Background: Formulary switches between agents in the same therapeutic class have become commonplace in the managed care setting as a strategy to reduce costs.
Objectives: We evaluated the impact of a formulary switch from conjugated to esterified estrogen tablets at the Fallon Community Health Plan, a mixed-model health maintenance organization.
Design: A retrospective study was conducted with the use of the automated database of the health plan.
Subjects: Study subjects were members of the health plan during the period from May 1, 1995, to December 31, 1997, who were dispensed > or =1 estrogen replacement product. From this population, a cohort of users of conjugated estrogens during the period from May 1, 1995, to October 31, 1995, was selected.
Measures: The cumulative incidence of switching from conjugated to esterified estrogen tablets and subsequent discontinuations of esterified estrogens was evaluated. The frequencies of ambulatory encounters during the 6 months before and after a switch or discontinuation were compared.
Results: During the period after promotion of the formulary switch, 2,149 of 2,984 patients (72%) originally dispensed conjugated estrogen tablets switched to esterified estrogen tablets. Among those patients switching to esterified estrogens, an excess of 20 office visits per 100 patients was noted in the postswitch period (P = 0.005). The risk of switching back to conjugated estrogen tablets was 15% by 2 years.
Conclusions: The findings of this study suggest that plan efforts were successful in switching most users of conjugated estrogens to esterified estrogens. The switch was associated with an increase in utilization of health care services.