A drug list (also called a formulary) tells you what drugs covered by a plan.
A drug list can change from year to year.
Part D plans can add or remove drugs from their drug list each year. The list can also change for other reasons. For example, if a drug is taken off the market. Your plan will let you know if there's a coverage change to a drug you're taking.
Many Part D plans have a tiered drug list.
Tiers divide drugs into groups based on cost. In general, drugs in low tiers cost you less than drugs in high tiers. Sometimes plans charge deductibles for certain drug tiers.