This is a register-based cohort study of 20,581 individuals in treatment for illicit substance use disorders in Denmark between 1996 and 2006. All in all, 1441 deaths were recorded during 111,445 person-years of follow-up. Standardized mortality ratios (SMRs) associated with different primary substance types were calculated and Cox-regression analyses were performed in order to establish hazard ratios (HR) associated with injection drug use and psychiatric comorbidity. SMRs for primary users of specific substances were: cannabis: 4.9 (95% confidence interval (CI): 4.2-5.8), cocaine: 6.4 (CI: 3.9-10.0), amphetamine: 6.0 (CI: 4.2-8.3), heroin: 9.1 (CI: 8.5-9.8), and other opioids 7.7 (CI: 6.6-8.9). For MDMA ('ecstasy') the crude mortality rate was 1.7/1000 person-years (CI: 0.4-7.0) and the SMR was not significantly elevated. Injection drug use was associated with significantly increased hazard ratios in users of opioids and cocaine/amphetamine. Overall, psychiatric comorbidity was not associated with increased mortality (HR: 1.1 [CI: 0.9-1.2], p=.28), but an association was found specifically among cocaine/amphetamine users (HR: 3.6 [CI: 2.1-6.4], p<.001).

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