Medicare Part D is the prescription drug benefit within Medicare, and it’s one of the most misunderstood pieces of the entire program. Unlike Parts A and B, which are run directly by the federal government, Part D is always delivered through private insurance companies that contract with Medicare. You choose a Part D plan, pay a monthly premium to the private insurer, and the plan provides drug coverage according to its formulary, which is the plan’s list of covered drugs organized into cost tiers. If you’re enrolled in Original Medicare and want prescription drug coverage, you add a standalone Part D plan. If you’re in a Medicare Advantage plan, drug coverage is usually bundled in, and that combined plan is called an MA-PD.
Most people don’t spend nearly enough time on their Part D decision. They either pick the cheapest premium without checking whether their medications are even covered, or they just stick with whatever plan Medicare auto-enrolls them in. Both approaches tend to cost more money than a 20-minute comparison would have. Part D has been one of the more complex components of Medicare since it launched in 2006, but 2025 brought the most significant structural changes in the program’s history, and those changes actually make Part D more favorable for most beneficiaries than it’s ever been.
The New 2025 Part D Structure: What Actually Changed
The most important change to Part D in its nearly 20-year history took effect January 1, 2025: a $2,000 annual out-of-pocket cap on covered drug costs for all Part D enrollees. Before this change, there was no true hard cap. People with expensive specialty medications, think cancer drugs, biologics, or MS treatments, could face $5,000 to $10,000 or more per year in out-of-pocket drug costs. That’s gone now. The $2,000 cap is adjusted annually for inflation, but 2025 marks the first year it’s in effect, and it’s a genuine game-changer for beneficiaries with high drug costs.
The 2025 Part D benefit has two main phases. The initial coverage phase runs from $0 to $2,000 in out-of-pocket drug costs. During this phase you pay your cost-sharing, which is whatever copays or coinsurance your plan assigns to each drug tier, as you fill prescriptions throughout the year. Once your out-of-pocket spending hits $2,000 for the calendar year, you enter the catastrophic coverage phase. In catastrophic coverage you pay nothing for covered Part D drugs for the rest of the year. Zero. The plan and Medicare split 100% of your drug costs once you’ve cleared $2,000, and you’re done writing checks until January 1 resets everything.
The prior “donut hole,” the coverage gap that sat between initial coverage and catastrophic coverage and generated enormous confusion and hardship for years, has been fully eliminated as of 2025. This is a significant simplification of the benefit structure. People who were previously spending thousands in the coverage gap, particularly those on expensive biologics and specialty medications, are the biggest winners. If you or someone you know was dreading Part D costs because of horror stories about the donut hole, that chapter is closed.
How Part D Formularies Work
A formulary is your Part D plan’s official list of covered drugs. Every Part D plan has its own formulary, and they differ substantially between plans. Medicare requires all Part D formularies to cover at least two drugs in each therapeutic category, so you typically have some options within most drug classes. But which specific drugs are included, what tier they’re assigned to, and what the cost-sharing is for each tier varies widely from plan to plan. This is why the “cheapest premium” approach to picking a Part D plan so often backfires.
Drug tiers determine your cost-sharing level for each medication. A typical Part D formulary has five tiers. Tier 1 covers preferred generic drugs and usually has the lowest copays, often $0 to $5. Tier 2 includes non-preferred generics with slightly higher copays. Tier 3 covers preferred brand-name drugs, with copays commonly in the $40 to $50 range. Tier 4 covers non-preferred brand-name drugs with higher copays or coinsurance, often 25% to 50% of the drug cost. Tier 5 is specialty drugs, the most expensive category, typically with 25% to 33% coinsurance. On a medication that costs $8,000 per month, 33% coinsurance is $2,640 per fill before you hit your cap. That’s why understanding tier placement for your specific medications matters so much.
Because formularies vary between plans, the right Part D plan for you is the one whose formulary places your specific drugs at the lowest tier and lowest cost-sharing. A drug that’s Tier 3 on one plan might be Tier 1 on another, or might not be covered at all on a third. This is not hypothetical. It’s routine. You can’t know which plan is cheapest for you without entering your actual medications into the comparison tool.
Using the Medicare Plan Finder to Compare Plans
Medicare.gov provides a Plan Finder tool that does the heavy lifting for you. You enter all your medications, the exact dosages, and the frequency you fill them, and the tool calculates your estimated annual drug cost on every Part D plan available in your zip code. It shows you the total estimated cost: premium plus your expected cost-sharing, which is what actually matters. The plan with the lowest total annual cost for your specific drug list is your best financial option, and it’s almost never the plan with the lowest advertised premium.
Run this comparison every year during the Annual Enrollment Period, which runs October 15 through December 7. Don’t assume last year’s optimal plan is still optimal. Formularies change. Tier assignments change. Premiums change. Pharmacy networks change. Plans add and drop drugs from their formularies every January 1, and your plan is required to notify you of material formulary changes, but that notice arrives after enrollment season has already started. Don’t wait for the notice. Log into Plan Finder and run your comparison proactively each October so you have time to switch if something changed in a way that hurts you.
Also check pharmacy access when you’re comparing plans. Each Part D plan has a network of preferred pharmacies where cost-sharing is lowest and non-preferred pharmacies where you pay more. If you use a specific retail pharmacy chain or want to use mail-order, verify it’s a preferred pharmacy under the plans you’re considering. Mail-order pharmacy fills, typically 90-day supplies, usually come with lower per-dose cost-sharing than retail fills and the convenience of home delivery. For maintenance medications you take long-term, this can reduce your total annual costs by a meaningful amount.
