How Does Medicare Part D Work?
Quick answer: Medicare Part D is prescription drug coverage offered through private plans — either as a standalone plan paired with Original Medicare, or built into a Medicare Advantage plan. Each plan has its own formulary (list of covered drugs), tier structure, and pricing, which means the right plan depends entirely on your specific medications. The most important rule of Part D: choose based on your actual medication list, not on premium alone, because a low-premium plan can cost far more in total if it places your drugs in expensive tiers or doesn't cover them. Part D also has a late-enrollment penalty that's permanent if you go without creditable drug coverage when first eligible, and plans change their formularies and pricing every year — so an annual review during the Medicare open enrollment period (October 15 to December 7) matters. For Columbus, Ohio retirees, matching the plan to your medications and reviewing annually are the two highest-value Part D habits. This article is educational; specific plan decisions should be made using the Medicare Plan Finder, with a licensed insurance professional, or Ohio's free OSHIIP counseling.
Key Takeaways
- Part D is prescription drug coverage through private plans — standalone (with Original Medicare) or built into Medicare Advantage.
- Choose a Part D plan based on your specific medication list, not on premium alone.
- Each plan has its own formulary and drug tiers, so the same medication can cost very differently across plans.
- Part D has a permanent late-enrollment penalty if you go without creditable drug coverage when first eligible.
- Plan formularies and pricing change every year — review your plan annually during open enrollment.
- The Medicare open enrollment period runs October 15 to December 7 each year.
- The Medicare Plan Finder and Ohio's OSHIIP program help compare plans against your actual prescriptions.
Table of Contents
- What Medicare Part D Is
- Standalone Part D vs. Advantage-Included Coverage
- How Part D Plans Are Structured
- Why You Choose by Medication, Not Premium
- The Coverage Phases
- The Part D Late Enrollment Penalty
- Why Annual Review Matters
- How to Compare Part D Plans
- Frequently Asked Questions
What Medicare Part D Is
Medicare Part D is the part of Medicare that covers prescription drugs. It's one of the more practical, recurring parts of Medicare — for retirees who take regular medications, it directly affects monthly costs and which pharmacy they use.
The basics:
Part D coverage is offered through private insurance plans approved by Medicare, not directly by the government. You enroll in a plan, pay a premium (in addition to your Part B premium), and the plan covers your prescriptions according to its formulary and cost structure.
Two ways to get Part D:
- Standalone Part D plan — a separate prescription drug plan that pairs with Original Medicare (and typically a Medigap supplement)
- Built into Medicare Advantage — most Medicare Advantage plans include Part D drug coverage as part of the bundle
You don't have both — your Part D coverage comes either through a standalone plan (if you have Original Medicare) or through your Medicare Advantage plan.
Why it matters:
For retirees managing chronic conditions or taking multiple medications, Part D can be a significant recurring cost. And because plans differ so much in what they cover and how they price it, the plan choice can mean the difference of thousands of dollars a year for the same medications.
For Columbus-area retirees, Part D is one of the more "get the details right" parts of Medicare — less about a single big decision and more about matching the plan to your specific situation and reviewing it each year.
This article is part of my broader guide on how to plan for healthcare in retirement in Ohio, which covers how prescription coverage fits with the rest of your healthcare planning.
Standalone Part D vs. Advantage-Included Coverage
Your Part D coverage path depends on which Medicare coverage structure you chose.
If you have Original Medicare (plus Medigap):
You'll typically add a standalone Part D prescription drug plan. This is a separate plan with its own premium, chosen specifically for its drug coverage. You can shop standalone Part D plans independently of your other coverage, choosing the one that best covers your medications.
If you have Medicare Advantage:
Most Medicare Advantage plans include Part D coverage built in. In this case, your drug coverage is part of the Advantage plan you chose — you don't select it separately. This means that when you chose your Advantage plan, the drug coverage was (or should have been) part of that evaluation.
The planning implication:
- With Original Medicare, you have flexibility to choose the standalone Part D plan that best fits your medications, independent of everything else
- With Medicare Advantage, the drug coverage is tied to the plan, so the drug formulary should be part of how you evaluate Advantage plans in the first place
A common mistake among Medicare Advantage enrollees is choosing the plan for its premium or extra benefits without checking whether its drug formulary covers their specific medications well. The drug coverage matters just as much in an Advantage plan as in a standalone plan — it's just bundled rather than separate.
How Part D Plans Are Structured
Understanding how Part D plans are built helps explain why plan choice matters so much.
The formulary:
Each Part D plan has a formulary — a list of the drugs it covers. Plans aren't required to cover every drug, and formularies differ from plan to plan. A medication covered by one plan may not be covered by another, or may be covered at a very different cost.
