In this 17-year Canadian cohort study, researchers studied individuals aged ≥18 years diagnosed with schizophrenia (ICD-9 code 295) who were treated concurrently with antipsychotics between January 1998 and March 2015 (N=1996). Adherence to methadone maintenance therapy was the primary independent variable, operationalized using the medication possession ratio. The primary dependent variable was antipsychotic adherence, operationalized using medication possession ratio ≥80%, calculated in each 120-day period beginning with the date of concurrent use of methadone maintenance therapy and antipsychotic medication.
A clear temporal relationship between methadone maintenance therapy and antipsychotic adherence was observed. In the first cycle of follow-up, 1022 individuals (51.2%) were adherent. Subsequent to that, 916 individuals (45.9%) were adherent to methadone maintenance therapy. Considering the entire period, 562 individuals (28.2%) continuously met methadone maintenance therapy adherence criteria. The researchers note that many patients being treated with methadone maintenance therapy were concurrently prescribed antipsychotic medications for conditions other than schizophrenia (78.4%).
This study was limited by the definition of adherence that used the medication possession ratio, which was based on pharmacy refill records. Without access to biomarkers, these results provide limited evidence that opioid agonist treatment may promote antipsychotic adherence. They also show that antipsychotic adherence was twice as likely in periods preceded by a period of methadone maintenance therapy adherence, which is consistent with studies reporting that adherence to opioid agonist treatment improves pharmacotherapy of comorbid non-psychiatric conditions.
The researchers indicated that the use of opioid agonist treatment medication possession ratio is a valid predictor of antipsychotic adherence. Integrating methadone maintenance therapy and antipsychotics may help ensure higher overall adherence to both medications. Improvements to treatment and outcomes for patients with comorbidities are clearly achievable through psychosocial care.
Rezansoff SN, Moniruzzaman A, Somers JM. Temporal associations between medication adherence for patients with schizophrenia and opioid dependence: A 17-year Canadian cohort study [published online June 12, 2019]. Schizophr Res. doi: 10.1016/j.schres.2019.05.031