BackgroundSubstance users being admitted to inpatient treatment experience a high level of mental distress. In this study we explored changes in mental distress during treatment.MethodsMental distress, as measured by the HSCL-10, was registered at admission and at discharge among 164 substance users in inpatient treatment in Northern Norway. Predictors of reduction in mental distress were examined utilizing hierarchical regression analysis.ResultsWe found a significant reduction in mental distress in the sample, but the number of patients scoring above cut-off on the HSCL-10 at discharge was still much higher than in the general population. A more severe use of substances as measured by the AUDIT and the DUDIT, and being female, predicted a higher level of mental distress at admission to treatment as well as greater reduction in mental distress during treatment. Holding no education beyond 10 year compulsory school only predicted a reduction in mental distress.ConclusionsThe toxic and withdrawal effects of substances, level of education as well as gender, contributed to the differences in change in mental distress during treatment. Regression to the mean may in part explain some of the findings.