Does retention matter? Treatment duration and improvement in drug use

AIM: This study examines whether there is a minimum threshold, continuous or non-linear relationship between the duration of addiction treatment and improvements in drug use. DESIGN: Longitudinal cohort study of 62 drug treatment units and 4005 clients in the US National Treatment Improvement Evaluation Study, fielded from 1993 to 1995. SUBJECTS: Baseline and 1-year follow-up interviews with clients in methadone maintenance, out-patient non-methadone, short-term residential and long-term residential treatment programs. MEASURES: Improvement in drug use is the difference between the client-reported peak frequency of drug use (in days per month) in the year prior to the baseline interview minus the peak frequency in the year after discharge. Primary drug, and overall use of the major illicit drugs (heroin, cocaine powder, crack cocaine, and marijuana) are considered separately. RESULTS: Controlling for multiple factors, treatment duration had a positive linear relationship with primary drug use improvement among methadone clients and an inverted-U-shaped relationship with overall and primary drug use improvements among out-patient and long-term residential clients. Improvement with longer duration is greatest for long-term residential clients. CONCLUSIONS: Contrary to previous arguments for a sharp retention threshold for onset of treatment effects, we find smooth curves relating treatment duration to drug use improvements in methadone maintenance, out-patient non-methadone and long-term residential modalities. These relationships are effectively linear for durations typically observed in single treatment episodes, but unusually long retention in out-patient non-methadone and long-term residential units appear steadily less predictive of improvement.