Part D is prescription coverage, bought either as a standalone plan alongside Original Medicare or bundled inside a Medicare Advantage plan. It is the part of Medicare that changes most from year to year, and the part people check least.
The formulary is the plan
A Part D plan is, functionally, a list of covered drugs with a price attached to each. That list is the formulary, and it is organised into tiers — roughly, generics at the bottom and specialist drugs at the top.
Two plans with similar monthly costs can treat your specific prescription completely differently. One may cover it on a low tier, one on a high tier, one not at all.
This is why “which Part D plan is best” has no general answer. The only useful question is which plan is best for your list of drugs, at your doses, at a pharmacy you can get to.
The check that has to happen every year
Formularies are revised annually. A drug covered this year can move tier, acquire a restriction, or come off the list entirely for next year.
Plans are required to tell you. The notice arrives in autumn, in an envelope that looks like every other envelope arriving that autumn, and it is routinely thrown away unopened.
So the single most valuable habit on this page is: re-run your drug list against next year’s formulary, every year, during the autumn enrollment window. It takes a few minutes with the Medicare Plan Finder and it is the difference between knowing in October and finding out at the pharmacy counter in January.
Restrictions that are not obvious
Even a covered drug can carry conditions:
- Prior authorisation — the plan wants clinical justification before it pays.
- Step therapy — you must try a cheaper alternative first.
- Quantity limits — a cap on how much is covered per period.
None of these mean you cannot get the drug. They mean there is a process, and the process takes time you may not have budgeted. If you are on something with a restriction, ask your prescriber’s office whether they have dealt with that plan before.
The penalty for going without
If you go without creditable prescription coverage for long enough after becoming eligible, a late-enrollment penalty can attach when you do join — and it generally stays attached rather than expiring.
“Creditable” is the operative word. Some employer and union drug coverage counts; some does not. The plan is required to tell you which, in an annual notice. Keep it. If you have never received one, ask.
Where standalone and bundled differ
If you take Original Medicare, you buy Part D separately and you can change it independently each year.
If you take Medicare Advantage, drug coverage is usually built in — which means changing your drug coverage means changing your whole plan, including its network. That is a bigger decision, and it is a reason the formulary check matters even more on that road.
The practical routine
- Keep a current list: drug name, dose, form, how often.
- Each autumn, run that list against next year’s formulary for your plan.
- If something moved tier or came off, compare alternatives before the window closes.
- If you are on Advantage, remember that changing drug coverage changes the whole plan.
We will run that comparison with you for the plans we can offer, and we will tell you if staying where you are is the better answer. For every plan available where you live, including ones we cannot sell, the Plan Finder and 1-800-MEDICARE are free and cover the whole market.
Where to check this independently
Nothing on this page is advice about your situation, and none of it replaces the free sources. The Medicare Plan Finder,1-800-MEDICARE, your SHIP counsellor andHealthCare.gov are free, independent, and show plans we cannot sell.
Healthbase is a licensed insurance agency, not a government agency and not connected with Medicare or any federal or state program.