Does Medication Adherence Following a Copayment Increase Differ by Disease Burden?

Objectives. To compare changes in medication adherence between patients with high- or low-comorbidity burden after a copayment increase. Methods. We conducted a retrospective observational study at four Veterans Affairs (VA) medical centers by comparing veterans with hypertension or diabetes required to pay copayments with propensity score-matched veterans exempt from copayments. Disease cohorts were stratified by Diagnostic Cost Group risk score: low-(<1) and high-comorbidity (>1) burden. Medication adherence from February 2001 to December 2003, constructed from VA pharmacy claims data based on the ReComp algorithm, were assessed using generalized estimating equations. Results. Veterans with lower comorbidity were more responsive to a U.S.$5 copayment increase than higher comorbidity veterans. In the lower comorbidity groups, veterans with diabetes had a greater reduction in adherence than veterans with hypertension. Adherence trends were similar for copayment-exempt and nonexempt veterans with higher comorbidity. Conclusion. Medication copayment increases are associated with different impacts for low- and high-risk patients. High-risk patients incur greater out-of-pocket costs from continued adherence, while low- risk patients put themselves at increased risk for adverse health events due to greater nonadherence.