Evaluation of a formulary switch from conjugated to esterified estrogens in a managed care setting

BACKGROUND. Formulary switches between agents in the same therapeutic class have become commonplace in the managed care setting as a strategy to reduc e costs. OBJECTIVES. We evaluated the impact of a formulary switch from conjugated t o esterified estrogen tablets at the Fallen Community Health Plan, a mixed- model health maintenance organization. DESIGN. A retrospective study was conducted with the use of the automated d atabase 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 greater than or equal to 1 estrogen replacement product. From this population, a cohort of users of conjugated estrogens during the period from May 1, 1995, to Octobe r 31, 1995, was selected. MEASURES. The cumulative incidence of switching from conjugated to esterifi ed estrogen tablets and subsequent discontinuations of esterified estrogens was evaluated. The frequencies of ambulatory encounters during the 6 month s before and after a switch or discontinuation were compared. RESULTS. During the period after promotion of the formulary switch, 2,149 o f 2,984 patients (72%) originally dispensed conjugated estrogen tablets swi tched to esterified estrogen tablets. Among those patients switching to est erified 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 conjug ated estrogen tablets was 15% by 2 years. CONCLUSIONS. The findings of this study suggest that plan efforts were succ essful in switching most users of conjugated estrogens to esterified estrog ens. The switch was associated with an increase in utilization of health ca re services.