The tier structure:
Within the formulary, drugs are organized into tiers, typically something like:
- Lower tiers for preferred generic drugs (lowest cost to you)
- Middle tiers for preferred brand-name drugs
- Higher tiers for non-preferred and specialty drugs (highest cost to you)
The same medication can be placed in different tiers by different plans, which means your cost for that drug varies depending on the plan.
The cost components:
- Monthly premium — what you pay for the plan each month
- Annual deductible — what you pay before the plan starts sharing costs (some plans have no deductible)
- Copays and coinsurance — what you pay for each prescription, varying by tier
- Pharmacy network — preferred pharmacies cost less; out-of-network may cost more or not be covered
The interaction:
These components interact in ways that make premium alone a poor measure of total cost. A plan with a low premium might have a high deductible, place your drugs in expensive tiers, or exclude your medications entirely. A plan with a higher premium might cover your specific drugs at low tiers with no deductible, making it cheaper overall for you.
This structure is exactly why the next section — choosing by medication rather than premium — is the single most important Part D principle.
Why You Choose by Medication, Not Premium
This is the most important practical lesson of Part D: the right plan depends on your specific medication list, not on which plan has the lowest premium.
The reasoning:
Because plans differ in their formularies, tiers, and pricing, the cheapest plan for one person can be the most expensive for another. Two retirees living next door to each other, taking different medications, may have completely different "best" Part D plans.
An illustrative example:
Imagine two standalone Part D plans:
- Plan A: Low monthly premium, but a high deductible, and your specific medication is in a high tier
- Plan B: Higher monthly premium, but no deductible, and your specific medication is in a low tier
For someone taking that medication regularly, Plan B might cost far less in total over the year despite the higher premium — because the lower drug costs and absent deductible more than offset the premium difference. But for someone who takes no regular medications, Plan A's low premium might win.
The point: there's no universally "best" or "cheapest" Part D plan. There's only the best plan for your specific medication list.
The right way to choose:
- List all your current medications, including dosages
- Use the Medicare Plan Finder (or OSHIIP counseling) to enter your medications and compare plans
- Look at the total estimated annual cost for each plan — premium plus deductible plus your drug costs — not just the premium
- Verify your preferred pharmacy is in the plan's preferred network
- Choose the plan with the lowest total cost for your specific situation
The mistake to avoid:
Choosing a Part D plan (or a Medicare Advantage plan's drug coverage) based on the premium alone, or on a friend's recommendation, without checking your specific medications. The friend's best plan may be terrible for your drug list.
For Columbus-area retirees, this medication-specific comparison is the single highest-value Part D habit. The Medicare Plan Finder makes it straightforward, and OSHIIP counselors can do it with you for free.
The Coverage Phases
Medicare Part D coverage moves through phases over the course of the year, which affects what you pay at different points.
The general structure:
Part D coverage has historically moved through several phases during the year — a deductible phase, an initial coverage phase, a coverage gap phase (often called the "donut hole"), and a catastrophic coverage phase. The amount you pay for medications can change as you move through these phases based on total drug spending during the year.
Recent changes:
The structure of Part D — particularly the coverage gap and the catastrophic phase — has been changing under recent federal legislation, including the introduction of an annual out-of-pocket cap on prescription drug spending. These changes are generally favorable for retirees with high drug costs, but the specifics evolve year to year.
Why this matters for planning:
- Your monthly drug costs may not be level throughout the year — they can be higher early (during the deductible phase) and change as you progress
- The annual out-of-pocket cap provides a ceiling on your prescription spending, which helps with budgeting and protects against catastrophic drug costs
- The specific dollar thresholds for each phase change annually
The practical takeaway:
Rather than memorizing the phase structure (which changes), focus on the total estimated annual cost when comparing plans, and understand that there's now a cap on annual out-of-pocket drug spending. For the current year's specific thresholds and cap amount, check Medicare.gov or consult OSHIIP — these are exactly the figures that change each year.
The Part D Late Enrollment Penalty
Like Part B, Part D has a late-enrollment penalty that's permanent — making timely enrollment important even if you don't currently take many medications.
How the penalty works:
If you go without creditable prescription drug coverage for a period after becoming eligible for Part D (and don't have other qualifying coverage), you may face a permanent penalty added to your Part D premium. The penalty grows the longer you go without coverage, and it's added to your premium for as long as you have Part D.
The "creditable coverage" concept:
The penalty generally doesn't apply if you had "creditable" prescription drug coverage during the gap — coverage at least as good as standard Part D. Common sources of creditable coverage include qualifying employer or union drug coverage. If you have creditable coverage, keep documentation of it, since you may need to prove it later.
