Medicare Part D is the piece that trips people up most, because unlike Part A and Part B, it is not one government-run benefit — it is a menu of private insurance plans, each with its own list of covered drugs, its own pricing tiers, and its own premium. Picking the wrong one, or picking once and never comparing again, is one of the more common and avoidable ways Medicare enrollees overpay. This is general information, not medical or financial advice; verify current plan details and costs directly with Medicare.gov or a plan provider.
What changed in 2026
- The annual out-of-pocket cap on covered drug costs continues to apply, meaning once you hit the cap in a given year, covered drugs cost nothing further for the rest of that year — verify the current-year dollar figure directly, as it is adjusted periodically.
- Formularies and premiums reset every plan year, so a plan that was the cheapest fit last year is not guaranteed to be this year — the annual open enrollment window exists specifically for this reason.
- More drug manufacturers and Medicare have continued direct price negotiations for certain high-cost drugs, which can shift which plans offer the best value for people on those specific medications.
How Part D actually works
You do not get prescription drug coverage automatically with Medicare — you enroll separately in either a standalone Part D plan (if you have original Medicare) or a Medicare Advantage plan that bundles drug coverage in. Each plan publishes a formulary: the specific list of drugs it covers, organized into tiers, with generics typically cheapest and specialty drugs most expensive. Your actual out-of-pocket cost for any given prescription depends entirely on which tier that plan puts it in.
Why comparing plans matters
Two Part D plans can have similar premiums but wildly different costs for the specific drugs you actually take, because their formularies and tier placements differ. Someone on a common generic might do fine on almost any plan; someone on an expensive specialty medication can see costs vary by hundreds of dollars a month depending on the plan chosen. This is why re-checking your plan every year during open enrollment, rather than letting it auto-renew indefinitely, is consistently recommended.
Coverage phases at a glance
| Phase |
What happens |
Your cost |
| Deductible |
You pay full negotiated price until the plan deductible is met |
Varies by plan, capped by federal limit |
| Initial coverage |
Plan and you share the cost per formulary tier |
Copay or coinsurance per tier |
| After the out-of-pocket cap |
You have hit the annual cap on covered drug costs |
$0 for covered drugs rest of year |
The late enrollment penalty
If you do not enroll in Part D when first eligible, and you go without other "creditable" drug coverage (such as through an employer) for 63 days or more, you can face a late enrollment penalty added permanently to your premium once you do enroll. This penalty compounds the longer you wait, and it does not go away — it is one of the few Medicare decisions where delaying has a genuinely lasting cost, distinct from the general Medicare-versus-Medicaid distinctions covered in Medicare vs Medicaid explained.
What Part D does not cover
Part D covers outpatient prescription drugs specifically. It does not cover drugs administered in a hospital or doctor's office (those typically fall under Part B), and it does not address the custodial long-term care gap covered in what is long-term care insurance or the cost-sharing gaps a Medigap policy is designed to fill.
FAQ
Do I need Part D if I rarely take prescriptions?
Likely yes to avoid the late enrollment penalty later, since needs can change — a low-premium plan can serve as a safety net even with minimal current drug use.
Can I switch Part D plans every year?
Yes, during the annual open enrollment window, and doing so is often worthwhile since formularies and pricing change annually.
Does Medicare Advantage include Part D automatically?
Many Medicare Advantage plans bundle drug coverage in, but not all — check the specific plan before assuming.
What is the out-of-pocket cap actually protecting against?
It caps what you pay for covered drugs in a calendar year, protecting against catastrophic costs for people on expensive ongoing medications.
Where to go next
Related reading: Medicare vs Medicaid explained, what is a medigap policy, and what is long-term care insurance.