BACKGROUND: Formulary restrictions—non-coverage, prior authorization, and step therapy—are increasingly employed by Medicare Part D plans. OBJECTIVES: To evaluate effects of formulary restrictions on utilization and costs of oral hypoglycemic agents (OHAs), statins, and renin-angiotensin system (RAS) antagonists among low-income subsidy (LIS) recipients who were randomly assigned to Part D plans.  METHODS: We chose to study the randomized LIS recipients in order to: 1) remove the confounding effects of plans’ cost-sharing rules on study outcomes (LIS recipients pay the same nominal copays); and 2) take advantage of the random plan assignment to reduce selection bias. The study outcomes included generic dispensing rate (GDR), mean cost per prescription, and proportion of days covered (PDC). A 5% sample of 2012 Medicare administrative data was linked to a customized dataset that captured beneficiaries’ histories of plan assignment, dating back to 2006. Random intercept regression models were estimated. RESULTS: Three study cohorts were selected, including 28,082 users of OHAs, 53,864 users of statins, and 57,289 users of RAS antagonists. After covariate adjustment, beneficiaries subject to formulary restrictions on brand-name pioglitazone and single-source brand-name DDP-4 inhibitors (saxagliptin, sitagliptin, and sitagliptin-metformin) had 3.0 percentage-points higher GDR, $10.8 lower cost per prescription fill but similar PDC compared to those who faced no restrictions. Restricting access to brand-name atorvastatin and single-source brand-name statins (rosuvastatin and ezetimibe-simvastatin) was associated with 14.9 percentage-points higher GDR and $29.6 lower cost per prescription fill but had no impact on PDC. Restricting use of single-source brand-name RAS antagonists (olmesartan, valsartan, and valsartan-hydrochlorothiazide) was associated with 15.0 percentage-points higher GDR, $27.2 lower cost per prescription fill, and 1.3 percentage-points lower PDC. CONCLUSIONS: Placing formulary restrictions on single-source brand-name medications shifts utilization toward generic drugs and has minimal impact on adherence for OHAs, statins and RAS antagonists among LIS recipients in Part D plans.