PROJECT SUMMARY/ABSTRACT This project seeks to determine whether 3 natural experiments that reduce out-of-pocket costs for diabetes patients improve acute and chronic diabetes complications over the longer term. These natural experiments include: (1) employer adoption of preventive drug lists (PDLs) that make diabetes-related medications $0, (2) state legislation capping patient out-of-pocket costs for insulin, and (3) employer transitions from high- deductible health plans (HDHPs) with substantial financial barriers for most services to generous low- deductible plans. Results could provide evidence about actionable interventions at multiple levels that improve the health of diabetes patients. Studies will leverage a large, national health insurance claims database from 2007-2023 that the research team has refined under previous projects. The Specific Aims of the project are to: 1. Assess 15-year national trends in (a) diabetes complications and (b) uptake of federal, state, and employer natural experiments of interest. Analyses will assess 2009-2023 trends in acute preventable and chronic microvascular and macrovascular complications, including stratification by key subgroups of interest. Measures related to natural experiment uptake will comprise annual PDL adoption, state-level insulin cost trends, and HDHP to low-deductible plan switches. These analyses will use a longitudinal, calendar time series design with joinpoints, adjusted for changing demographics, socio-economic status, and co-morbidity. 2. Determine if reducing drug out-of-pocket costs improves diabetes complications. Over 4 follow-up years, Aim 2a will examine whether employer switches from health savings account (HSA)-HDHPs with high drug out-of-pocket costs to PDL HSA-HDHPs with $0 preventive drugs reduce acute and chronic diabetes complications. Aim 2b will assess these outcomes before and after approximately 10 US states limit insulin cost sharing. Analyses will use a highly rigorous interrupted time series with control series study design and Aim 2a will also apply a segmented survival with control group design. 3. Determine if reducing outpatient and emergency department out-of-pocket costs improves diabetes complications. Analyses will assess if switching from HDHPs with high cost sharing for outpatient and emergency department care to generous low-deductible plans reduces the risk of acute and chronic diabetes complications over 4 follow-up years. This aim will also use rigorous controlled interrupted time series and segmented survival designs. 4. Examine the long-term impact of reduced out-of-pocket costs on key diabetes subgroups including low-income and high-morbidity members. Aim 4 will involve stratifying Aims 2 and 3 analyses by characteristics such as income, morbidity, race/ethnicity, and diabetes severity. This aim will use the same study designs as for Aims 2 and 3 but stratified by the factors of interest.