Background and methods

Opioid use disorder (OUD) is a significant contributor to morbidity and premature mortality and represents a major public health problem worldwide. Drug overdose is the leading cause of death among people with OUD and interventions such as opioid agonist treatment (OAT) have been widely implemented to circumvent preventable fatalities within this population.1 Methadone maintenance treatment (MMT) is the most common OAT in Ireland and has been shown to be cost-effective, safe, and beneficial in reducing risk behaviour and improving health and social outcomes.

Despite the strong evidence base supporting the use of MMT, during the first four weeks of treatment initiation and following treatment cessation, the mortality risk for clients remains high.2 This risk is reduced if individuals are retained in MMT, therefore, efforts to improve retention are essential. A 2020 systematic review demonstrated that retention rates in MMT decrease over time, with most dropouts occurring early in treatment, and a number of factors that influence retention were identified.3 However, despite clients often transitioning in and out of treatment, most of the studies included did not consider these risk factors as time-varying covariates and also failed to model the time to dropout of successive treatment episodes as recurrent event data.

In a 2021 research paper, published in the journal Drug and Alcohol Dependence,4 the authors conducted an observational cohort study of individuals who had experienced at least one episode of MMT lasting greater than seven days in Irish specialist addiction services (Dublin Southwest and Kildare) between 1 January 2010 and 31 December 2015. Data were gathered through the Central Treatment List and the Methadone Treatment Scheme. Client records were also linked to the General Medical Services (GMS) pharmacy claims database and the National Drug-Related Deaths Index. Using a statistical model, this study identified determinants of time to dropout of MMT at three months and at 12 months across multiple treatment episodes.4

Results

The 2,035 clients included in the study experienced a total of 4,969 MMT episodes over the observation period. The median age was 34 years at the time of initial treatment episode and 68% were men. Almost 42% of clients received a median methadone dose below the recommended optimal range of 60–120 mg/day. The study observed:

• 2,724 dropout events occurred during the six-year observation period.
• 82.8% of all dropouts occurred within the first 12 months of treatment.
• 49.7% of dropout events occurred in the first three months.

Dropout at three months was associated with low dose methadone (<60 mg/day) and a history of previous dropout. Adherence to treatment, defined as not missing doses over the previous 30 days, was shown to be protective. Low dose methadone and previous dropout remained as increased risks of dropout at 12 months. Being male, having a prescription for benzodiazepines, and a higher number of comorbidities were also identified as additional risk factors in the longer-term model. Once again, adherence was protective at this time point.

Conclusions

The factors identified in this study could serve as indicators to risk stratify clients and provide service enhancements to increase engagement and retention in treatment. Previous dropout or missing doses are suggestive of instability and these clients should be identified as high risk of both early and later dropout. It is noted that no study thus far has examined the persistent prescribing of low dose methadone, but it is suggested by the study authors that a review of prescribing practices should be considered. Adherence was protective at three and 12 months and should be actively encouraged. The authors recognise both the strengths and limitations of this current research and note that further testing and validation is required to build on the associations identified here in understanding the determinants of dropout from OAT.

2 Sordo L, Barrio G, Bravo MJ, et al. (2017) Mortality risk during and after opioid substitution treatment: systematic review and meta-analysis of cohort studies. BMJ, 357: j1550.

3 O’Connor AM, Cousins G, Durand L, Barry J and Boland F (2020) Retention of patients in opioid substitution treatment: a systematic review. PLoS One, 15(5): e0232086.
https://www.drugsandalcohol.ie/32009/

4 Durand L, Boland F, O’Driscoll D, et al. (2021) Factors associated with early and later dropout from methadone maintenance treatment in specialist addiction clinics: a six-year cohort study using proportional hazards frailty models for recurrent treatment episodes. Drug Alcohol Depend, 219: 108466. https://www.drugsandalcohol.ie/33611/