Effect of prescription copayments on adherence and treatment failure with oral antidiabetic medications

Previous studies have shown that an increase in cost sharing by patients for medications results in reduced medication use. The purpose of our study was to determine whether the amount of members' copayments predicted oral antidiabetic treatment failure in a managed care population and to analyze the relationship between copayments, adherence to therapy, and hemoglobin A(1c) levels in patients with type-2 diabetes.

Health plan members 18 years of age or older with type-2 diabetes and who were newly initiated on an oral antidiabetic drug (OAD) between January 1, 2002, and January 31, 2006, were identified from a managed care population in the U.S. Members were required to have continuous eligibility for six or more months before and 12 months after the index prescription and were placed into four treatment groups. These patients were followed for one year. The time during which they took the index OAD was measured until treatment failure (discontinuation of therapy or a switch of the index OAD) or censoring of patient data. The medication possession ratio (MPR), which was used to calculate adherence, was defined as the number of days that a patient had a supply of the index OAD during the year after the index fill, divided by 365 days. Copays were identified for every prescription, and the amount for each 30 days of treatment was calculated. We used multivariate analyses to assess the impact of copays on treatment failure, adjusting for differences in member characteristics.

Adherence was poor, with an overall mean MPR of 0.57, decreasing from 0.58 for the group with a copay of less than $10 to 0.52 for patients with a copay of $20 or more. Initial treatment failed for 13,091 patients (70%), with approximately 60% of all members discontinuing treatment. This discontinuation rate ranged from 55% in the lowest copayment group (below $10) to 67% in those with a copay of $30 or more. For every $10 increase in copay, OAD treatment was 26% more likely to fail (95% confidence interval, 22.3-29.8%; P < 0.0001).

Higher copays for health plan members were a significant predictor of treatment failure. With the increased trend toward more cost sharing by members of health plans, ongoing evaluation will be necessary to determine the impact on treatment persistence in patients with chronic conditions.