Drop-out is an important problem in the treatment of people with multiple substance use disorder. There is growing agreement for substance abuse and dependence, being a chronic and relapsing condition [1–3]. The effectiveness of treatment to stabilize or overcome a chronic condition is strongly related to treatment compliance . Among alcohol-dependent people, diabetics or persons with hypertension it is common to find a compliance rate below 50% .
In a therapeutic community environment for drug addicted people, evidence has been found that family-[5, 6], social network- [7, 8], and staff-involvement [9, 10], improve retention levels. In the broader area of health care, patient-based measures in routine practice combined with feedback were found to improve significantly the process and outcome of patient care . In mental healthcare, feedback to clinicians about patients' progress based on routine outcome monitoring affected outcome and even the number of sessions in psychological interventions [12, 13].
In the realm of outpatient treatment of substance use disorder, compliance is also associated with session attendance a/o adherence to treatment. Regular treatment often relies on assessment, but outside its use in planning, monitoring, and evaluating interventions, assessment and feedback as part of the treatment itself and its positive effect on patient adherence is under-investigated. Kypri et al. focused on this issue, related to contamination of clinical trials , and found that routine screening and minimal assessment themselves may produce some benefit . In line with the studies based on routine outcome monitoring in psychotherapy [12, 16], Hawkins et al.  developed a similar approach in the treatment of substance use disorder. Their focus was on feedback to clinicians about lack of progress in order to reduce the risk of patient drop-out.
In this study, assessment and feedback is introduced as part of the treatment process itself. It is aimed to offer opportunities to support counselling by establishing a collaborative relationship and a patient-centred focus [18, 19]. This therapeutic method of assessment  seeks to integrate key features of therapeutic alliance between clinician and patient in assessment and feedback sessions. Feedback is given to the patient each time at the next session after assessment. It consists of communicating the status of substance use, readiness to change, and personal resources that may facilitate or hinder change. Repeated assessment and graphical representation is considered to provide opportunities for feeding back of progress, treatment recommendation, and further action. It is also providing the patient with new ways of thinking and feelings about self and others.
This study addresses the question whether continuing rounds of assessment with personal feedback to patients that replace a number of regular outpatient sessions in the treatment of people with multiple substance use disorder, improves adherence, compared to an outpatient approach without such within treatment assessment and feedback. It is the most important aim of this study to investigate the effectiveness of within treatment assessment and personal feedback to patients on outpatients' adherence to treatment. The reporting follows the CONSORT-guidelines [21–23].