Gradual improvement in functioning and mental distress during long-term outpatient SUD treatment – A prospective pre-post study

•Outpatient treatment is a preferred treatment form for SUD, but lacks research.•We followed 67 patients during 5 months of a longer outpatient treatment course.•Patients reported a significant improvement in perceived functioning from T0 to T1.•Patients with the lowest initial scores showed the largest improvement.•Other variables, e.g. distress, sleep, restlessness, improved significantly. Globally, outpatient programs for substance use disorder (SUD) treatment have gained prominence. To assess the broader clinical implications of this trend we investigated shifts in functioning experienced by outpatients undergoing treatment. We describe the clinical characteristics of a cohort of 93 SUD patients in a Norwegian outpatient treatment clinic. Using paired-samples t-tests, we examined changes in perceived functioning, mental distress, and other clinically relevant outcome variables in a 5-month time interval during the treatment course. We obtained follow-up data for 67 (72%) of the included patients, with no significant difference in patient-related factors between those who completed the treatment course and those who were not assessed at follow-up. Perceived functioning increased significantly from study inclusion (Time 0) (mean 19.8, standard deviation ± 8.8) to its conclusion (Time 1) (24.3, ±9.3; t (66) = 4.5, (95% CI: 2.5–6.5, p