OBJECTIVES: To investigate statin usage patterns in terms of termination, switch and/or augmentation, and to associate these patterns with changing market conditions. METHODS: The study is based on a 20% sample of California Medicaid (Medi-Cal) fee-for-service claims on statin prescriptions from 1995 through 2002. The length of the first statin therapy phase is presented with Kaplan-Meier curves. The switch and/or augmentation pattern is studied by tracking each patient for 12 months from the date of the first fill (index date). Results are presented by the year of index date. There are six statins included in the study: Cerivastatin, Fluvastatin, Atorvastatin, Lovastatin, Pravastatin and Simvastatin. RESULTS: Atorvastatin (N = 15,686, Median survival days = 244) and Simvastatin (N = 9,162, Median survival days = 200) had longer therapy phases while Cerivastatin (N = 2,022, Median survival days = 101) and Lovastatin (N = 8,910, Median survival days = 99) had shorter therapy phases. About 40% of Atorvastatin and Simvastatin patients were still on their initial medication after one year, but only 20% of patients initiated on Lovastatin or Cerivastatin remained on their medication. The introduction of Atorvastatin was associated with a sudden increase of switch/augmentation events among patients initiated on other statin brands. When Cerivastatin was withdrawn from the US market and Lovastatin was phased out of the Medi-Cal formulary, the proportion of switch/augmentation events also increased. CONCLUSIONS: The introduction of Atorvastatin, the withdrawal of Cerivastatin and the phase-out of Lovastatin may be key factors in statin persistence patterns. Market forces should not be overlooked when analyzing medication compliance and medication usage patterns. Because changes in the market are more likely to be unique events, their effect may overshadow other adjustments in comparison of medication compliance.