OBJECTIVES: This project evaluated a therapeutic interchange program of a pharmacy benefit management firm that initiated an amlodipine to felodipine therapeutic interchange program. The purpose of this study was to investigate the effect of switching hypertension therapy in a group of managed care patients on health-related quality of life, patient satisfaction, and prescription utilization and cost. METHOD: Three groups of managed care patients were prospectively selected from five large primary care medical groups in Texas and Oklahoma during the last six months of 1997. A study group of patients undergoing interchange (switching from amlodipine to felodipine), and two comparison groups of patients not undergoing a medication change were used (continuous amlodipine and continuous felodipine). Baseline (before a planned interchange) patient questionnaires were mailed to a total of 802 patients (Groups 1-3) and eight-week follow-up (after the interchange) patient questionnaires were mailed to 295 patients who responded to the first survey. The patient questionnaires included the SF-36, symptom measures, and satisfaction with care. Prescription claims data were used for the utilization and cost analysis. RESULTS: Patients maintaining therapy on felodipine continuously were older than amlodipine or switched patients and were more likely to be female. Switch patients had lower total prescription utilization after the switch compared to the continuous-use groups. Switch patients also had fewer hypertension-related prescriptions after the switch. There were no statistically significant differences in quality of life. It appeared that switch patients were more likely to change hypertension medications after their initial switch. CONCLUSIONS: The clear difference in patient characteristics between the treatment groups implies an important selection bias. These differences may mean that patients subject to the therapeutic interchange were selected for clinical or other reasons not immediately apparent. Subsequent analyses should control for these effects.