Medicare Prescription Drug Coverage: Where It Comes From and How It Works

Which part pays depends on where you get the drug

Which Part of Medicare Pays for Your Drugs? is the distinction that causes most of the confusion, and it is about setting rather than medication. This page is part of our Medicare 101 series. The same drug can be covered by different parts of Medicare depending on who administers it and where.

• Part A: Drugs given during an inpatient hospital stay or a covered skilled nursing stay. They form part of that bill, so no drug plan is involved.
• Part B: A limited set, mostly administered by a clinician: some infusions and injections, flu and pneumonia vaccines, certain oral cancer drugs, and drugs used with durable medical equipment. You usually pay coinsurance after the Part B deductible.
• Part D: Almost everything you collect from a pharmacy or receive by post. Original Medicare does not cover these at all on its own.

Two ways to get Part D

Private insurers approved by Medicare offer drug coverage in two forms, and you choose one, not both. A stand-alone prescription drug plan, often called a PDP, attaches to Original Medicare and is what people pair with a Medigap policy. Alternatively, most Medicare Advantage plans (Part C) include it. Medicare Advantage plans build drug coverage in, and if you join one of those you generally cannot add a separate PDP alongside it. Special Needs Plans, for people with particular conditions or with both Medicare and Medicaid, always include drug coverage.

How a plan's drug list actually works

Every Part D plan publishes a formulary, its own list of covered drugs. Each must cover a range across the categories people on Medicare use most, but the specific drugs are the plan's choice, which is why two plans can differ enormously for the same person. A formulary can also change mid-year, and the plan must tell you about changes affecting you. Most plans sort drugs into tiers, with generics on lower tiers costing less and brand-name and specialty drugs higher up, and your copay or coinsurance follows the tier rather than the drug.

The three coverage rules behind most pharmacy surprises

A drug being on the formulary does not always mean you can collect it today. Plans may attach conditions, and knowing the names makes them far easier to challenge.

• Prior authorization: You or your doctor must get the plan's approval before it covers the drug.
• Step therapy: You must try a lower-cost drug for the same condition first.
• Quantity limits: The plan caps how much you can get at one time.
• You can ask for an exception: If your doctor believes you need a drug that is not listed, or that a rule should not apply to you, they can send a statement explaining why. Step therapy and quantity limits are often waived this way.

What you pay

Your costs are a monthly premium, possibly a deductible, and cost-sharing per prescription. Federal limits cap the worst of it. In 2026 no plan's deductible can exceed $615 and your out-of-pocket spending on covered drugs is capped at $2,100 for the year, after which you pay nothing more for them. Those limits rise to $700 and $2,400 in 2027. Covered insulin is capped at $35 a month, and recommended adult vaccines cost nothing. Our Medicare Part D page explains each stage. If your income and resources are limited, Extra Help lowers all of it, and anyone on one of the Medicare Savings Programs qualifies for the Extra Help program automatically.

• Extra Help, if money is tight: The federal Extra Help program lowers premiums, deductibles and copays. In 2026 the income limits are $23,475 for an individual and $31,725 for a couple, with resource limits of $18,090 and $36,100.
• It can be automatic: Qualify for a Medicare Savings Program and you get Extra Help without applying separately.

When you can join

When You Can Join follows the same windows as the rest of Medicare, starting with your Initial Enrollment Period (IEP), with one important difference: the penalty clock runs on days, not on enrollment periods.

• Initial Enrollment Period: The 7 months around your 65th birthday.
• Annual Enrollment Period: October 15 to December 7, for coverage starting January 1.
• Special Enrollment Period: After certain events, such as moving or losing employer drug coverage.

The penalty for waiting

Go 63 days or more in a row without Part D or other creditable drug coverage once your Initial Enrollment Period has ended and a late enrollment penalty is added to your premium for as long as you hold drug coverage. It is 1% of the national base beneficiary premium, $41.33 for 2027, for every full uncovered month. Creditable coverage means coverage expected to pay at least as much as standard Part D, which many employer and VA plans are. Keep the notice your plan sends each year stating whether it counts, because that document is what settles the question later.

If you already have drug coverage

If You Have Other Drug Coverage the rules change. Plenty of people reach Medicare with coverage already in place through an employer, a union, a retiree plan, TRICARE or the VA, and in many cases you can keep it and delay Part D with no penalty, provided it is creditable. One warning before you change anything: ask the plan administrator how it works alongside Medicare first. Some employer plans end drug coverage for a retiree who joins a separate Part D plan, and in some cases that decision also affects a spouse or other family members on the same policy. That is very hard to undo.

Choosing a plan without guessing

The plan with the lowest premium is rarely the cheapest over a year. Our Tips for Choosing a Drug Plan go further, but work through this order first.

• List every medicine: With the dose and how often you take it.
• Check each against the formulary: Confirm it is covered, and note which tier.
• Look for the three rules: Prior authorization, step therapy or quantity limits on anything you take.
• Ask about generics: Your doctor can say whether a lower-tier alternative would work.
• Check your pharmacy: Confirm it is in network, and compare mail order pricing.
• Compare the yearly total: Premiums plus deductible plus your actual prescriptions, not the premium alone.
• Review every fall: Formularies and tiers change annually, and the plan that fitted last year may not.

Common questions

More in Medicare 101

• Turning 65: Your complete guide to starting Medicare
• Medicare Eligibility: Who Qualifies, and When
• How to Enroll in Medicare, Step by Step
• Medicare and Working Past 65: When to Enroll and What to Keep
• The Parts of Medicare, Explained: A, B, C and D
• Medicare Enrollment Periods: Every Window Explained
• Medicare Late Enrollment Penalties, and How to Avoid Them
• Medicare Costs: What You'll Actually Pay
• Medicare Advantage vs. Medicare Supplement: How to Choose