Why this catches people:
A common mistake is skipping Part D enrollment because you don't currently take regular medications. The logic seems reasonable — why pay for drug coverage you don't use? But if you skip Part D and later need it (which most people eventually do), you face the permanent penalty plus the cost of the coverage. Enrolling in even a low-premium Part D plan when first eligible avoids the penalty and provides coverage for when you do need it.
The takeaway:
Unless you have creditable drug coverage from another source, enrolling in a Part D plan when first eligible — even a low-cost one — avoids a permanent penalty. The modest cost of a basic plan is usually worth avoiding the lifelong penalty.
Why Annual Review Matters
Part D is unusual among Medicare decisions in that it genuinely needs to be reviewed every single year — not set once and forgotten.
Why annual review is necessary:
- Formularies change — plans can add or drop drugs from their formulary each year
- Tier placement changes — your medication might move to a more expensive tier
- Premiums change — plan premiums are adjusted annually
- Your medications change — new prescriptions, dropped prescriptions, dosage changes
- New plans become available — and existing plans are discontinued
Because of all this change, the plan that's optimal for you this year may not be optimal next year. A plan that perfectly covered your medications could move one of your key drugs to a higher tier, making a different plan cheaper.
The open enrollment window:
The Medicare open enrollment period runs from October 15 to December 7 each year. During this window, you can switch Part D plans (or Medicare Advantage plans) for the coming year. This is the time to review.
The annual review habit:
Each fall, during open enrollment:
- Update your current medication list
- Run it through the Medicare Plan Finder against available plans
- Compare your current plan's cost for next year against alternatives
- Switch if a better-fitting plan is available
- Confirm your pharmacy is still in the preferred network
Why people skip it (and shouldn't):
Many retirees enroll once and never review, assuming their plan stays the same. But because plans change annually, this passivity can cost hundreds or thousands of dollars over time. The annual review takes perhaps an hour and can produce real savings.
For Columbus-area retirees, building the annual Part D review into your fall routine — alongside other year-end planning — is one of the simplest ways to control prescription costs over a long retirement.
How to Compare Part D Plans
The mechanics of comparing Part D plans are straightforward once you know the tools.
The Medicare Plan Finder:
The Medicare Plan Finder at medicare.gov is the primary tool. You enter your medications and dosages, your preferred pharmacies, and your location, and it shows you available plans ranked by estimated total annual cost for your specific situation. This is the single most useful tool for Part D decisions.
What to enter:
- All your current medications, with dosages
- Your preferred pharmacies (including mail-order if you use it)
- Your ZIP code
What to look at in the results:
- Total estimated annual cost — premium plus deductible plus your estimated drug costs (this is the number that matters, not premium alone)
- Whether all your medications are covered
- The tier placement of your key medications
- Whether your pharmacy is in the preferred network
- Plan ratings and customer service considerations
Free help from OSHIIP:
Ohio's SHIP program, OSHIIP (Ohio Senior Health Insurance Information Program), offers free, unbiased counseling that includes help comparing Part D plans against your specific medications. OSHIIP counselors can run the comparison with you, which is especially helpful if the Plan Finder feels overwhelming. Because OSHIIP doesn't sell plans, their guidance is unbiased.
Licensed insurance professionals:
Licensed agents can also help with Part D plan selection, though agents may be compensated by the plans they sell. OSHIIP's unbiased counseling is often the better starting point for an objective comparison.
Where Blue Advisors fits:
I help Columbus-area retirees understand how prescription drug costs fit into the overall retirement budget and income plan. For the plan selection itself — running your medications through the Plan Finder and choosing a specific plan — I point clients to the Medicare Plan Finder, OSHIIP, and licensed insurance professionals. I'm not an insurance agency or Medicare broker; the value I add is coordinating the cost into the broader plan.
Frequently Asked Questions
What is Medicare Part D?
Medicare Part D is prescription drug coverage offered through private plans approved by Medicare. It's available either as a standalone plan paired with Original Medicare, or built into most Medicare Advantage plans. You pay a premium and the plan covers your medications according to its formulary and cost structure.
Do I need Part D if I don't take any medications?
Even if you don't currently take regular medications, enrolling in a Part D plan when first eligible avoids a permanent late-enrollment penalty (unless you have other creditable drug coverage). Most people eventually need medications, and the penalty for waiting is permanent, so enrolling in even a low-cost plan is usually worthwhile.
How do I choose the right Part D plan?
Choose based on your specific medication list, not on premium alone. Use the Medicare Plan Finder to enter your medications and compare plans by total estimated annual cost — premium plus deductible plus your drug costs. Verify your pharmacy is in the preferred network. The best plan is the one with the lowest total cost for your specific medications.
