OBJECTIVES: To evaluate the effectiveness of a zero dollar copay tier by measuring the proportion of days covered for Medicare beneficiaries. The 2015 average PDC for zero dollar copay medications will be compared against the average PDC for the same medications in 2014. Further stratification will determine if the same relationship holds true for all drug classes recognized by CMS adherence measures. METHODS: Beneficiaries were included in the study if they received at least two fills, in either 2014 or 2015, of a CMS STAR medication listed on the 2015 zero dollar copay tier and plan eligible for the entire calendar year when claims were filled. Average PDC score regardless of drug class was compared between 2014 and 2015 using a random sample and the application of a t-test. Finally, a random sample of beneficiaries’ PDC scores per drug class per year were compared using t-tests to determine if the implementation of a zero dollar copay tier resulted in significantly higher adherence scores. RESULTS: The results of the t-tests showed a significant difference (P < .0001) between the mean PDC scores for 2014 and 2015, 87.1% in 2015, 84.8% in 2014. Zero dollar statin medications displayed a significant difference (P < .0001) with a 2.3 % increase in mean PDC score, 86.9% in 2015, and 84.6% in 2014. RASA medications displayed a significant difference (P < .0001) with a 1.8 % increase in mean PDC score, 87.5% in 2015 and 85.7% in 2014. Diabetes medications displayed a 1.6% increase in mean PDC score, 85.4% in 2015 and 83.8% in 2014 (P < .0001). CONCLUSIONS: Lowering the cost of access to medications can have a significant impact on a beneficiary’s adherence. By allowing an individual to have their medication without out-of-pocket expenses, adherence scores increased in select CMS STAR medications.