Impact of Cost Sharing on Therapeutic Substitution: The Story of Statins in 2006.

BACKGROUND: Cost sharing is widely used to encourage therapeutic substitution. This study aimed to examine the impact of increases in patient cost-sharing differentials for brand name and generic drugs on statin utilization on entry into the Medicare Part D coverage gap.

METHOD AND RESULTS: Using 5% Medicare Chronic Condition Warehouse files from 2006, this quasi-experimental study examined patients with hyperlipidemia who filled prescriptions for atorvastatin or rosuvastatin between January and March 2006. Propensity score matching and difference-in-difference regressions were used to compare changes in statin utilization for the study group (patients who were not eligible for low-income subsidies [non-LIS] and had generic-only gap coverage) to those of a control group (LIS patients who faced the same cost sharing before and during the Part D coverage gap). In the final sample, 801 patients in the study group were matched to 801 patients in the control group. We found that, compared to the control group, the study group had a larger decline in any monthly brand-name statin use (-0.24 30-day fills, P<0.001). This was only partially offset by increased monthly generic statin use (+0.06 30-day fill, P<0.001), with an overall drop in any monthly statin use (-0.18 30-day fills, P<0.001). Overall adherence with statins declined (OR 0.81, P<0.001), and statin discontinuation increased (OR 1.62, P<0.001) in the study group as compared to the control group.

CONCLUSIONS: Increases in cost-sharing differentials for brand name and generic drugs on coverage gap entry were associated with discontinuation of statins in Medicare Part D patients with hyperlipidemia.