Why shouldn't I just pick the cheapest premium?
Because a low-premium plan can have a high deductible, place your medications in expensive tiers, or not cover them at all — making it more expensive overall. The premium is only one component of total cost. Two plans with very different premiums can have reversed total costs depending on your specific drug list.
What is the Part D late enrollment penalty?
If you go without creditable prescription drug coverage for a period after becoming eligible (and don't have other qualifying coverage), you may face a permanent penalty added to your Part D premium for as long as you have Part D. The penalty grows the longer you go without coverage.
Do Part D plans change every year?
Yes. Plan formularies, drug tier placements, premiums, and pharmacy networks can all change annually. This is why you should review your Part D plan every year during the open enrollment period (October 15 to December 7), comparing your current plan against alternatives for your medications.
When can I change my Part D plan?
The Medicare open enrollment period runs from October 15 to December 7 each year, during which you can switch Part D plans for the coming year. There are also some special enrollment circumstances. Review your plan each fall against your current medications.
Is Part D included in Medicare Advantage?
Most Medicare Advantage plans include Part D prescription drug coverage built in. If you have Medicare Advantage, your drug coverage is typically part of that plan. If you have Original Medicare, you add a separate standalone Part D plan. Either way, the drug formulary should be evaluated against your specific medications.
Is there a cap on what I'll pay for prescriptions?
Recent federal legislation introduced an annual out-of-pocket cap on Part D prescription drug spending, which provides a ceiling on your annual prescription costs. The specific cap amount and coverage phase thresholds change year to year — check Medicare.gov or consult OSHIIP for current figures.
Where can I get free help comparing Part D plans?
Ohio's SHIP program, OSHIIP (Ohio Senior Health Insurance Information Program), offers free, unbiased counseling that includes help comparing Part D plans against your specific medications. Because OSHIIP doesn't sell plans, their guidance is unbiased, unlike agents who may be compensated by the plans they sell.
Match the Plan to Your Medications, Review Every Year
For Columbus-area retirees, Medicare Part D is less about a single high-stakes decision and more about getting two practical habits right: choosing the plan that best fits your specific medications, and reviewing that choice every year during open enrollment.
The pattern that produces better outcomes: list your medications, run them through the Medicare Plan Finder (or get OSHIIP's help), compare plans by total annual cost rather than premium alone, confirm your pharmacy is in-network, enroll on time to avoid the permanent penalty, and revisit the comparison each fall because plans change. These habits can save hundreds or thousands of dollars over a retirement.
The goal isn't to find the cheapest premium — it's to find the plan that covers your specific medications at the lowest total cost, and to keep it optimized as both the plans and your medications change over time.
At Blue Advisors, I help Columbus-area retirees fit prescription drug costs into the overall retirement budget and income plan. Blue Advisors is a fee-only fiduciary registered investment advisory firm based in Columbus, Ohio. I'm not an insurance agency or Medicare broker — for Part D plan selection itself, I point clients to the Medicare Plan Finder, OSHIIP, and licensed insurance professionals.
Schedule a conversation: If you're a Columbus-area retiree or pre-retiree thinking through how prescription and healthcare costs fit into your retirement plan, you can book an introductory call here: calendly.com/jimblue/blue-advisors-meeting.
By James Blue, Fee-Only Advisor | Blue Advisors
James Blue is the founder of Blue Advisors, a fee-only registered investment advisory firm based in Columbus, Ohio, serving retirees, pre-retirees, and busy professionals across Central Ohio and nationally.
This content is provided for informational and educational purposes only and should not be construed as personalized investment, tax, legal, insurance, or medical advice. Medicare Part D rules, formularies, premiums, coverage phases, out-of-pocket caps, penalties, and enrollment periods change periodically and depend on individual circumstances. Blue Advisors is a fee-only registered investment advisory firm and is not an insurance agency, Medicare broker, tax preparation firm, or law firm. Readers should consult Medicare (medicare.gov), the Medicare Plan Finder, the Ohio Senior Health Insurance Information Program (OSHIIP), a licensed insurance professional, and where applicable a qualified tax professional or attorney before making Medicare Part D decisions. The views expressed are those of the author as of the date published and are subject to change without notice. Advisory services are offered only pursuant to a written advisory agreement and to clients in the State of Ohio, the Commonwealth of Pennsylvania, and other jurisdictions where Blue Advisors is properly registered or exempt from registration. Past performance is not indicative of future results. Specific premium amounts, deductibles, coverage phase thresholds, and the annual out-of-pocket cap have been kept general — consult current Medicare guidance for specific figures.