Medicare Part D explained in plain language
The drug coverage piece of Medicare is the one most likely to quietly cost you money through a mismatched formulary rather than a high premium.
At a glance
- Part D is prescription drug coverage, sold either as a standalone plan or bundled into a Medicare Advantage plan.
- Every plan has its own formulary — the specific list of covered drugs and their cost tier — which is why premium alone is a poor way to compare plans.
- An annual out-of-pocket cap on covered drugs now limits how much anyone pays in a calendar year, replacing the old multi-phase "donut hole."
- Extra Help (the Low-Income Subsidy) can sharply reduce costs for those who qualify on income and assets.
Part D gets less attention than the choice between Original Medicare and Medicare Advantage, but for someone taking several regular medications, it can matter just as much financially. The core issue is that "drug coverage" isn't one standardized product — each plan publishes its own formulary, deciding which drugs it covers and at what cost tier, and those lists genuinely differ in ways that aren't obvious from the premium alone.
How Part D plans actually differ
Two plans can have nearly identical premiums and still cost a specific person very different amounts, because of three variables:
- Formulary — whether a specific drug is covered at all, and which tier it's placed in.
- Pharmacy network — plans often have "preferred" pharmacies with lower copays than out-of-network ones.
- Utilization management rules — some drugs require prior authorization, step therapy (trying a cheaper drug first), or quantity limits.
This is why Medicare's Plan Finder tool asks for exact drug names, dosages and a preferred pharmacy — it's calculating a real estimated annual cost using the actual formulary, not just comparing premiums.
Plain-language glossary
- Formulary
- A plan's specific list of covered drugs, organized into cost tiers (generic, preferred brand, non-preferred brand, specialty).
- Tier
- The cost category a drug falls into within a formulary — lower tiers generally mean lower copays.
- Prior authorization
- A requirement that a doctor get the plan's approval before it will cover a specific drug.
- Step therapy
- A rule requiring a patient to try a lower-cost drug first, and show it doesn't work, before the plan covers a more expensive alternative.
- Extra Help (LIS)
- A federal subsidy that lowers or eliminates Part D premiums and copays for people who meet income and asset limits.
What you'll actually pay through the year
Part D costs generally move through phases over a calendar year, though the structure has been simplified in recent years. Exact dollar thresholds change annually and should be confirmed for the current year, but the general shape looks like this:
| Phase | What typically happens |
|---|---|
| Deductible | You pay full negotiated price up to the plan's deductible (some plans have a $0 deductible). |
| Initial coverage | You pay a copay or coinsurance per tier; the plan pays the rest, up to a total drug spending threshold. |
| Annual out-of-pocket cap reached | Once your own out-of-pocket spending on covered drugs hits the yearly cap, you generally pay $0 for covered drugs for the rest of the calendar year. |
That annual cap — a flat dollar limit on out-of-pocket spending for covered drugs — is a meaningful change from the older system, where costs could fluctuate through a confusing multi-phase "coverage gap" partway through the year. It's still worth confirming the current year's exact cap, since it's adjusted periodically.
Choosing a plan around actual medications
The practical approach: list every current medication with exact name, dosage and quantity, then run that list through Medicare's Plan Finder for the specific ZIP code and preferred pharmacy. Sort by estimated total annual cost, not premium, and double check that any specialty or newer medications aren't subject to step therapy or prior authorization under the plan you're leaning toward.
Questions to ask a pharmacist or plan
- Is this exact medication, dose and quantity covered under this plan's formulary, and at what tier?
- Is there a lower-tier generic or therapeutic alternative that would work just as well for this condition?
- Does this drug require prior authorization or step therapy under this plan?
- Is my regular pharmacy a "preferred" pharmacy for this plan, or will I pay more there?
- Would I qualify for Extra Help based on my income and assets?
What to do if a drug's cost changes mid-year
Formularies can shift during the year — a drug can move tiers, or a plan can add new utilization rules. If a medication suddenly costs much more than expected, ask the pharmacist to check for a formulary exception process, a generic or therapeutic alternative, and whether the plan's manufacturer or state assistance programs apply. Outside of specific exceptions, switching plans generally has to wait for the next open enrollment period.
If there's employer or retiree drug coverage already
Some people entering Medicare already have drug coverage through a current employer, a spouse's plan, or a retiree health plan from a former employer. The question that matters is whether that coverage counts as "creditable" — meaning it's expected to pay, on average, at least as much as standard Part D coverage. A plan is required to send a notice each year stating whether its coverage is creditable. If it is, you can generally delay enrolling in Part D without a late penalty later; if it isn't, delaying can trigger a permanent premium penalty once you do enroll. Keep those creditable-coverage notices somewhere findable — proving creditable coverage later, if a penalty is ever questioned, is much easier with the original notice in hand than without it.
If retiree coverage ever ends or changes, that typically opens a special enrollment period to sign up for Part D outside the normal annual window, so it's worth acting relatively promptly rather than assuming the usual October-to-December timeline applies.
Common questions
Is there still a Medicare "donut hole" in 2026?
The old multi-phase coverage gap has effectively been replaced by an annual out-of-pocket cap on covered drugs — once you hit that cap for the year, you generally pay nothing more for covered drugs through year's end. Confirm the current year's exact cap on Medicare.gov.
What is Extra Help for Medicare Part D?
Extra Help (the Low-Income Subsidy) reduces or eliminates Part D premiums, deductibles and copays for people who meet income and asset limits. Apply through the Social Security Administration or your state Medicaid office.
Can I change my Part D plan every year?
Yes — generally once a year during the annual Open Enrollment Period, since formularies and pricing can shift meaningfully year to year even within the same plan.
