Adherence to oral antidiabetic therapy in a managed care organization: a comparison of monotherapy, combination therapy, and fixed-dose combination therapy

BACKGROUND: Although medication adherence is one of the most important aspects of the management of diabetes mellitus, low rates of adherence have been documented. OBJECTIVE: This study sought to examine medication adherence among patients with diabetes mellitus in a managed care organization who were receiving antidiabetic monotherapy (metformin or glyburide), combination therapy (metformin and glyburide), or fixed-dose combination therapy (glyburide/metformin). METHODS: Medication adherence was evaluated through a retrospective database analysis of pharmacy claims. The adherence rate was defined as the sum of the days' supply of oral antidiabetic medication obtained by the patient during the follow-up period divided by the total number of days in the designated follow-up period (180 days). Health plan members were included in the analysis if they had an index pharmacy claim for an oral antidiabetic medication between August 1 and December 31, 2000, were continuously enrolled in the health plan, and were aged > or =18 years. A 6-month pre-index period was used to classify patients as newly treated or previously treated. Patients were grouped according to their medication-use patterns. RESULTS: After adjustment for potential confounding factors, including overall medication burden at index, there were no significant differences in adherence rates among 6502 newly treated patients receiving monotherapy, combination therapy, or fixed-dose combination therapy. Among the 1815 previously treated patients receiving glyburide or metformin monotherapy who required the addition of the alternative agent, resulting in combination therapy, adherence rates were significantly lower (54.0%; 95% CI, 0.52-0.55) than in the 105 patients receiving monotherapy who were switched to fixed-dose combination therapy (77.0%; 95% CI, 0.72-0.82). The 59 previously treated patients receiving combination therapy who were switched to fixed-dose combination therapy had a significant improvement in adherence after the switch (71.0% vs 87.0%; P < 0.001). CONCLUSIONS: In a managed care organization, previously treated patients receiving monotherapy with an oral antidiabetic medication who required additional therapy exhibited significantly greater adherence when they were switched to fixed-dose combination therapy compared with combination therapy. Patients receiving combination therapy who were switched to fixed-dose combination therapy exhibited significantly greater adherence after the switch.