How temporal patterns of medication adherence to antidepressants, bisphosphonates and statins are associated with healthcare cost

We evaluate benefits of measuring temporal patterns of medication use using group-based trajectory modelling (GBTM) by finding associations between medication adherence (MA) and healthcare costs for which inconsistent results have been reported. We conducted a retrospective cohort study of 9,287, 1,660 and 10,242 users of antidepressants, bisphosphonates and statins, respectively, between 2012 and 2016, who participated in the 45 and Up Study in New South Wales, Australia. Associations between MA and subsequent 1-year healthcare cost components from generalized linear models and two-part models were compared across medications and MA measure types including GBTM and proportion of days covered (PDC). Compared to adherers, antidepressant users with declining adherence showed increased inpatient costs ($643 at p < 0.05). Bisphosphonate users with declining and consistently low adherence showed decreased osteoporotic inpatient costs ($387 at p < 0.05 and $419 at p < 0.05, respectively). All types of nonadherence were associated with reduced cost of the respective medications. GBTM revealed relationships between specific nonadherence types and healthcare costs, even in cases where no relationship was identified using the combined nonadherence PDC measure. With additional insights into the dynamics of nonadherence, GBTM could help clinicians and health policymakers design effective MA interventions, leading to improved health outcomes and pharmaceutical use.