Medicare Part B vs. Part D Drugs: Which Plan Covers Your Medication?

One of the most common points of confusion in Medicare is why some drugs are covered under Part B while others fall under Part D — and what that distinction means for your out-of-pocket costs. The answer isn’t always intuitive, but understanding it can save you hundreds or even thousands of dollars annually. This guide explains how Medicare’s two drug-coverage systems work, what determines which one covers your medication, and the financial implications of each.

The Core Distinction: How the Drug Is Administered

The single biggest factor in whether a drug falls under Part B or Part D is how it is administered and in what setting:

Part B generally covers drugs that are administered by a healthcare provider (injected, infused, or otherwise administered in a clinical setting) and drugs directly related to certain medical equipment or procedures
Part D covers drugs you take at home — pills, capsules, patches, self-injectable biologics, and most outpatient prescription drugs

But this principle has important exceptions and nuances that can significantly affect your costs.

What Part B Covers

Infused and Injected Drugs in a Clinical Setting

Part B covers drugs that require administration by a healthcare professional in a physician’s office, hospital outpatient department, or infusion center. Examples include:

Chemotherapy drugs administered intravenously (IV chemo falls under Part B; oral chemotherapy pills generally fall under Part D)
Biologic infusions for rheumatoid arthritis, Crohn’s disease, and other autoimmune conditions (infliximab/Remicade, vedolizumab/Entyvio, natalizumab/Tysabri)
Bone-strengthening infusions such as zoledronic acid (Reclast) for osteoporosis
IVIG (intravenous immunoglobulin) for immune deficiencies, CIDP, and other conditions
Erythropoiesis-stimulating agents (Epogen, Aranesp) administered in dialysis centers or physician offices
Ophthalmology injections — anti-VEGF drugs like ranibizumab (Lucentis), aflibercept (Eylea), and bevacizumab (Avastin) for wet AMD and diabetic macular edema

Insulin Administered via Insulin Pump

Medicare Part B covers insulin used in an externally worn insulin pump (durable medical equipment). The 2023 Inflation Reduction Act capped insulin cost-sharing at $35 per month under both Part B (pump insulin) and Part D (self-injected insulin). If you use an insulin pump, your insulin is covered under Part B as a supply for the DME — not under Part D.

Drugs administered directly during a covered procedure (e.g., anesthesia drugs, contrast agents, drugs injected during endoscopy)
Antigens administered by a physician
Hepatitis B vaccine for at-risk patients administered in a clinical setting
Flu and pneumonia vaccines (when administered by a provider enrolled in Medicare)
Immunosuppressive drugs for Medicare beneficiaries who received a Medicare-covered kidney transplant — extended coverage added in 2023 makes this lifetime coverage rather than 36 months

Oral Drugs for ESRD Patients

Medicare Part B covers certain oral drugs for end-stage renal disease (ESRD) patients receiving dialysis — specifically oral drugs that are “equivalent” to injectable drugs used in dialysis treatment. This is an exception to the general oral-drug-goes-to-Part-D rule.

Under original Medicare, Part B drug coverage works like any other Part B service:

You pay the Part B deductible ($240 in 2025) if not yet met
Then 20% coinsurance on the Medicare-approved amount, with no out-of-pocket cap

For infused biologics and cancer drugs, the 20% can be enormous. A drug with a monthly infusion cost of $10,000 means $2,000 in coinsurance per month — $24,000 per year — with no annual cap under original Medicare. This is the primary reason Medigap coverage is financially critical for anyone with a high-cost infused medication. Medigap Plan G covers the Part B 20% coinsurance, converting that $24,000 exposure to near-zero beyond the Medigap premium.

Site of Care and Part B Drug Costs

Where a Part B drug is infused can dramatically affect how much Medicare pays — and thus how much your 20% coinsurance amounts to:

Hospital outpatient department (HOPD): Medicare pays the hospital’s facility fee plus a separate drug fee, often at higher rates under the Outpatient Prospective Payment System (OPPS)
Physician office or freestanding infusion center: Medicare pays under Part B fee schedule (generally lower), resulting in lower total costs and lower 20% coinsurance

For example, the same biologic infusion might cost $8,000 in a hospital outpatient department but $5,500 in a physician office. Your 20% coinsurance is $1,600 vs. $1,100 — a $500 difference per infusion. Over a year of monthly infusions, site-of-care choice can save $6,000 in out-of-pocket costs.

Ask your oncologist or infusion specialist whether you can receive infusions in their office or a freestanding infusion center rather than the hospital outpatient setting. Medically, the drug is the same; financially, the difference is substantial.

What Part D Covers

Part D covers the vast majority of outpatient prescription drugs you take at home:

Oral medications: pills, tablets, capsules of all kinds
Self-injectable biologics: medications like adalimumab (Humira), etanercept (Enbrel), or insulin pens you inject at home
Patches and transdermal drugs: hormone patches, pain patches, nicotine patches (if prescribed)
Oral cancer drugs: the Oral Chemotherapy Parity Act requires Medicare Advantage prescription drug plans to cover oral cancer drugs at the same cost-sharing as IV drugs, but standalone Part D plans are not yet required to do the same — though many do
Most inhaled medications: bronchodilators, corticosteroids, combination COPD inhalers
Topical medications that are prescription-only

The 2025 Part D redesign significantly changed the cost structure:

Annual out-of-pocket cap: $2,000 per year is the maximum you’ll pay in Part D drug costs in 2025 (down from the prior catastrophic threshold of roughly $8,000+)
Manufacturer discounts: Drug manufacturers now contribute to the catastrophic tier, effectively reducing what beneficiaries pay for branded drugs
Monthly Installment Plan: You can spread your $2,000 maximum over monthly payments rather than paying large amounts upfront early in the year

The $2,000 cap is particularly meaningful for Medicare beneficiaries on high-cost specialty drugs — cancer drugs, biologics, and specialty medications that previously drove huge annual out-of-pocket burdens.

