Temporal associations between medication adherence for patients with schizophrenia and opioid dependence: A 17-year Canadian Cohort Study

Abstract

The current study investigated whether a previously reported beneficial effect of methadone maintenance therapy (MMT) on antiretroviral adherence is also present in relation to antipsychotic treatment for schizophrenia. Administrative data were linked over a 17-year period for 1996 people who were dually diagnosed with schizophrenia and opioid dependence and, as an indicator of further marginalization, experienced at least one episode of correctional supervision in British Columbia. Adherence was estimated using the medication possession ratio (MPR\u202f≥\u202f0.80), calculated in each 120-day period beginning with the first date of concurrent use of MMT and antipsychotic medication. Generalized Estimating Equations were used to estimate the association between independent and dependent variables. The probability of antipsychotic adherence doubled in periods that were preceded by a period of MMT adherence (AOR: P: 2.07; 95% CI: 1.90-2.26). Subgroup and sensitivity analyses yielded results similar to those derived through the primary analysis, examining: conviction history; length of follow-up; initiation of MMT prior to antipsychotic induction; excluding participants who died during the study period; and restricted to participants who received methadone exclusively as part of a MMT program. Despite a strong temporal association between MMT and antipsychotic adherence, overall MPRs for both prescriptions remained <0.50 throughout the study period. Antipsychotic adherence was more than twice as likely following periods of adherence to MMT among dually-diagnosed patients. Research is needed to identify the conditions responsible for MMT adherence, and to further clarify the relationship between opioid agonist treatment and antipsychotic pharmacotherapy in this vulnerable and under-studied population.