropriate to their allocation.

At baseline, information will be collected from the patient regarding their drug and alcohol use, emotional wellbeing, social network, family environment, quality of life, school attendance, selfreported crime, health care use, social services contact, and demographics. The same set of information will be collected at 3 months and 12 months follow up postrandomisation assessments. Interviews will be undertaken by the research fellows, covering the main and secondary outcome measures mentioned earlier. In addition, qualitative interviews will be used to explore the acceptability and wider context of the impact of the intervention to the young people and their family members/social network. Economic evaluation will also be conducted to inform larger definitive trial.

Inclusion Criteria:

(i) Young people with drug and/or alcohol problems newly referred and accepted for treatment by the two agencies during the period of recruitment. (ii) Willing to provide written informed consent. (iii) Able to provide written informed consent.

Exclusion Criteria:

(i) Concurrent severe mental illness that precludes them from active participation.

(ii) Severe physical illness that precludes them from active participation. (iii) Unable or unwilling to give written informed consent.