Differential medication adherence among patients with schizophrenia and comorbid diabetes and hypertension

Objectives: Global patient characteristics may affect adherence across all medications in a regimen, making medication-specific risk factors for adherence problems less important. Medication adherence was examined among patients with schizophrenia and comorbid physical conditions for consistency across therapeutic classes. Methods: A national sample of veterans was selected according to use of medication for schizophrenia, diabetes, and hypertension (N=1,686). Adherence to each medication type was assessed with medication possession ratios (MPRs). Multilevel logistic models were used to study the impact of medication type on adherence, as well as the effect of other medication characteristics (such as the average days of medication supplied per refill), health service use, and patients' sociodemographic characteristics. Results: Adherence was only modestly correlated across types of medication. Information about antipsychotic adherence explained only 13% and 16% of the variance in patients' antihypertensive and hypoglycemic MPRs, respectively. In unadjusted analyses, patients were more likely to have poorer adherence (MPR less than .8) to their antipsychotics (35%) than to their hypoglycemic (29%) or antihypertensive medications (26%) (p <.001). However, when analyses controlled for the average days' supply and other regimen characteristics, hypoglycemic and antihypertensive medications were associated with an increased risk of poor adherence relative to antipsychotics (both adjusted odds ratios=1.5, p <.001). Conclusions: Patients with schizophrenia and comorbid physical conditions demonstrated important differences in adherence across medications in their regimen, reinforcing the importance of medication-specific factors in determining adherence behavior. The lower levels of adherence observed for antipsychotics may be associated with the shorter refill intervals for these medications.