Risk of fatal overdose during and after specialist drug treatment: the VEdeTTE study, a national multi-site prospective cohort study

Background: Specialist drug treatment is critical to overdose prevention; methadone maintenance is effective, but we lack evidence for other modalities. We evaluate the impact of a range of treatments for opiate dependence on overdose mortality. Methods: Prospective cohort study of 10,454 heroin users entering treatment 1998-2001 in Italy followed-up for 10,208 person-years in treatment and 2,914 person-years out of treatment. Standardized overall mortality ratios (SMR) estimate excess mortality risk for heroin users in and out of treatment compared to the general population. Cox models compare the hazard ratio (HR) of overdose between heroin users in treatment and out of treatment. Results: There were 41 overdose deaths, 10 during treatment and 31 out of treatment, generating annual mortality rates of 0.1% and 1.1% and SMRs of 3.9 [95% confidence interval (CI) 2.8-5.4] and 21.4 (16.7-27.4), respectively. Retention in any treatment was protective against overdose mortality (HR 0.09 95% CI 0.04-0.19) compared to the risk of mortality out of treatment, independent of treatment type and potential confounders. The risk of a fatal overdose was 2.3% in the month immediately after treatment and 0.77% in the subsequent period; compared to the risk of overdose during treatment the HR was 26.6 (95% CI 11.6-61.1) in the month immediately following treatment and 7.3 (3.3-16.2) in the subsequent period. Conclusions: We demonstrate that a range of treatments for heroin dependence reduces overdose mortality risk. However, the considerable excess mortality risk in the month following treatment indicates the need for greater health education of drug users and implementation of relapse and overdose death prevention programmes. Further investigation is needed to measure and weigh the potential benefits and harms of short-term therapies for opiate use.