Original Medicare pays about 80 percent of the cost of covered doctor and outpatient services, and the person on the plan owes the other 20 percent. For a routine visit, that share is small change. The catch that surprises many retirees is what happens when the bill is not routine: Original Medicare puts no ceiling on that 20 percent, so a single serious illness can leave a beneficiary owing an open-ended amount with no annual limit to stop the bleeding.
The 80/20 split that never stops
Under Original Medicare, Part B handles physician services, outpatient care, tests and many procedures. After a beneficiary meets the annual Part B deductible, the program generally covers 80 percent of the approved amount for those services, and the enrollee is responsible for the remaining 20 percent in coinsurance. On an ordinary office visit or a routine test, that leaves a modest out-of-pocket cost that most retirees absorb without much thought.
What sets Original Medicare apart from a typical employer health plan is what it lacks. A workplace plan usually includes an annual out-of-pocket maximum, a point past which the insurer pays everything for the rest of the year. Original Medicare has no such cap. According to Medicare’s own cost breakdown, the 20 percent coinsurance keeps applying no matter how high the total climbs, which means the exposure on a major medical event is theoretically unlimited.
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Why 20% of a big bill is the real danger
The percentage sounds manageable until the underlying number grows. A cancer course, a lengthy hospital stay with heavy outpatient follow-up, or a run of specialist care can generate bills that reach well into the tens or hundreds of thousands of dollars. Twenty percent of a $100,000 treatment is $20,000 out of pocket, and Original Medicare does nothing to stop that figure from going higher if the care continues.
For a household drawing most of its income from Social Security, that kind of open-ended liability is exactly the risk that retirement planning is supposed to remove. The Social Security benefit that anchors most retirees’ budgets is designed to replace only part of a working paycheck, leaving little slack to cover a surprise five-figure medical share. An uncapped 20 percent turns a health crisis into a financial one, which is why so few people rely on Original Medicare alone.
How Medigap plugs the hole
The traditional fix is a Medicare Supplement policy, widely known as Medigap. Sold by private insurers under standardized letter plans, Medigap is built to cover the gaps Original Medicare leaves, including much or all of that 20 percent coinsurance, along with deductibles and other cost-sharing depending on the plan chosen. In exchange for a monthly premium, a beneficiary trades an unpredictable and potentially unlimited share for a fixed, budgetable cost.
Timing matters with Medigap. The strongest guaranteed-issue rights generally apply during a one-time enrollment window that opens when someone is 65 or older and first enrolled in Part B. During that window an insurer cannot deny coverage or charge more because of health history. Apply later, and in many states the insurer can review medical history and turn an applicant down or raise the price. Retirees weighing supplemental coverage can review the options through Medicare’s guidance on joining a plan before that protection lapses.
Medicare Advantage takes a different route
Medicare Advantage, the private alternative to Original Medicare, handles the cap problem in its own way. Unlike Original Medicare, Advantage plans are required by law to include an annual out-of-pocket maximum, so a beneficiary’s spending on covered in-network services stops at a set ceiling each year. That built-in limit is one of the main reasons many retirees choose an Advantage plan over Original Medicare without a supplement.
The trade-offs are different, though. Advantage plans typically use provider networks, may require referrals, and can change their rules, premiums and out-of-pocket limits from year to year. A retiree gets protection from catastrophic bills but gives up some of the freedom to see any provider that accepts Medicare. The right choice depends on health needs, budget and how much a person values the ability to keep a specific doctor or hospital.
What retirees should weigh
The core decision comes down to whether the open-ended 20 percent is a risk worth carrying. For a healthy retiree with ample savings, the coinsurance on routine care may never amount to much. For someone on a tight fixed income, or with a family history that raises the odds of a costly illness, the uncapped share is the sort of exposure that can undo years of careful saving in a single bad year.
Both paths, a Medigap supplement layered on Original Medicare or a Medicare Advantage plan with its statutory cap, exist to solve the same problem the headline points to. The mistake is assuming Original Medicare already includes a safety net it does not have. Understanding that the 20 percent has no ceiling is the first step toward deciding how to put one in place.
This article was produced with AI assistance and reviewed before publication.
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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.
