Assesses how changing to a three-tiered co-payment system would affect drug costs and pharmacy utilization, in particular for specific high-cost classes of medications; As part of an effort to redesign the TRICARE pharmacy benefit to save costs, the Department of Defense is considering moving from a two-tiered to a three-tiered co-payment system, which will increase the co-payment for some classes and brands of drugs. Providers would, theoretically, have an incentive to prescribe less-costly options. To predict how this move would affect costs and pharmacy utilization, the authors use an existing data resource to determine how beneficiaries age 45 to 64 in private-sector health plans responded to similar changes.