Medicare Part D exists to cover prescription drugs that Original Medicare (Parts A and B) does not. If you take any medications regularly, or expect to in the future, Part D is how Medicare helps you pay for them. Even if you’re healthy now, skipping Part D can cost you more later through permanent penalties added to your premiums.
What Part D Actually Covers
Parts A and B of Medicare cover hospital stays and doctor visits, but they cover very few prescription drugs. Part B pays for a handful of medications administered in a doctor’s office or clinic, plus flu and pneumonia vaccines. Everything else falls to Part D: medications you pick up at a pharmacy, insulin, insulin delivery supplies like syringes and pen needles, and most vaccines not covered under Part B (like shingles or tetanus shots).
The key rule is simple: if a drug could be paid for under Part A or Part B, Part D won’t cover it. Part D fills the gap for everything else prescribed by your doctor for a medically accepted use. That includes brand-name drugs, generics, biologics, and specialty medications.
How Part D Protects You Financially
The biggest reason to enroll is cost protection. Starting in 2025, Part D plans cap your total out-of-pocket drug spending at $2,000 per year. Once you hit that limit, you pay nothing for covered prescriptions for the rest of the year. This is a major change. Before 2024, there was no hard cap at all. People taking expensive medications for cancer, autoimmune conditions, or other chronic illnesses could spend thousands of dollars annually even after reaching the so-called catastrophic coverage phase, where they still owed 5% of drug costs indefinitely.
That 5% doesn’t sound like much until you’re taking a specialty drug that costs $10,000 a month. The $2,000 cap, introduced through the Inflation Reduction Act, eliminates that open-ended exposure entirely.
How Drug Costs Are Structured
Part D plans organize medications into tiers, and your cost depends on which tier your drug falls into. Generic drugs on the lowest tier typically cost between $0 and $10 per prescription. Preferred brand-name drugs cost more, usually a fixed copay. Specialty drugs, those costing over $950 per prescription in 2025, carry coinsurance instead of a flat copay, typically 25% to 33% of the drug’s cost in standalone Part D plans.
Every plan maintains a formulary, which is its list of covered drugs. Not all plans cover the same medications, and the tier placement can vary between plans, so a drug that’s $10 on one plan might cost $47 on another. This is why checking whether your specific medications are on a plan’s formulary, and which tier they’re on, matters more than just comparing monthly premiums.
The benefit has phases. You start by paying a deductible (if your plan has one), then move into the initial coverage phase where you typically pay 25% of drug costs. Once your out-of-pocket spending reaches $2,000, you enter catastrophic coverage and pay nothing for the rest of the year.
The Late Enrollment Penalty
This is the reason many people enroll in Part D even when they don’t take any medications. If you don’t sign up when you’re first eligible and you go 63 or more consecutive days without what Medicare considers “creditable” drug coverage, you’ll face a permanent penalty when you eventually do enroll. The penalty is 1% of the national base beneficiary premium for every month you were uncovered. That adds up fast.
Say you waited five years (60 months) without creditable coverage. Your penalty would be 60% of the national base beneficiary premium, added to your Part D premium every month for as long as you have Part D. The national base beneficiary premium is $38.99 in 2026, so a five-year gap would cost you roughly an extra $23 per month on top of your regular premium, permanently.
The penalty resets each year based on the current national base premium, so it can increase over time. This is designed to encourage people to enroll early, even if they’re healthy, because drug needs tend to increase with age.
What Counts as Creditable Coverage
You can avoid the penalty without enrolling in Part D if you already have drug coverage that’s considered “creditable,” meaning it’s expected to pay at least as much as a standard Part D plan. Employer or union health plans, TRICARE, and VA coverage can all qualify. Your current plan is required to tell you each year whether your drug coverage is creditable. If you get a notice saying it is, keep it. You’ll need that documentation if you later switch to Part D to prove you don’t owe a penalty.
If your coverage is not creditable, or if you’re not sure, treat it as if you’re uncovered. The 63-day clock starts ticking from the moment you lose creditable coverage or become eligible for Medicare without having it.
When You Can Enroll
Your first chance to sign up is the Initial Enrollment Period: a seven-month window that starts three months before the month you turn 65, includes your birthday month, and extends three months after. If you miss that window, you’ll generally need to wait for the Annual Open Enrollment Period, which runs from October 15 through December 7 each year.
Certain life events trigger a Special Enrollment Period that lets you sign up or switch plans outside those windows. The most common triggers include losing employer or union drug coverage, moving to a new area where different plans are available, losing Medicaid eligibility, or being released from incarceration. You can also get a Special Enrollment Period if your current plan loses its Medicare contract or receives consistently poor quality ratings.
Help Paying for Part D
If your income and savings are limited, you may qualify for Extra Help (also called the Low-Income Subsidy), which covers most or all of your Part D premiums, deductibles, and copays. In 2026, the income limit is $23,940 for an individual or $32,460 for a married couple. Your countable resources (savings, investments, and similar assets, but not your home or car) must be below $18,090 for an individual or $36,100 for a couple.
People who qualify for Extra Help also get a Special Enrollment Period, meaning they can switch Part D plans once per quarter rather than waiting for open enrollment. Even if you’re not sure you qualify, applying costs nothing and can save thousands of dollars a year in drug costs.