OBJECTIVES: The CMS Five Star Rating system for Medicare Advantage (MA) informs beneficiaries about plan performance and determines Quality Bonus Payments.  High Risk Medication (HRM) use is a triple weighted measure defined as the percent of beneficiaries aged 65+ who received two or more fills for a drug with high risk of serious side effects in the elderly.  This analysis evaluates differences in HRM use between dual eligible (DE) and non-DE members and examines the contribution of socio-demographic and clinical characteristics to observed disparities. METHODS: The study used a nationally representative administrative claims database of 1.5 million MA members in 2011-2012 (measured in member years): 232,273 DE (female: 66.3%; average age 75.4) and 1,251,145 non-DE (female 57.4%; average age 75.2).  The Linear Probability Model (LPM) and Blinder-Oaxaca decomposition techniques with Neumark weighting formula were performed. RESULTS: HRM rate was 32.2% higher in DEs (16.8% vs. 12.7%).  The decomposition analysis found member characteristics accounted for only 48% of the performance gap (“explained gap”); 52% was attributed to differential effects of member characteristics on HRM use  (“unexplained gap”).  The Charlson Severity Score indicates more complex comorbidities in DEs (2.34 vs. 1.77) and explained 70.6% of the difference in HRM use; disability as original reason for entitlement explained 23.3%.  Members of a Preferred Provider Organization (PPO) and older members were less likely to use HRMs, while females and low income members were associated with higher use.   CONCLUSIONS: This study provides information about the contribution of socio-demographic and clinical characteristics to higher use of HRMs in DEs, and can support targeted interventions to reduce the performance gap.  The analysis further demonstrates that more than half the disparity is not explained by the member characteristics evaluated and points to the need for further research to understand the factors behind the unexplained DE gap.