Do interruptions to the continuity of methadone maintenance treatment in specialist addiction settings increase the risk of drug-related poisoning deaths? A retrospective cohort study.

AIMS

To examine the risk of mortality associated with interruptions to the continuity of methadone maintenance treatment (MMT), including transfers between services, in opioid-dependent individuals attending specialist addiction services.

DESIGN

Retrospective cohort study using addiction services and primary care dispensing records, the National Methadone Register and National Drug-Related Death Index (NDRDI).

SETTING

Geographically defined population in Dublin, Ireland.

PARTICIPANTS

A total of 2899 people prescribed and dispensed methadone in specialist addiction services between January 2010 and December 2015. There were five exposure groups: weeks 1-4 following transfer between treatment providers; weeks 1-4 out of treatment; weeks 5-52 out of treatment; weeks 1-4 of treatment initiation; and weeks 5+ of continuous treatment (reference category).

MEASUREMENTS

Primary outcome: drug-related poisoning (DRP) deaths. Secondary outcome: all-cause mortality (ACM). Mortality rates calculated by dividing number of deaths (DRP; ACM) in exposure groups by person-years exposure. Unadjusted and adjusted Poisson regression (covariates age, sex, incarceration, methadone dose and comorbidities) estimated differences in mortality rates.

FINDINGS

There were 154 ACM deaths, 55 (35.7%) identified as DRP deaths. No deaths were observed in the first month following transfer between treatment providers. The risk of DRP mortality was highest in weeks 1-4 out of treatment [adjusted relative risk (aRR = 4.04, 95% confidence interval (CI) = 1.43-11.43, P = 0.009] and weeks 1-4 of treatment initiation (ARR = 3.4, 95% CI = 1.2-9.64, P = 0.02). Similarly, risk of ACM was highest in weeks 1-4 out of treatment (ARR = 11.78, 95% CI = 7.73-17.94, P < 0.001), weeks 1-4 of treatment initiation (aRR = 5.11, 95% CI = 2.95-8.83, P < 0.001) and weeks 5-52 off treatment (aRR = 2.04, 95% CI = 1.2-3.47, P = 0.009).

CONCLUSIONS

Interruptions to the continuity of methadone maintenance treatment by treatment provider do not appear to be periods of risk for drug-related poisoning or all-cause mortality deaths. Risk of drug related poisoning and all-cause mortality deaths appears to be greatest during the first 4 weeks of treatment initiation/re-initiation and after treatment cessation.