This study tracks the diffusion of generic fluoxetine after its release in August 2001 within the largest U.S. pharmacy benefit manager (PBM). Within two weeks of the generic's release, prescriptions exceeded those of brand-name Prozac. The main winners proved to be Barr Laboratories, the first entrant to the generic market; large purchasers, who reaped substantial cost savings after Barr's period of exclusivity expired; and the PBM. The major loser was Eli Lilly, the manufacturer of Prozac. Consumers and makers of other antidepressants largely remained on the sidelines, with surprisingly little short-term impact evident from Prozac's patent expiration.