OBJECTIVES: This study compares the fill patterns among patients with and without 90-day prescriptions at a national retail pharmacy chain. Specifically, the objective of this study is to determine whether 90-day supplies of Proton-Pump Inhibitors (PPI) and Statins are associated with higher rates of patients changing from single source brand (SSB) medications to generic alternatives, and to determine the cost implications to patients and their benefit plans.  METHODS: This retrospective cohort study includes a random sample of patients with 1 or more fills for SSB PPIs (N=96,927) or Statins (N=26,057) at Walgreens pharmacy in the first quarter of 2011. Study patients are assigned to the 90-day group, if they had a 90-day fill of PPI or Statin within 270 days of their index fill of SSB PPI or Statin, otherwise they are assigned to the 30-day group.The primary outcome measure is the change rate (CR) from SSB to generic medications within 3 fills of the index date. RESULTS: The 90-day group has a significantly higher CR than the 30-day group:  9.3% vs. 5.8% for PPI (p <0.0001) and 10.3% vs. 7.5% for Statin (p value <0.0001).  Co-pay savings per drug change were considerably higher for the 90-day group than the 30-day group ($46 vs. $30 for PPI and $39 vs. $27 for Statin); plan savings per drug change were also higher for the 90-day group ($259 vs. $121 for PPI and $164 vs. $83 for Statin). CONCLUSIONS: Compared with the 30-day group, patients in the 90-day group were observed to have a significantly higher rate of change from SSB drugs for generic drugs in both the PPI and Statin therapeutic classes. These physician approved medication changes translate into significant savings for both patients and their benefit plans.