Most people assume that once they are on Medicare, there is a limit to how much a bad year of health can cost them. On a private or employer health plan, that assumption is usually correct, because those plans include an annual ceiling on out-of-pocket spending. Original Medicare works differently, and the difference is easy to miss until a serious illness makes it painfully clear. The traditional program leaves beneficiaries responsible for a share of most bills with no annual cap on that share, which means the exposure on a truly expensive year is, in theory, without a ceiling.
How the 80/20 split works
Original Medicare splits the cost of most doctor and outpatient care between the program and the patient. After a beneficiary meets the annual deductible under Part B, the program generally pays 80 percent of the Medicare-approved amount for covered services, and the beneficiary is responsible for the remaining 20 percent, known as coinsurance. On a routine office visit or a simple test, that 20 percent is a small, manageable figure that most retirees pay without a second thought.
The reason the arrangement feels harmless most of the time is that most medical bills are small. As Medicare’s breakdown of costs lays out, the coinsurance is a percentage rather than a flat fee, so it scales directly with the size of the bill. When the underlying care is inexpensive, so is the share. The trouble begins when the care is not inexpensive, because the same percentage that costs a few dollars on a checkup becomes a very different number on a major course of treatment.
Free for readers: Social Security and Medicare change every year, and nobody sends a memo. The free Retirement Shield newsletter breaks down what changed and what to do. Get it free in your inbox.
The missing out-of-pocket maximum
What sets Original Medicare apart from nearly every other kind of health coverage is the absence of an out-of-pocket maximum. Employer plans and marketplace plans are required to include one, a threshold beyond which the insurer covers the full cost for the rest of the year. That backstop is what turns a catastrophic diagnosis into a large but survivable expense rather than an open-ended one. Original Medicare has no such limit built into it.
The practical meaning is stark. A lengthy hospitalization followed by heavy outpatient treatment, a cancer regimen, or a chronic condition requiring ongoing specialist care can generate bills that reach well into the tens or hundreds of thousands of dollars. Twenty percent of a very large number is itself a very large number, and there is nothing in the structure of Original Medicare to stop that share from climbing as the treatment continues. For a household living mostly on Social Security, that kind of uncapped liability is precisely the risk retirement planning is meant to eliminate.
The exposure is not limited to a single catastrophic year, either. Chronic conditions such as heart disease, kidney failure, or cancer can generate substantial coinsurance year after year, so the twenty percent share becomes a permanent line in the budget rather than a one-time shock. Because Original Medicare also splits inpatient hospital costs through Part A deductibles and daily coinsurance for long stays, a person who is hospitalized more than once in a year can face those charges repeatedly. None of it is capped, and none of it pauses simply because a previous illness already cost a great deal.
How a supplement caps the exposure
The traditional way to close the gap is a Medicare Supplement policy, commonly called Medigap. Sold by private insurers under standardized letter-named plans, these policies are designed specifically to pay the costs Original Medicare leaves behind, including much or all of that 20 percent coinsurance along with certain deductibles, depending on the plan chosen. In return for a predictable monthly premium, a beneficiary swaps an unlimited and unpredictable share for a fixed, budgetable cost.
Timing is what makes Medigap worth understanding early. As Medicare’s guidance on Medigap explains, the strongest guaranteed rights to buy a policy generally apply during a one-time enrollment window that opens when a person is 65 or older and first enrolled in Part B. During that window, insurers cannot deny coverage or charge more based on health history. Apply after it closes, and in many states an insurer can review medical records and either raise the price or turn an applicant away, which can leave someone who waited stuck with the uncapped exposure.
Deciding whether to add protection
The core question for a retiree on Original Medicare is whether the open-ended 20 percent is a risk worth carrying alone. For a healthy person with substantial savings, the coinsurance on routine care may never amount to much, and the monthly cost of a supplement might feel like an unnecessary expense. For someone on a tight fixed income, or with a family history that raises the odds of an expensive illness, the uncapped share is the sort of exposure that can undo years of careful saving in a single difficult year.
Either way, the mistake to avoid is assuming a safety net that is not there. Original Medicare covers a great deal, but it does not place a ceiling on the beneficiary’s share of the bill. Recognizing that limit is the first step toward deciding, on purpose, whether to add a supplement, choose a different form of coverage, or accept the risk with eyes open, rather than discovering the gap in the middle of a health crisis.
Free for readers: One number can cost or save hundreds a month in retirement. The free Retirement Shield newsletter surfaces the ones worth knowing. Sign up free.
This article was researched and drafted with AI assistance and reviewed against the linked primary sources.
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.
