Most private health plans put a ceiling on what a member can lose in a bad year. Original Medicare does not. A retiree covered only by Parts A and B faces no annual cap on out-of-pocket spending, which means the 20 percent share Medicare leaves to the patient has no upper limit. In an ordinary year that is a manageable cost; in a year with a cancer diagnosis, a major surgery, or a long course of treatment, the same 20 percent can climb into tens of thousands of dollars.
The 20 percent with no ceiling
Under Part B, Medicare generally pays 80 percent of the approved amount for doctor visits, outpatient care, and many drugs administered in a clinical setting, and the patient owes the remaining 20 percent as coinsurance. Medicare’s own overview of what Original Medicare costs confirms there is no annual out-of-pocket maximum on that share. The percentage stays fixed, but the dollar amount behind it is open-ended, rising in lockstep with the size of the bills.
That design is easy to underestimate because most working-age adults have spent decades in employer plans that cap annual out-of-pocket costs. Moving to Original Medicare removes that safety net, and a retiree who does not add supplemental coverage carries the full 20 percent exposure alone.
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Why a single serious illness is the real risk
The absence of a cap matters most in a catastrophic year. A course of treatment with a large approved cost leaves the patient responsible for a fifth of it, and there is no point at which Medicare steps in to stop the meter. On top of the Part B coinsurance, Part A imposes its own hospital deductibles and daily coinsurance charges for long inpatient stays, so a serious illness can strike from more than one direction at once.
The pattern that worries planners is the retiree who feels well and covered for years, then confronts a single high-cost event. Because nothing limits the running total, the same diagnosis that is a medical crisis becomes a financial one, and the bill lands on a fixed retirement income with little room to absorb it.
The hospital side of the exposure
The out-of-pocket risk is not only about the 20 percent coinsurance on the doctor-and-outpatient side. Part A, which covers inpatient hospital stays, is organized around “benefit periods” rather than the calendar year, and that structure surprises many retirees. A benefit period begins when a patient is admitted and ends only after they have been out of a hospital or skilled-nursing facility for a set stretch of time. A person who is hospitalized, recovers, and is then readmitted for a separate event later in the same year can start a brand-new benefit period, and owe the inpatient deductible again, because the clock resets each time. There is no annual limit on how many benefit periods a year can contain, so a difficult year with multiple hospitalizations can stack these charges on top of one another.
For very long inpatient stays, Part A also shifts from covering the bulk of the cost to charging daily coinsurance after a certain number of days, and that daily charge climbs the longer a stay runs. Combined with the open-ended 20 percent on the Part B side, the result is a program that can bill a seriously ill patient from two directions at once, with neither side capping the total the way an employer plan or a Medicare Advantage plan would.
How Medicare Advantage and Medigap cap the exposure
Two routes exist to put a lid on the risk, and they work differently. A Medicare Advantage plan replaces Original Medicare with a private plan that, by federal rule, must include an annual out-of-pocket maximum, so a member’s spending on covered in-network care cannot exceed that limit in a year. The tradeoff is a network of providers and plan-specific rules that Original Medicare does not impose.
The other route keeps Original Medicare and adds a Medigap supplement policy, which pays much of the coinsurance and deductibles that would otherwise fall on the patient. Buying Medigap on favorable terms depends heavily on timing, and Medicare’s page on guaranteed issue rights lays out the limited situations in which an insurer cannot deny a policy or charge more based on health. Outside those windows, a retiree who applies later can face medical underwriting, which is why the decision about supplemental coverage is best made when a person first joins Medicare rather than after a health problem appears.
The timing point deserves emphasis because it is where retirees most often get caught. During the months when a person first enrolls in Part B, an insurer generally must sell a Medigap policy regardless of health history. Let that initial window close without buying one, and outside the specific guaranteed-issue situations, a later applicant can be turned down or charged more for a pre-existing condition, the very condition that made the coverage feel urgent in the first place. The protection is easiest and cheapest to lock in before it is needed, not after, which is the opposite of how most people instinctively approach insurance.
Weighing the coverage decision at enrollment
The choice between an Advantage plan’s out-of-pocket cap and Original Medicare paired with Medigap comes down to how a retiree wants to trade cost, flexibility, and certainty. What the numbers make clear is that Original Medicare on its own leaves a person exposed to an unbounded share of a catastrophic bill, and the protections that close that gap are easiest to secure at the start of Medicare enrollment. Understanding that the 20 percent has no ceiling is the first step toward deciding which form of coverage removes it.
This article was researched and drafted with the assistance of AI and reviewed by The Financial Wire editorial team.
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David M. Keller is a finance writer based in Columbus, Ohio, covering personal finance and consumer-focused economic topics. He earned his degree in journalism from Ohio University and began his career reporting on local business and economic trends for a regional media outlet. Since then, he has contributed to a variety of online publications, focusing on clear, practical coverage of topics such as cost of living, debt, and everyday financial decision-making.
