Most people assume that once they are on Medicare, there is a ceiling on what a bad year of health can cost them. There is not. Original Medicare has no annual limit on out-of-pocket spending, which means a single serious illness can generate bills with no upper bound. That gap is the reason so many retirees pay extra each month for a supplement, and the reason the timing of that decision matters more than almost anything else in Medicare.
Original Medicare has no out-of-pocket maximum
Under Original Medicare, meaning Part A and Part B together, there is no cap on how much a person can pay out of pocket in a year. After meeting the Part B deductible, an enrollee generally owes 20 percent coinsurance on most Part B services, and that 20 percent keeps running with no ceiling. For a routine year, that is manageable. For a year with major surgery, a long hospital stay, cancer treatment, or an extended course of specialist care, 20 percent of a very large bill with no stopping point can reach into the tens of thousands of dollars. There is no built-in backstop in Original Medicare that says the enrollee has paid enough.
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What a Medigap policy is built to cover
This is the coverage gap that Medicare Supplement Insurance, better known as Medigap, is designed to fill. A Medigap policy is sold by private insurers and works alongside Original Medicare to pick up some or all of the costs that Medicare leaves to the enrollee, such as deductibles, coinsurance, and copayments. Medicare’s own guidance explains that a Medigap policy can cover those out-of-pocket shares that Original Medicare does not, turning an open-ended exposure into a predictable monthly premium. In effect, a retiree trades the risk of an unlimited bill for a fixed, budgetable cost.
Medigap plans are standardized and sold under lettered names, so a plan with a given letter offers the same core benefits no matter which company sells it. That standardization makes it easier to compare policies on price and service rather than trying to decode different benefit structures. A Medigap policy pairs with Original Medicare and is separate from a standalone drug plan, which a person generally still needs for prescription coverage.
The six-month window that protects the best price
The single most important thing to understand about Medigap is when to buy it. The best time is the Medigap Open Enrollment Period, a six-month window that starts the first month a person is both 65 or older and enrolled in Part B. During that window, coverage is guaranteed issue, which means an insurer must sell the applicant a policy, cannot turn them down, and cannot charge more because of current or past health problems. Medicare spells out this protection on its page about when to buy a Medigap policy.
Miss that window, and the rules change sharply. Outside the open enrollment period, in most states an insurer can use medical underwriting, which lets it deny coverage, delay it, or charge a higher premium based on a person’s health. Someone with a chronic condition who waits could find that the affordable, guaranteed policy they could have bought at 65 is now expensive or out of reach entirely. The window is short and it does not come back on the same terms, so the decision about whether to buy Medigap is really a decision to make during those six months, not later.
How Medigap and Medicare Advantage handle the risk differently
There is an alternative path that handles the no-cap problem in a completely different way. Medicare Advantage plans, the private plans that bundle Medicare coverage, do include an annual out-of-pocket maximum, so they solve the unlimited-exposure problem on their own. The tradeoff is that Advantage plans typically restrict care to a network of doctors and hospitals and may require referrals or prior approval, whereas Original Medicare paired with Medigap lets a person see any provider that accepts Medicare.
These two approaches do not combine. A person generally cannot use a Medigap policy to supplement a Medicare Advantage plan, so the choice is between Original Medicare with a Medigap policy on one side and a Medicare Advantage plan on the other. Each answers the same underlying risk in a different way: Medigap by covering the coinsurance so there is little left to pay, and Advantage by capping the total.
For a retiree weighing the two, the practical takeaway is that Original Medicare alone leaves a real financial risk, and the cleanest time to address it with Medigap is during that first six-month window. Understanding the gap early, rather than discovering it during a health crisis, is what keeps a single bad year from turning into an unlimited bill.
This article was produced with AI assistance and reviewed by a human editor. Figures are linked to their primary sources; where a claim could not be verified from the public record, we say so.
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