OBJECTIVES: Numerous studies have examined prescription utilization and medication adherence using continuous enrollment criteria with claims data. Benefit design changes may have an impact on both measures. The purpose of this analysis was to indentify the multiple changes of benefit plan designs that occurred in a commercially insured population continuously enrolled for 30 months. METHODS: Retrospective study examining eligibility data and health plan benefit design records. Commercially insured beneficiaries aged 18 or older and enrolled from January 1st, 2011 through June 30th, 2013 were included. The benefit designs for all beneficiaries were identified from administrative records. RESULTS: