Two people on Medicare each take an expensive medication. One picks up pills at the pharmacy; the other sits in an infusion chair at the doctor's office. Their bills follow completely different rules, because one drug is covered by Part D and the other by Part B — a distinction that determines whether annual drug costs are capped or effectively unlimited under Original Medicare. Here is how to tell which part pays in 2026, and why it matters.
The Short Answer
The dividing line is generally who administers the drug:
- Part B covers a limited set of drugs you generally cannot give yourself — infusions and injections administered by a provider, plus a few specific categories
- Part D covers drugs you pick up at a pharmacy and take yourself — pills, self-injected insulin, and most vaccines
The stakes: Part D out-of-pocket costs are capped at $2,100 in 2026, after which you pay $0 for covered drugs for the rest of the year. Part B drug costs have no annual cap under Original Medicare — and Part B drug spending does not count toward the $2,100 Part D cap.
What Part B Covers
Part B pays for drugs that generally must be administered by a professional or that are tied to Part B-covered equipment or services:
- Infusions and injections given in a doctor's office or hospital outpatient department — chemotherapy and many biologics are the most common examples (see our guide to how Medicare covers cancer treatment)
- Drugs used with Part B-covered durable medical equipment — insulin delivered through a pump, and nebulizer medications
- Oral anti-cancer drugs that have injectable equivalents
- Oral anti-nausea drugs taken within 48 hours of chemotherapy
- Immunosuppressants, but only after a Medicare-covered transplant
- Dialysis drugs
- Certain vaccines — flu, pneumococcal, COVID-19, and hepatitis B for at-risk patients
What Part D Covers
Part D covers self-administered outpatient drugs dispensed by a pharmacy:
- Pills, capsules, and other oral medications you take at home
- Insulin in pens or vials for self-injection, plus supplies such as syringes, needles, and alcohol swabs
- Most vaccines not covered by Part B — notably, the shingles vaccine is $0 under Part D, with no cost-sharing
By law, Part D cannot pay for a drug payable under Part A or Part B. Some drugs can be either B or D depending on use — an immunosuppressant may bill to Part B after a Medicare-covered transplant but to Part D otherwise. If you are new to drug plans, start with our Part D overview.
Why the Difference Matters: Capped vs. Uncapped Costs
Part B drugs: you generally pay 20% coinsurance after the $283 Part B deductible (2026), with no annual limit under Original Medicare. For an infused biologic costing $10,000 a month, that 20% is $2,000 a month — every month — without supplemental coverage.
Part D drugs: out-of-pocket costs are capped at $2,100 in 2026, with a maximum plan deductible of $615. Once you reach the cap, you pay $0 for covered drugs for the rest of the year. Our guides to the Part D out-of-pocket cap and Part D coverage phases explain the mechanics.
The two buckets do not mix: Part B drug spending does not count toward the $2,100 Part D cap. Two common ways to limit Part B exposure:
- Medicare Advantage: Part B drug cost-sharing counts toward the plan's out-of-pocket maximum, which cannot exceed $9,250 in-network in 2026
- Medigap: a Medicare Supplement policy may cover the 20% coinsurance on Part B drugs, depending on the plan
Insulin: One Price Rule for Both Parts
Insulin is the notable exception. Under the Inflation Reduction Act, insulin is permanently capped at $35 for a month's supply under both Part B and Part D, with no deductible applying — whether it is pump insulin billed to Part B or pen/vial insulin billed to Part D. For the full picture, see our guide to how Medicare covers diabetes supplies and equipment.
The Hospital "Self-Administered Drugs" Trap
A common billing surprise: during an ER visit or observation stay, the hospital gives you your regular daily medications — blood pressure pills, a statin. Part B does not pay for these "self-administered drugs", because they are drugs you would normally take yourself, and hospitals may bill them directly at rates far above pharmacy prices. What you may be able to do:
- Submit the itemized receipts to your Part D plan for possible reimbursement — but the hospital pharmacy is usually out-of-network and the drug must be on your formulary, so reimbursement may be partial
- Ask whether you can bring your own labeled medications with a doctor's order — some hospitals allow it
The trap is closely tied to observation status — see our article on urgent care vs. emergency room costs.
How to Tell Which Part Pays
A quick test before starting a new medication:
- Will a provider administer it (infusion, office injection)? It likely bills to Part B
- Will a pharmacy dispense it for you to take yourself? It likely bills to Part D — check your plan's formulary to confirm coverage and tier
- Is it a gray-area drug (immunosuppressants, certain oral cancer drugs, insulin)? Ask your doctor's billing office or your Part D plan which part applies
Asking up front helps you anticipate whether the cost is capped or open-ended.
How to Get Help and Learn More
To confirm how a specific drug is covered, these official resources can help:
- Medicare.gov — See the official page on outpatient prescription drug coverage at medicare.gov/coverage/prescription-drugs-outpatient.
- 1-800-MEDICARE (1-800-633-4227) — Medicare's official helpline can answer coverage and billing questions. TTY users can call 1-877-486-2048.
- State Health Insurance Assistance Program (SHIP) — SHIP offers free, unbiased counseling on how your medications are covered. Find your local program at shiphelp.org or by calling 1-800-MEDICARE.
Summary and Next Steps
Which part pays comes down to how a drug is administered — and that determines whether costs are capped. Key points:
- Part B covers provider-administered drugs: infusions, injections, drugs used with covered equipment, certain oral cancer and anti-nausea drugs, post-transplant immunosuppressants, dialysis drugs, and certain vaccines
- Part D covers self-administered pharmacy drugs — pills, self-injected insulin and supplies, and most other vaccines, including the shingles vaccine at $0
- Part D costs are capped at $2,100 in 2026; Part B drug costs have no cap under Original Medicare, and Part B spending does not count toward the Part D cap
- Insulin is $35 per month's supply under both parts, with no deductible
- In Medicare Advantage, Part B drug cost-sharing counts toward the plan's out-of-pocket maximum (no more than $9,250 in-network in 2026); Medigap may cover the 20% on Part B drugs
- Watch for hospital-billed "self-administered drugs" during ER and observation stays
Before starting a high-cost medication, ask: "Does this bill to Part B or Part D?" If you are unsure what the answer means for your coverage, a SHIP counselor can walk through it with you at no charge.
