What Is a Drug Formulary? How Medicare Plans Decide What They Cover
Two Part D plans can have nearly identical premiums and still cost you wildly different amounts, because premium is only half the picture. The other half is the formulary — each plan's own list of which drugs it covers and how much you'll pay for each one. Checking the formulary before you enroll, not after, is the single highest-leverage thing you can do when picking a plan.
What's actually in a formulary
A formulary isn't a simple yes/no list — it's a structured list where every covered drug sits on a tier, and the tier determines your copay or coinsurance. A typical structure looks something like:
- Tier 1 — Preferred generics. The lowest copay, often just a few dollars.
- Tier 2 — Generics. A somewhat higher, still modest copay.
- Tier 3 — Preferred brand. Noticeably higher cost-sharing.
- Tier 4 — Non-preferred drug. Higher still — often the tier a plan uses to discourage a drug it would rather you not take when an alternative exists.
- Specialty tier. The highest cost-sharing tier, for very expensive drugs (often biologics or drugs for rare conditions), frequently priced as coinsurance (a percentage) rather than a flat copay.
The exact tier names, count, and pricing are set by each individual plan — there's no single national formulary, which is exactly why the same drug can cost you very differently depending on which plan you pick.
Restrictions that can apply even to a covered drug
- Prior authorization — your doctor must get plan approval before it's covered, usually to confirm it's medically necessary for your situation.
- Step therapy — you may be required to try a cheaper drug first, and only "step up" to the requested one if the cheaper option doesn't work.
- Quantity limits — a cap on how much of a drug the plan covers in a given period, for safety or cost reasons.
None of these mean the drug isn't covered — they mean there's a process attached, and it's worth knowing about before you're standing at the pharmacy counter.
Formularies can change mid-year
Plans are allowed to make certain formulary changes during the plan year (typically removing a drug or moving it to a higher tier when a new generic becomes available, for example), with advance notice to affected members. If a plan removes or re-tiers a drug you depend on, you can request a formulary exception, and if approved during the plan year, the change generally can't retroactively make you pay more for that drug for the rest of the year.
How to actually check before you enroll
- Get your exact drug list (name, dose, and how often you take it) before comparing plans — "compare formularies" without specifics isn't actionable.
- Check each plan's formulary directly (through Medicare's Plan Finder or the plan's own website) for every drug you take, not just the expensive one — a formulary gap on a cheap generic can still mean an inconvenient switch.
- Note the tier, not just "is it covered" — two plans that both cover your drug can still differ by a large copay gap depending on the tier it lands on.
- If you take a specialty drug, check specifically whether it requires prior authorization or step therapy, since those add time before you can actually fill it under a new plan.
Why this matters more than the premium
A plan with a slightly higher premium but your medications on preferred tiers, with no prior authorization roadblocks, is very often the cheaper choice overall once you count what actually leaves your wallet at the pharmacy across a full year.