In Medicare Part D plans, a drug formulary is divided into tiers, with each representing a different level of cost-sharing by the insured. Some plans may have more tiers and some may have less (e.g., tiers 1, 2, 3 and/or a specialty tier).

Under the Medicare Part D prescription drug plan, this is a specific list of drugs. A plan may have several tiers, and the patient’s copayment amount depends on which tier the drug is listed in. The plans are variable because patients can choose their own tiers. Each plan benefit booklet contains the list of drugs pertinent to a specific plan.