Factors related to antipsychotic oversupply among Central Texas Veterans

There have been many studies of underadherence to antipsychotics, but antipsychotic overadherence, or medication oversupply, in which patients receive more prescription medications than are needed, has been overlooked. Both underadherence and oversupply can have an important impact on clinical outcomes.

This study examined adherence (based on the medication possession ratio [MPR]) among patients treated with antipsychotics in the Central Texas Veterans Health Care System (CTVHCS) and investigated factors associated with their adherence status.

Data from September 1995 to October 2002 were extracted from the computerized patient record system of the CTVHCS for continuously enrolled adult outpatients receiving antipsychotic monotherapy and filling at least 2 prescriptions within a year of the index date. Patients' prescription records were tracked for up to 12 months. Underadherence was defined as an MPR <0.8, good adherence as an MPR from 0.8 to 1.2, and oversupply as an MPR >1.2.

Of 3268 eligible patients, 49.9% had good adherence, 42.6% were underadherent, and 7.6% had medication oversupply. The overall mean (SD) MPR was 0.83 (0.33). Multinomial logistic regression analysis revealed that compared with patients with good adherence, underadherent patients were significantly more likely to be nonwhite (P < 0.001), younger (P < 0.01), and receiving chlorpromazine therapy (P < 0.05), and were less likely to be receiving fluphenazine (P < 0.01), olanzapine (P < 0.05), or risperidone (P < 0.05). Patients with medication oversupply were significantly more likely to be receiving olanzapine (P < 0.001), quetiapine (P < 0.01), or risperidone (P < 0.05) than those with good adherence.

Although half of adult outpatients receiving antipsychotic monotherapy in the CTVHCS were adherent to their treatment regimens, a large proportion were underadherent, and a small proportion had medication oversupply. Patients receiving second-generation antipsychotics were more likely to be adherent and were more likely to have medication oversupply than patients receiving first-generation antipsychotics.