This project will examine current trends in the use of mail order pharmacies in Medicare Part D and its potential impact on costs to the program and beneficiaries. The project will quantify the use of mail order pharmacies in Medicare Part D over time and examine if product selection differs between mail order and retail pharmacies. The project will compare the costs associated with dispensing multisource drugs in mail order versus retail pharmacy and the rate at which branded drugs were dispensed over a generic alternative using dispense-as-written (DAW) codes, which document if a branded product was dispensed as a generic or as written by the provider. The project also will investigate if rebate size drives this behavior. The researchers will use Medicare Part D pharmaceutical claims data between 2014–2019 and SSR Health data on drug rebates.