Objectives: This study analyzed annual trends in the distribution of beneficiaries entering each benefit phase and the utilization of and expenditures for prescription drugs among Medicare Part D beneficiaries from 2008 to 2015.

Study design: Retrospective, repeated cross-sectional analysis using Medicare Current Beneficiary Survey data.

Methods: The study population included elderly Part D beneficiaries without a low-income subsidy, with continuous enrollment in a Part D plan, and with at least 1 prescription fill for a given year. We assessed annual trends for 3 outcomes: (1) proportion of beneficiaries entering each benefit phase and the number of days taken to enter these phases, (2) number of 30-day prescription drug fills, and (3) total and out-of-pocket spending on prescription drugs.

Results: The proportion of beneficiaries reaching the catastrophic coverage phase increased after the Affordable Care Act (ACA), and they reached the threshold earlier in the year. The overall number of 30-day drug fills increased over the study period, although no statistically significant changes in utilization were seen among those reaching the catastrophic coverage phase. Total drug spending steadily increased over time, particularly after the ACA, with the largest increase seen in those reaching the catastrophic threshold; however, out-of-pocket spending significantly decreased.

Conclusions: Although this study provides support for reductions in financial barriers to prescription drugs under the ACA, substantial increases in both total drug spending and the proportion of high-cost beneficiaries in the Part D program indicate a growing burden of Part D spending on the Medicare program, which is expected to continue to grow in the future.