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The 20% with no cap: why Part B coinsurance is the biggest Medicare risk

How Part B's 20% coinsurance works, what it costs on serious illness, and the ways people limit it with Medigap or Medicare Advantage.

After the $283 deductible, Part B pays 80% of the Medicare-approved amount for doctor services, outpatient care, imaging, lab work, durable medical equipment and many drugs given in a clinic. You pay the other 20%. There is no annual limit.

What 20% looks like

Your 20% Part B share on approved costs (no supplement, 2026) $0 $5,000 $10,000 $15,000 $20,000 $25,000 $426 $1,000 $1,226 $5,000 $2,226 $10,000 $5,226 $25,000 $10,226 $50,000 $20,226 $100,000 You pay
Your share on different levels of approved Part B costs in one year: deductible plus 20% of the rest.

Routine years are cheap. But cancer treatment, dialysis, or a run of outpatient surgeries can create Part B bills of $50,000 to $100,000 or more. At 20%, that is $10,000 to $20,000 out of pocket, every year the treatment continues.

Part B drugs

Drugs given in a doctor's office or infusion centre, such as many chemotherapy and immunotherapy drugs, are Part B, not Part D. They fall under the 20% coinsurance. Some Part B drugs have reduced coinsurance under recent federal rules, and insulin is capped at $35 a month, but most expensive infused drugs remain at 20%.

Three ways to cap it

  1. Medigap: Plans C, D, F, G, M and N pay the full 20% (N with small copays); K and L pay part, up to a cap. See plan letters explained.
  2. Medicare Advantage: plans must have an annual in-network out-of-pocket maximum, with networks and prior authorization as the trade-off.
  3. Employer or retiree coverage, or Medicaid for people who qualify.

Who is most exposed

People on Original Medicare alone. That is common for people who missed their Medigap open enrollment and cannot pass underwriting, which is why the six-month window after Part B starts matters so much. If you are in Washington or Florida and want to compare supplement options, Maru Insurance is an independent agency licensed in both states.

Questions people ask

Is there an out-of-pocket maximum in Original Medicare?

No. Part B's 20% coinsurance has no cap.

Are office-administered drugs Part B or Part D?

Usually Part B, so they fall under the 20% coinsurance.

Does Medigap Plan G cover the 20%?

Yes, fully, after you pay the Part B deductible.

Does Medicare Advantage have a cap?

Yes, every plan has an annual in-network out-of-pocket maximum.

Independent educational site. Not affiliated with or endorsed by the U.S. government, the federal Medicare program, or any insurance company. Figures are 2026 amounts published by CMS unless noted; check Medicare.gov for your situation.