Medicare Part B vs. Medicare Part D Drugs

By Marcia Mantell

What I asked:

What do I need to know about the difference between Part D and Part B medications?

What I got back from AI (aka Chatty Cathy):

Medicare Part B vs. Part D Drug Coverage: Key Differences

Medicare Part B typically covers drugs that are:

Administered in a clinical setting (like a doctor’s office or outpatient hospital)

Not usually self-administered

Often associated with specific medical conditions or procedures

Examples of Part B drugs include:

Infusion drugs given through durable medical equipment (e.g., insulin pumps)

Injectable medications like chemotherapy drugs

Vaccines such as the flu shot, pneumonia shot, and COVID-19 vaccine

Certain medications for ESRD (End-Stage Renal Disease) if you’re on dialysis

Medicare Part D covers drugs that are:

Typically self-administered

Prescribed and picked up at a pharmacy

Covered under a Part D prescription drug plan or a Medicare Advantage plan with drug coverage

Examples of Part D drugs include:

Oral medications like antibiotics or blood pressure meds

Insulin (non-pump) and diabetic supplies like needles and syringes

Shingles vaccine (Shingrix)—covered only under Part D

Why the Distinction Matters

Billing Errors: If a drug is incorrectly billed under the wrong part, claims can be denied.

Higher Out-of-Pocket Costs: You might pay more out-of-pocket if you or your provider assumes coverage under the wrong part.

Prior Authorization: Some medications might require approval or step therapy depending on which part covers them.

Tips to Avoid Confusion

Ask your provider or pharmacist whether the drug is covered under Part B or Part D.

Know how the drug is administered—that’s usually the biggest clue.

Check your plan’s formulary (for Part D) and Part B coverage rules.

Consider working with a Medicare-savvy pharmacist or agent if you take specialty drugs.

Here’s a simple side-by-side chart comparing Medicare Part B vs. Part D drug coverage:

Assessment of the AI answer:

Overall, it’s the same information one will get if searching on Medicare.gov or using Google. The information is correct and technically accurate, but not nearly complete. It does a good job of showing consumers the craziness in the vaccine world. Why would the flu and COVID-19 vaccine be paid under Part B while the shingles vaccine is under Part D? You get all three of these shots at your local pharmacy or at a clinic. Who knows?

However, and unfortunately, Chatty Cathy misses the most critical point. It’s the one about how much you will pay depending on how you get your Part B coverage. Did you choose Original Medicare Part B along with a Medigap plan? Or did you think you were going to save a lot of money with a Medicare Part C (Medicare Advantage or MAPD) plan?

If you are one of the

57% of 65+ folks who end up with cancer that requires chemo, or

47% of 65+ folks with diabetes who develops kidney disease that may lead to dialysis, or

27% of women over 65 who will develop osteoporosis and need treatment

you could pay substantially more if you are in a Part C (Advantage) plan than with Original Medicare Part B with a Medigap plan.

Medicare Part C plans require a 20% coinsurance

Those who have chosen a substitute to Medicare via Part C and wind up needing injectable treatments are often exposed to paying the plan’s maximum out-of-pocket (OOP) costs. The maximum OOP allowed by law in 2025 is $9,350 for in-network providers and $14,000 for out-of-network usage.

So much for $0 and low dollar monthly premium plans.

The Medicare Advantage Prescription Drug Plan (MAPD) you have chosen may have a lower maximum OOP. Perhaps $4,500 in network and $10,000 out-of-network. Insurers can do better than the legal maximums.

Let’s say you get a cancer diagnosis in September. Chemotherapy starts in October every other week for the next six months. That means you’ll cross over a calendar year. You’ll pay your 20% share of the cost of the Part B drug treatment in the fourth quarter of the current year and the first quarter of the next year. MAPDs are single-year contracts and restart each January.

If the MAPD you have has a lower out-of-pocket maximum of, say, $4,500 per year, and the cost of the treatment is $50,000 per injection, you’ll need to write a check for $4,500 this year. And another $4,500 next year. $9,000 flies out of your bank account or health savings account pretty fast.

But, if your in-network plan charges the maximum $9,350, you’ll shell out the remaining balance of $9,350 less anything you’ve already paid this year. And another $9,350 plus any increase to the max OOP amount early next year. We’ll round your required expenses to about $18,000. All payable in a short six-month period.

Your OOP costs are known and manageable with a Medigap plan

On the other hand, if you had kept your Original Part B along with a Medigap Plan G, or Supplement 1 or 1A in Massachusetts, you would have simply paid your monthly premium. In most states in 2025, the monthly premium is between $150 and $250 per month. For the entire year, you’d pay $1,800 to $3,000. And you’d rarely, if ever, see any kind of bill.

Medigaps do not come with $0 monthly premiums. Rather, they charge a flat monthly premium that you’ll pay every month. You know the amount you’ll be paying. You can estimate annual inflation increases (I use 6% as a placeholder to forecast future years) and have a predictable budget.

That way, if you’re the one who needs the injectable Evenity for managing your osteoporosis, Part B covers 80% and your 20% share is paid by your Medigap Plan G. This is a drug that retails for $3,150/month, or $37,800/year. You’ll just pay your monthly insurance premium.

Can you switch out of a Part C and buy a Medigap?

Chatty Cathy also forgot to mention how difficult it can be to get out of a Part C plan (MAPD) once you get a diagnosis that comes with high OOP costs. In all but four states, don’t count on getting into a Medigap plan once your initial 6-month period runs out. You’ll be subject to medical underwriting where the new insurance company will want to find out how healthy you are. If you already have a cancer diagnosis, you aren’t so healthy.

The insurer may allow you to buy a Medigap, but with a 6-month or longer exclusion period. Meaning, during the first six months, they will not cover your share of Part B costs.

In Massachusetts, New York, Connecticut, and Maine, you can generally get out of a MAPD and still buy a Medigap. Even in your 80s and 90s. Or if diagnosed with an expensive disease. These states offer continuous or nearly-continuous open enrollment. A few other states are starting to ease restrictions on moving. But it’s important to think about your decision as a 30-year health insurance commitment.

About the author: Marcia Mantell, RMA, NSSA

Marcia Mantell, RMA®, NSSA®, is the founder and president of Mantell Retirement Consulting, Inc., a retirement business and education company. She’s the author of “What’s the Deal with Retirement Planning for Women,” “What’s the Deal with Social Security for Women,” “Cookin’ Up Your Retirement Plan,” “Creating Your Medicare Recipe,” and blogs at BoomerRetirementBriefs.com.