Original Medicare is often described as comprehensive coverage, yet it carries a gap that many households discover only after a serious illness: there is no cap on what a beneficiary can be charged in a single year. Part A and Part B each leave the enrollee responsible for a deductible and for ongoing coinsurance, and those charges keep accumulating no matter how large the bills become. For a retiree living on a fixed income, that open-ended exposure is the precise risk a Medigap policy was built to close.
Why Original Medicare Leaves a Bad Year Uncapped
Unlike most employer plans and most Medicare Advantage plans, Original Medicare does not include an annual out-of-pocket maximum. A person who spends months in and out of the hospital, or who needs an extended course of specialist care, continues to owe a share of every covered service. The program pays its portion, but the remaining slice never stops.
The structure is spelled out on Medicare’s own explanation of what beneficiaries pay under Parts A and B. Part A carries a deductible for each benefit period tied to inpatient hospital stays, and long stays add daily coinsurance once the covered days run out. Part B carries its own annual deductible and then generally leaves the enrollee paying a percentage of the cost of doctor visits, outpatient procedures and durable medical equipment, with no ceiling on that running total.
In an ordinary year, those amounts stay manageable. The danger is the outlier year, the one that involves a cancer diagnosis, a major surgery with complications, or a long rehabilitation. Because the coinsurance percentage applies to services that can run into six figures, the retiree’s unbounded share is exactly where a single bad year can quietly drain a lifetime of savings.
Free retirement updates: The gap between capped and uncapped coverage is the kind of single number that can cost or save a household hundreds a month once the bills start. Plain-English breakdowns of Medicare’s moving parts are free at Retirement Shield.
What a Medigap Policy Is Actually Built to Absorb
Medicare Supplement Insurance, sold by private insurers and known as Medigap, exists for one job: to pay the deductibles, copayments and coinsurance that Original Medicare hands to the beneficiary. Medicare’s guide to Medigap describes the standardized plans, each labeled with a letter, that fill different combinations of those gaps. Some of the more complete plans cover the Part A hospital coinsurance and the Part B coinsurance in full, turning an unpredictable share into something close to a fixed, budgetable cost.
That predictability is the value. A retiree with a strong Medigap plan trades the risk of an unlimited bill for a known monthly premium. When the outlier year arrives, the supplement absorbs the overflow that Original Medicare alone would have passed straight through to the household. A person weighing the premium against the protection is really pricing insurance against the worst case, not the average one.
Medigap works only alongside Original Medicare, not with a Medicare Advantage plan. It also does not include prescription drug coverage, which enrollees add separately through a Part D plan. Those boundaries matter when a household is deciding how the pieces fit together.
The Enrollment Window When Medigap Is Guaranteed
The catch is timing. The best chance to buy a Medigap policy is the six-month Medigap Open Enrollment Period, which Medicare’s page on when to buy explains begins the month a person is 65 or older and enrolled in Part B. During that window, insurers must sell a policy regardless of health history and cannot charge more because of a pre-existing condition.
Once that window closes, the protection weakens. Outside of certain guaranteed-issue situations, an insurer can use medical underwriting, meaning it can deny coverage or raise the price based on health. A retiree who waits until a diagnosis makes the need obvious may find the door has already narrowed. That sequence is why the decision is often better made when a person is healthy and the premium looks like an unnecessary expense.
How the Medigap Choice Differs From Medicare Advantage
The uncapped-cost problem is also the reason many people choose Medicare Advantage instead, since those plans are required to include an annual out-of-pocket maximum. Medicare’s comparison of the two paths lays out the tradeoff: Medigap pairs with Original Medicare’s wide access to providers but costs a monthly premium on top of Part B, while Advantage plans cap spending but route care through networks and prior authorizations.
Neither answer is universally right. A retiree who values freedom to see any provider that accepts Medicare, and who wants a hard shield against a catastrophic year, tends to favor Medigap. Someone comfortable with a network and focused on a lower monthly outlay may prefer the built-in cap of an Advantage plan. What the headline number makes clear is that Original Medicare by itself leaves the ceiling off, and the household has to decide which structure puts it back on.
This article was researched and drafted with the assistance of AI and reviewed by The Financial Wire editorial team.
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Warren Cohen is a finance writer based in Phoenix, Arizona, covering personal finance topics including credit, banking, and beginner investing. He earned his degree in business administration from Arizona State University and began his career working in consumer finance, where he gained direct experience with lending and credit systems. He now writes for personal finance websites and fintech platforms, focusing on clear, practical content that helps readers make informed financial decisions.
