Medicare Part D vs. Part B: Why the Difference Costs You Thousands
Medicare Part D vs. Part B: Why the Difference Costs You Thousands
6 minute read
If you are on Medicare and taking a specialty drug, there is a good chance you have run into a confusing situation: your medication is covered by a different part of Medicare than you expected, or suddenly it is not covered by the part it used to be under. The difference between Medicare Part B and Part D coverage for prescription drugs is one of the most misunderstood and financially consequential distinctions in American healthcare.
This is not obscure fine print. For people who take biologics, chemotherapy agents, infusion drugs, or specialty injectables, getting this wrong can mean thousands of dollars in unexpected out-of-pocket costs. Understanding how these two benefits work, and how to respond when coverage shifts, is worth your time.
The Basic Difference: Part B vs. Part D
Medicare Part B is the medical benefit. It covers physician services, outpatient care, and certain drugs that are administered by a healthcare provider in a clinical setting. Think of Part B drugs as drugs that come with a needle and a medical professional.
Medicare Part D is the prescription drug benefit. It covers drugs you pick up at a pharmacy and take at home.
The distinction sounds simple, but it gets complicated fast. Here is why it matters financially: Part B drugs are typically billed at 80 percent coverage after the Part B deductible, with a 20 percent coinsurance that has no annual out-of-pocket maximum. Part D drugs have their own deductible, coverage gap (though significantly reduced since the Inflation Reduction Act of 2022), and copay tiers, but they do have a catastrophic cap.¹
For an expensive specialty drug, the absence of a Part B out-of-pocket cap can be devastating. A drug that costs $15,000 per infusion, with 20 percent coinsurance and no cap, leaves you with $3,000 per treatment. That adds up to $36,000 per year if you receive monthly infusions, even with a Medicare supplement (Medigap) plan, which may not cover all of that gap depending on your plan type.
Which Drugs Fall Under Which Benefit
The rule of thumb is that drugs administered by injection or infusion in a doctor's office or hospital outpatient setting are generally covered under Part B. Drugs you self-administer at home are generally covered under Part D.
But the exceptions are numerous and clinically significant.
Part B drugs typically include:
Part D drugs typically include:
Here is where it gets particularly confusing: the same drug can fall under Part B or Part D depending entirely on how it is administered. An infliximab infusion given in your rheumatologist's office is Part B. A subcutaneous biologic you self-inject at home is Part D. If your rheumatologist switches your administration route, your coverage benefit can shift, and the financial implications may be significant.
What Happens When Coverage Shifts
Coverage shifts happen for several reasons: a change in how your drug is administered, a formulary update, a change in where you receive treatment, or a Medicare plan switch.
When coverage shifts from Part D to Part B (or vice versa), the consequences depend on your specific plan structure. If you have a Medicare Advantage plan, the plan is required to cover both Part B and Part D benefits, but the cost-sharing may differ significantly between the two. If you have traditional Medicare with a Part D prescription drug plan, a shift to Part B means your Part D plan no longer applies and your cost-sharing is governed entirely by Part B rules.
If you discover that a drug you thought was covered under Part D is being billed under Part B (or not covered at all under your current plan structure), here is what to do:
Verify the Part B vs. Part D status with your prescriber. Ask your physician's billing office how the drug has historically been billed. Medicare has published guidance on which drugs fall under each benefit, and your prescriber's office deals with this regularly.
Review your Medicare Summary Notice. This document, which arrives quarterly, shows all claims processed by Medicare and which benefit paid for them. If something looks different from what you expected, the Medicare Summary Notice is your starting point.
Contact Medicare directly. The Medicare helpline (1-800-MEDICARE) and the Medicare Plan Finder tool can help you verify coverage status for specific drugs.
Consider the implications during open enrollment. If you know you are taking a high-cost Part B drug, your Medicare Supplement and Part D plan choices during open enrollment (October 15 through December 7 each year) can significantly affect your total annual cost. Some Medigap plans cover Part B coinsurance more generously than others.
The Inflation Reduction Act Changes
The Inflation Reduction Act of 2022 made significant changes to Medicare Part D, most of which took effect between 2023 and 2025. The most important change for people on high-cost specialty drugs is the $2,000 out-of-pocket cap on Part D expenses, which took effect in 2025.² This cap fundamentally changes the calculus for Part D drugs — but it does not apply to Part B.
If you are currently receiving an expensive drug under Part B with high coinsurance and no cap, it may be worth asking your physician whether there is a clinically equivalent option that could be administered in a way that falls under Part D instead, where the new $2,000 cap would apply. This is a conversation between you and your doctor, not a decision to make unilaterally.
What This Means for You
If you are on Medicare and taking any specialty medication, understanding which part of Medicare covers it and what your cost-sharing looks like is not optional — it is essential financial self-defense. Surprises in this space tend to be expensive ones.
Ellen can help you decode your Medicare coverage situation and understand your options if a coverage shift has left you with unexpected costs. Start here →
Frequently Asked Questions
How do I find out if my drug is covered under Part B or Part D?
Medicare publishes a drug coverage database at medicare.gov where you can look up specific drugs. Your prescribing physician's billing office can also tell you how the drug is typically billed. If you have a Medicare Advantage plan, your plan's formulary will specify coverage category and cost-sharing.
Does my Medigap plan cover Part B drug coinsurance?
Most standardized Medigap plans cover the 20 percent Part B coinsurance, but the specifics depend on your plan letter. Plans C, D, F, G, M, and N include Part B coinsurance coverage in various forms. If your drug costs are high, reviewing your Medigap plan's coverage of Part B coinsurance is worth the time.
What is the Medicare Part B drug coverage gap?
Unlike Part D, Part B does not have a traditional coverage gap. However, Part B has no out-of-pocket maximum, meaning that 20 percent coinsurance accumulates without a cap unless you have a Medigap plan or Medicare Advantage plan that limits your annual out-of-pocket. The Inflation Reduction Act's $2,000 cap applies only to Part D.
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