Plan designs that encourage the use of generic drugs over brand-name drugs: an analysis of a free generic benefit.

Objective: To evaluate the effect on adherence and medical care expenditures of a pharmacy benefit change that included free generic drugs and higher copayments for brand-name drugs.

Study Design: Quasi-experimental pre-post study of patients with ischemic heart disease (1286 control and 555 intervention) and patients with diabetes mellitus (4089 control and 1846 intervention).

Methods: Medical and pharmacy claims data were analyzed for continuously enrolled members from January 1, 2005, through December 31, 2008.