What Part D Does NOT Cover

Part D explicitly excludes certain drug categories:

Drugs covered under Part B (as described above — you can’t double-bill)
Over-the-counter medications (except insulin, which requires a prescription)
Drugs used for weight loss or gain (though coverage is expanding for GLP-1 drugs for diabetes and cardiovascular risk)
Fertility drugs
Drugs used for erectile dysfunction
Prescription vitamins and mineral products (except prenatal vitamins and fluoride)
Barbiturates (some plans voluntarily cover these)
Benzodiazepines (some plans voluntarily cover these)

Finding Whether Your Drug Is in a Plan’s Formulary

Every Part D plan has a formulary — a list of covered drugs organized by tiers that determine your cost-sharing. Drug placement on a plan’s formulary is the most important factor in your annual drug costs. The Medicare Plan Finder at medicare.gov lets you enter your specific medications and compare total annual costs across all available Part D and Medicare Advantage plans in your area.

Use Plan Finder every October during the Annual Enrollment Period (October 15 – December 7) to see if your drugs’ formulary placement or tier has changed for the coming year.

The Gray Areas: When It’s Not Clear Which Part Applies

Oral Cancer Drugs

This is one of the most practically important gray areas. IV chemotherapy is covered under Part B. Oral chemotherapy for the same cancer type is generally covered under Part D. This can result in vastly different cost-sharing:

IV chemo under Part B: 20% coinsurance on Medicare-approved amounts, with Medigap potentially covering all of it
Oral chemo under Part D: Tier 4 or Tier 5 specialty drug cost-sharing, subject to the $2,000 annual cap

The Oral Chemotherapy Parity Act applies to Medicare Advantage plans with prescription drug benefits — it requires them to charge no more for oral chemo than they charge for IV chemo. Standalone Part D plans are not subject to this requirement, though many have adopted similar policies voluntarily.

If you have both a Medigap plan and a standalone Part D plan, and your oncologist is offering either an IV or oral treatment option with similar efficacy, the IV option may be cheaper for you because Part B 20% is covered by Medigap while Part D oral drugs are not. Always confirm the cost difference with your oncologist’s billing department before choosing between equivalent treatment options.

Self-Administered Injectable Biologics

Most self-injectable biologics are covered under Part D (you take them home and inject yourself), but some have Part B coverage in certain circumstances:

If a physician administers the first dose in the office and the patient later self-administers subsequent doses, the first dose may be billed under Part B
Certain biologics may be covered under Part B if they meet specific criteria

When your doctor’s office or infusion center handles billing, they determine which program is billed — you typically won’t make this choice yourself. But if you’re paying high Part B cost-sharing for a biologic your plan says is Part D, it’s worth confirming with your provider which billing code applies.

Vaccines

Vaccine coverage has been restructured:

Flu and pneumonia vaccines: Part B, no cost-sharing
COVID-19 vaccines: Part B, no cost-sharing
All other recommended adult vaccines (shingles/Shingrix, RSV vaccines, Tdap, etc.): Part D, no cost-sharing since 2023 under the Inflation Reduction Act

The no-cost-sharing rule for Part D vaccines applies even if you haven’t met your Part D deductible. You should not pay anything for a shingles vaccine — if you’re being charged, your provider may be billing incorrectly.

Practical Strategy Guide

If You Take High-Cost Infused Biologics

Get a Medigap Plan G or Plan N during your initial enrollment window to cover the Part B 20% coinsurance
Ask about site of care — physician office or freestanding infusion center instead of hospital outpatient if medically appropriate
If you have Medicare Advantage, check your plan’s prior authorization requirements and infusion provider network before starting a new biologic

If You’re Starting High-Cost Oral Medications

Use the Plan Finder every October to compare total drug costs across plans — specialty tiers vary significantly between plans
Consider the Medicare Savings Program (Extra Help) if your income is limited — it substantially reduces Part D premiums, deductibles, and cost-sharing
Track your spending toward the $2,000 cap — once you hit the cap, all covered drugs cost $0 for the rest of the year

If You’re Choosing Between IV and Oral Treatment Options

Confirm with your provider’s billing department the expected cost-sharing for each option under your specific coverage
With Medigap Plan G, IV options (Part B) often have lower net cost than oral options (Part D) for expensive drugs
With Medicare Advantage, the calculation depends on your specific plan’s drug tiers and prior authorization rules

Key Takeaways

Part B covers drugs administered by healthcare providers in clinical settings — IV chemo, infused biologics, ophthalmology injections, pump insulin, and some dialysis drugs
Part D covers drugs you take at home — oral medications, self-injectable biologics, and patches
Part B has no annual out-of-pocket cap — the 20% coinsurance with no ceiling is the reason Medigap is essential for high-cost infused medications
Part D has a $2,000 annual cap in 2025, protecting beneficiaries on high-cost specialty drugs
Site of care matters for Part B drugs — hospital outpatient costs more than physician office or freestanding infusion center
Use the Medicare Plan Finder every October to ensure your Part D plan still covers your medications at the lowest tier