The Medicare Extra Help Program
Medicare offers a subsidy program for Part D called Extra Help, also known as the Low-Income Subsidy or LIS, for Medicare beneficiaries with limited income and resources. Extra Help reduces or eliminates Part D premiums, deductibles, and copays. Some people are automatically eligible: those who receive Medicaid, Medicare Savings Program assistance, or Supplemental Security Income. Others can apply through Social Security if their income and assets fall below the program’s thresholds.
In 2025, full Extra Help covers most Part D costs entirely, and the $2,000 out-of-pocket cap is essentially irrelevant for full Extra Help recipients because their cost-sharing is near zero throughout the year. People who qualify for Extra Help can enroll in or switch Part D plans at any time throughout the year, not just during the standard enrollment windows. That’s a significant additional flexibility that most people don’t know about.
If you or a Medicare beneficiary you know has limited income, checking Extra Help eligibility is one of the most financially impactful steps available. A lot of eligible people never apply because they assume they won’t qualify, or they don’t know the program exists. You apply through Social Security either online at SSA.gov, by phone, or in person at a local Social Security office. The application is straightforward. Even a partial Extra Help subsidy can reduce Part D costs by hundreds of dollars per year.
The Part D Late Enrollment Penalty
If you don’t enroll in Part D when you first become eligible and you go 63 or more continuous days without creditable drug coverage elsewhere, you’ll face a permanent late enrollment penalty when you eventually do sign up. Creditable coverage means prescription drug coverage that’s at least as comprehensive as Part D, such as drug coverage through an employer group health plan, union coverage, VA drug benefits, or TRICARE. The penalty is 1% of the national base beneficiary premium for each month you were without creditable coverage. In 2025, that benchmark premium is $36.78, so each month of uncovered gap adds approximately $0.37 to your monthly Part D premium, permanently.
That may sound small, but it adds up. A two-year gap without creditable coverage means a 24% permanent penalty on the benchmark. If you go 36 months, it’s 36%. The penalty is recalculated annually as the benchmark premium changes, so your absolute dollar penalty fluctuates slightly year to year, but it’s always with you. Most people skip documenting their creditable coverage and regret it when they try to contest a penalty later. Your employer is required to notify you annually whether their drug coverage is creditable, typically before October 15. Keep those notices somewhere safe. If you’re ever assessed a penalty and you have documentation proving continuous creditable coverage, you can dispute and have it removed.
The practical rule is simple: if you don’t have creditable drug coverage elsewhere and you’re Medicare-eligible, enroll in a Part D plan. Even a low-premium plan with minimal benefits protects you from the penalty clock. You can always switch to a more comprehensive plan later during the Annual Enrollment Period. The $10 or $12 per month minimum plan exists for exactly this purpose.
Medicare Advantage Plans with Drug Coverage (MA-PD)
If you choose a Medicare Advantage plan instead of Original Medicare, your drug coverage is usually bundled into the plan as an MA-PD. You don’t need a separate Part D plan and you can’t add one if your Medicare Advantage plan already includes drug benefits. The same 2025 $2,000 out-of-pocket cap applies to MA-PD drug coverage, and the same formulary structure applies. MA-PD plans also use tiered formularies, preferred and non-preferred pharmacy networks, and mail-order options.
The Medicare Plan Finder tool compares MA-PD plans alongside standalone Part D plans when you run your medication comparison, so you can see how the bundled drug coverage in a Medicare Advantage plan compares to buying Original Medicare plus a standalone Part D plan. For many people with lower drug costs, an MA-PD that bundles drug coverage at low or zero premium can be the most cost-effective structure. For people with complex medication regimens and high-cost specialty drugs, the comparison is worth doing carefully, because formulary differences between MA-PD plans can be significant.
Annual Changes and the October Review Habit
Part D is not a set-it-and-forget-it decision. Plans issue an Annual Notice of Change every September explaining what’s changing for the next plan year: premium changes, formulary changes, pharmacy network changes, and cost-sharing changes. You’re required to receive it. Most people file it in the recycling bin and miss important changes that affect their drug costs. Don’t do that.
The habit that saves money is simple: every October, before November 1, go to Medicare.gov, open Plan Finder, enter your current medications, and run the full comparison. It takes about 20 minutes. If your current plan is still the best option, great, do nothing. If another plan would save you $300 or $700 or $1,200 for the year, switch before December 7 and the new coverage starts January 1. People who do this every year consistently pay less for their drug coverage than people who set it once and forget it, sometimes significantly less. It’s one of the few areas in Medicare where a small annual time investment has a direct and measurable financial payoff.
What Part D Does Not Cover
Part D covers prescription drugs intended for outpatient use, meaning drugs you pick up from a pharmacy and take at home. It does not cover drugs administered in a clinical setting like a hospital or doctor’s office, those are typically covered under Part B. Some high-cost medications like chemotherapy drugs administered intravenously in a clinical setting are Part B drugs, not Part D drugs, and the coverage and cost-sharing rules are completely different. If you’re starting a new medication and you’re not sure which part of Medicare covers it, ask your pharmacist or call your plan’s member services line. Getting this wrong can result in unexpected bills.
Part D also doesn’t cover drugs that aren’t on your plan’s formulary. If your doctor prescribes a drug that’s not on your plan’s formulary, you have options: your doctor can request an exception to have the drug covered, you can ask your doctor about a therapeutically equivalent alternative that is on the formulary, or you can pay full retail price out of pocket. Exception requests require medical necessity documentation and take time to process. If you’re starting a new prescription and you know it’s expensive, check formulary coverage before you fill it so you’re not blindsided at the pharmacy counter.