OBJECTIVES: Lipitor is one of the most commonly prescribed statins in the United States. The approval of generic Zocor (simvastatin) by the Food and Drug Administration on 06/23/2006, brought about the question of how this product may influence the switching pattern on existing Lipitor patients. METHODS: A large employer based pharmacy claim database was used in this study from 01/01/2005 to 12/31/2006. The impact of simvastatin on existing Lipitor patients was evaluated as follows. The rate ratio was computed in order to assess Lipitor switching. Those patients who were exclusively on Lipitor from 01/01/2005 to 06/30/2005 were included in control group and those exclusively taking Lipitor between 01/01/2006 and 06/30/2006 were selected in study group. Patients in both groups were followed-up for an additional six months and their respective switching rates were computed and compared using Chi-square analysis. RESULTS: A total of 24,137 control group patients and 23,869 study group patients were identified from administrative pharmacy claims database. About 865(3.6%) patients in the study group and 632(2.6%) patients in the control groups switched to alternative statins after their respective follow-up periods. Study group patients were 1.4 times more likely to switch to alternative statins than control group patients (P<0.001; 95% CI, 1.3-1.6). Specifically, the study group patients were 5.1 times (P<0.001; 95% CI, 4.0-6.6) more likely to switch to simvastatin, and 2.0 times (P<0.001; 95% CI, 1.5-2.6) more likely to switch to Crestor. In both groups, study group patients were 2.0 times (P<0.001; 95% CI, 1.5-2.8) less likely to switch to lovastatin and 1.3 times (P<0.001; 95% CI, 1.1-1.6) less likely to switch to vytorin. CONCLUSION: The results suggest that majority of the study group patients' tend to switch to simvastatin and their switching behavior could have resulted due to the fact that simvastatin became available as a generic version.