Insulin Costs and Use by Medicare Beneficiaries After the Inflation Reduction Act Out-of-Pocket Cap.

Importance: The Inflation Reduction Act capped out-of-pocket costs for insulin at $35 per 30-day supply for Medicare beneficiaries in 2023; the impact on insulin access and use is not known. Objective: To measure changes in insulin cost and use after the cap, overall and for insulin users with previously high out-of-pocket cost. Design, Setting, and Participants: Interrupted time series analysis using data from the United States from January 2021 through December 2023 to measure the additional changes experienced by insulin users with Medicare Part D insurance after the cap (2023) compared with before the cap (2021-2022). Exposure: The $35 out-of-pocket cap. Main Outcomes: Out-of-pocket cost per 30-day insulin supply (average and within-year range), 30-day insulin fills, adherence (proportion of days covered with basal insulin), and persistence (no days without basal insulin). Results: The study cohort included 2 860 394 insulin users (mean age, 70.7 years; 52% female). In 2023, almost no insulin fills exceeded the cap; in comparison, 13% of fills would have exceeded the cap in 2021-2022. Cost per 30-day supply of insulin was $22.95 before the policy and decreased to $18.16 after the policy (difference, -$4.79 [95% CI, -$4.84 to -$4.74]; relative reduction, 21%), and the within-year range declined from $22.73 to $11.77 (difference, -$10.96 [95% CI, -$11.07 to -$10.84]; relative reduction, 48%). Insulin fills, adherence, and persistence to basal insulin did not increase for the full cohort but did for people with high baseline cost for insulin. For people with baseline costs in the top decile (ie, above $58 on average per 30-day supply of insulin), 30-day insulin fills per year increased from 10.4 to 11.2 (difference, 0.8 fills [95% CI, 0.7 to 0.8]; relative increase, 8%), proportion of days covered increased from 59.7% to 62.4% (difference, 2.7 percentage points [95% CI, 2.5 to 2.8 percentage points], relative increase, 5%), and persistence increased from 7.5% to 8.5% (difference, 1 percentage point [95% CI, 0.8 to 1.2 percentage points]; relative increase, 13%). Conclusions and Relevance: The Medicare $35 insulin out-of-pocket cap decreased and stabilized insulin out-of-pocket cost, increasing insulin use for people with previously high out-of-pocket cost. These findings can inform the design of policies to boost insulin access among users with high out-of-pocket cost.