Prescription Costs & Part D
What is the difference between Part B and Part D drugs?
Last reviewed: · Big Sioux Benefits Data Desk
Part D covers prescriptions you pick up at a pharmacy and take yourself. Part B covers drugs administered to you by a health professional, such as infusions and injections given in a doctor's office or clinic, along with a few specific categories including most vaccines given as preventive care, certain immunosuppressants, and some drugs used with durable medical equipment such as nebulizer solutions.
The qualifier: this distinction is not cosmetic, because the two parts price drugs on entirely different systems. Under Part B you generally pay 20% coinsurance after the annual Part B deductible, which is $283 in 2026, and there is no annual out-of-pocket maximum in Original Medicare. Under Part D you pay through the three-phase structure that ends at a $2,100 cap. The same medication can therefore cost you very different amounts depending on where and how it is given.
Which part pays
| Situation | Usually covered by | Your cost structure |
|---|---|---|
| Pills you fill at a pharmacy | Part D | Deductible, then cost sharing, then $0 after $2,100 |
| Infusion given in a clinic | Part B | 20% coinsurance after the $283 deductible, no annual cap |
| Injection administered by your doctor | Part B | 20% coinsurance after the $283 deductible |
| Injection you give yourself at home | Usually Part D | Part D phases |
| Vaccines recommended for prevention | Part B or Part D depending on the vaccine | Often $0 either way, see below |
Two consequences worth planning around. Because Original Medicare's Part B side has no out-of-pocket maximum, a Medigap policy is what makes an expensive Part B drug financially predictable, which is one of the strongest arguments for Medigap in the first place, covered in the South Dakota Medigap guide. And because the $2,100 cap is a Part D cap, spending on Part B drugs does not count toward it. The vaccine rules are in are vaccines free under Medicare.
Sources
- Medicare.gov — Costs for Medicare drug coverage — accessed 2026-08-21
- CMS — 2026 Medicare Parts A & B Premiums and Deductibles — November 14, 2025; accessed 2026-08-21
- Medicare.gov — What Medicare Part D drug plans cover — accessed 2026-08-21
Related questions
Why does my infusion cost more than my pills even though the pills are pricier?
Because the infusion is almost certainly billed under Part B, where you pay 20% coinsurance with no annual out-of-pocket maximum in Original Medicare, while your pills run through Part D and stop costing you anything once you reach $2,100. The two parts price drugs on different systems, and Part B is the one without a ceiling.
Does a Medicare Advantage plan change the Part B and Part D split?
No, the underlying classification of a drug stays the same. What changes is that a Medicare Advantage plan applies its own copays and its own medical out-of-pocket maximum to the Part B side, which does create an annual ceiling that Original Medicare alone does not have.
Drug costs turn on your specific prescriptions, so the arithmetic is worth doing once, properly: book a free, no-pressure conversation, see your county's South Dakota plan lineup, or browse all answers. SHIINE, the state's free counseling program, is at 1-800-536-8197 and sells nothing.
Big Sioux Benefits is a licensed insurance agency, not affiliated with or endorsed by the U.S. government or the federal Medicare program. We do not offer every plan available in your area. Contact Medicare.gov, 1-800-MEDICARE, or SHIINE (South Dakota's SHIP) for all of your options.