This study examined the association between incentive-based formularies and antihypertensive drug selection and spending. We compared the use of drugs from five drug classes by the number of tiers and copayment differentials. We found that raising copayments within a single-tier formulary system had a relatively modest impact on use of antihypertensives, compared with raising them in multi-tier systems. Likelihood of using ACE inhibitors and angiotensin II receptor blockers was lower among two-tier plans with generic/brand differentials of dollars 10 relative to flat-copayment plans. Incentive formularies were associated with lower total antihypertensive spending by plans, but enrollees paid more out of pocket.