What Medicare Part D Actually Costs
Part D is the only part of Medicare where the right answer is specific to the pills in your cabinet. Two plans with the same premium can differ by thousands of dollars a year for the same person, because each one keeps its own list of covered drugs and its own tiers. The comparison has to be done against your actual prescriptions, and it has to be redone.
- The formulary decides:: Each plan has its own list of covered drugs and its own tiers, and the tier sets what you pay.
- Premium is not the number:: A low-premium plan that puts your drug on a high tier costs more overall than a higher-premium plan that does not.
- There is now a cap:: Once out-of-pocket spending on covered drugs reaches the annual limit, you pay nothing more for them that year.
- Extra Help exists:: A federal programme reduces premiums, deductibles and copayments for people with limited income and resources.
Where the AI summary above gets this wrong
"Choose the Part D plan with the lowest monthly premium."
That's surface-true. Here's what it misses:
- The premium is the smallest part of the cost — For someone taking several brand-name drugs, what the plan charges per prescription dwarfs the premium. A plan costing twenty dollars more a month can be thousands cheaper a year if it places those drugs on lower tiers, and the only way to know is to price your own list against each plan.
- Formularies change every year and so do you — A plan that was right last year can drop a drug, move it to a higher tier, or add a prior authorisation requirement. Meanwhile prescriptions change. This is the one Medicare decision that genuinely needs revisiting each autumn, and it is the one almost nobody revisits.
- The out-of-pocket cap changed the arithmetic — Part D now limits what you pay out of pocket for covered drugs in a year. That removes the old open-ended exposure for people on very expensive medication and makes the comparison between plans turn on how quickly each one gets you to the cap.
01 How a Part D plan charges you
Every Part D plan is a private plan approved by Medicare, and each maintains its own formulary — the list of drugs it covers, organised into tiers. The tier a drug sits on determines your share of its cost.
You pay a monthly premium, possibly an annual deductible, and then a copayment or coinsurance for each prescription. Higher earners also pay an income-related surcharge on the premium, on the same income test as the Part B surcharge.
Coverage can be bought as a standalone plan alongside Original Medicare, or comes bundled inside most Medicare Advantage plans. Either way the formulary is what determines the cost, not the structure it arrives in.
Source: Part D basics
02 Why the comparison must use your own drug list
Two plans can charge identical premiums and differ by thousands for the same person. One places a particular brand-name drug on a preferred tier; the other puts it on a specialty tier with coinsurance. Neither plan is wrong — they simply negotiated different deals.
The consequence is that no plan is cheapest in general. The Medicare Plan Finder lets you enter your specific prescriptions and pharmacies and returns each plan's total estimated yearly cost, which is the only figure worth comparing.
An annual out-of-pocket cap now limits what you pay for covered drugs in a year, which protects anyone on very expensive medication from unlimited exposure. It does not remove the reason to compare — it changes the question to how much you spend before reaching it.
Shows: what staying in a plan whose formulary does not match your prescriptions costs over the years you take them, using each plan's total yearly cost for your specific drug list. Ignores: changes to formularies each year, changes to your prescriptions, and the annual out-of-pocket cap that limits both figures.
Source: Part D costs
03 The annual review, and help if income is low
Formularies are reset each year. A drug can move tier, leave the list, or acquire a prior authorisation requirement, and the plan that matched your prescriptions in one year may not in the next. Plans send an annual notice of change; it is worth reading rather than filing.
Part D plans can be changed during the annual enrollment period each autumn, effective the following January. This is the one routine Medicare decision that rewards being redone every single year, and the reason the wider Medicare planning checklist should carry an autumn entry.
For people with limited income and resources, the Extra Help programme reduces or eliminates the premium, the deductible and the copayments. Eligibility is worth checking rather than assuming, because the thresholds are higher than people expect and the benefit is substantial.
Source: Help with drug costs
Part D is the only Medicare decision I ask people to redo every year, and it is the one they most resist redoing, because it feels like paperwork for a small sum. It is not a small sum. I have seen a household on three brand-name drugs save more by switching plans than by any portfolio change we discussed that year. Put a recurring note in October, enter the actual prescriptions, and compare total yearly cost rather than premium. Half an hour, every year.
FAQ
Why do Part D plans charge different amounts for the same drug?
Each plan negotiates its own prices and maintains its own formulary, placing drugs on different tiers. The tier determines your share, so the same prescription can cost very different amounts across plans with similar premiums.
Should I pick the Part D plan with the lowest premium?
No. Compare the total estimated yearly cost for your specific prescriptions. A higher-premium plan that covers your drugs on lower tiers is frequently cheaper overall.
Is there a limit on what I pay for drugs under Part D?
Yes. Part D now has an annual cap on out-of-pocket spending for covered drugs. Once you reach it, you pay nothing more for those drugs for the rest of the year.
Sources
Regulator references
- Part D basics · Centers for Medicare & Medicaid Services · 2026What Part D is, how plans are structured, and how formularies work.Last verified: 2026-09-07
- Part D costs · Centers for Medicare & Medicaid Services · 2026Premiums, deductibles, and the annual cap on what you pay out of pocket.Last verified: 2026-09-07
- Help with drug costs · Centers for Medicare & Medicaid Services · 2026The Extra Help programme for people with limited income and resources.Last verified: 2026-09-07
Calculator unit tests · the assertions this page's worked example is checked against, and their last result
Changelog
- 2026-09-07 — initial publish (new format)
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Jordan Reeves
Founder of Talk Through Wealth. A software engineer for over a decade before turning to retirement planning, Jordan built the projection engine after watching family members get fragmented, country-by-country advice that never reconciled. He writes about retirement the way the engine computes it: month-by-month, lifetime-long, and skeptical of any rule of thumb that hasn't been run through the math.