Generic initiation and antidepressant therapy adherence under Medicare Part D.

OBJECTIVES: To assess the effect of initiating antidepressant therapy with a generic prescription on adherence to antidepressant therapy among Medicare patients. A second objective is to assess how the effect might be moderated by the Medicare Part D coverage gap. STUDY DESIGN AND METHODS: Adherence to antidepressant therapy was measured by (a lack of) disruption in medication use defined by a gap of 30 days or more in antidepressant possession and monthly days of possession, both measured over 180 days since antidepressant initiation. We used a 5% random sample of Medicare fee-for-service beneficiaries who received a new depression diagnosis in the first half of 2007 and initiated antidepressant therapy within 60 days (n = 16,778). We estimated a Cox proportional hazard model for antidepressant disruption and a mixedeffects linear model for monthly possession. All analyses were stratified by 4 cohorts defined by Part D low-income subsidy (LIS) status and Medicare entitlement (aged vs disabled).
RESULTS: Generic initiation was associated with improved adherence among all 4 cohorts, with a stronger effect among the non-LIS patients. Hazard ratios for antidepressant disruption ranged from 0.71 (95% confidence interval [CI], 0.53-0.96) among non-LIS, disabled patients to 0.88 (95% CI, 0.79-0.98) among LIS, aged patients. Generic initiation was associated with increases in days of monthly possession in all 4 cohorts and an additional benefit during the coverage gap for non-LIS patients.
CONCLUSIONS: Generic initiation can be an important tool to improve adherence to antidepressant treatment among Medicare patients and to mitigate the negative effects of the Part D coverage gap.