OBJECTIVES: Our previous study suggested that Taiwan’s drug price adjustments result in prescription switching from cheaper angiotensin-converting enzyme inhibitors (ACEIs) to expensive angiotensin receptor blockers (ARBs), demonstrated by a macro level data that showed a decrease use in ACEIs and an increase use in ARBs after policy implementation and by a long-term increasing trend observed in annual prevalent ARB users that exceeded the cumulative incident ARB users. This study uses individual patient level data to examine the proportions of prescription switching from ACEIs to ARBs among the prevalent ARB users. METHODS: We identified 82,447 patients treated with ARBs between Feb 1998 and Dec 2008 from Taiwan’s Longitudinal Health Insurance Database (LHID2005). We examined the monthly prevalent users of ARBs (from 2/1998 - 12/2008), the incident users of ARBs (who receive ARBs first time in a given month from Feb 1998 to Dec 2008 and did not receive ACEIs or ARBs in the preceding months from 1997 to 2008), and the ACEIs-to-ARBs switchers (who used ARBs in a given month and used ACEIs or both drugs [ACEIs in combination with ARBs] in last month during 2/1998 - 12/2008). RESULTS: The number of monthly prevalent ARB users increased from 4 to 32,292 from Feburary 1998 to December 2008. The growth of ARB prevalent users was mainly contributed by the incident ARB users and the ACEIs-to-ARBs switchers. The number of monthly incident ARB users increased from 2 to 562 and that of monthly ACEIs-to-ARBs switchers increased from 1 to 477 from February 1998 to December 2008. The number of ACEIs-to-ARBs switchers exceeds that of incident ARB users (average monthly ratio of the former to the latter is 1.25). CONCLUSIONS: Prescription switching from cheaper ACEIs to expensive ARBs based on individual patient level data is evident. Policy makers should be aware of this problem.