OBJECTIVES: Antihypertensive medications are important in saving lives and decreasing disability. Formulary tiers, which determine copayment levels, may contribute to patients taking antihypertensive medications incompletely, inconsistently, or not at all, diminishing the potential benefit of treatment. Our objective was to determine whether copayment level is related to adherence to antihypertensive medications. METHODS: A retrospective claims database evaluation of hypertensive patients in a large health plan in Hawaii between January, 1999 and June, 2004 (n=114,232) was conducted. Generic copayments were $5, preferred brand copayments were $20, and copayments for non-preferred brands varied based on price with copayments ranging from $20-$165. Adherence was measured using medication possession ratios, with a ratio of 0.8 considered adherent. Adherence rates were calculated annually from the date of first prescription for up to three years. Multi-variable logistic regression was used to examine the impact of copayment on adherence, controlling for patient characteristics and specific therapeutic class. RESULTS: Overall, adherence was 66.8% for generics, 66.1% for preferred brands, and 54.6% for non-preferred brands. Differences in adherence between generic and preferred brands for specific classes were 4-7% and 9-18% for preferred and non-preferred brands. The largest difference in adherence was between generic and non-preferred beta blockers (22%). All the differences were statistically significant at the 0.05 level. CONCLUSION: Adherence rates were consistently highest for generic drugs, and lowest for non-preferred brands for all specific therapeutic classes. This suggests that copayment, which is directly tied to formulary tier, may play a significant role in adherence to antihypertensive medication.