Turning 65 and signing up for Medicare Part B starts a clock that most new enrollees never hear about until it has run out. For six months, a beneficiary can buy any Medigap supplement policy sold in their state at the best available price, with no medical questions and no chance of being turned down. Once that window closes, insurers in most states can screen applicants, raise the price, or refuse coverage outright based on health, making the six-month period one of the most consequential deadlines in retirement.
What the one-time window guarantees
The Medigap open-enrollment period begins the first month a person is both 65 or older and enrolled in Part B, and it lasts six months. During that stretch, as Medicare’s guidance on when to buy a policy explains, a beneficiary holds a guaranteed-issue right: any insurer must sell them any Medigap plan it offers, cannot charge more because of a health condition, and cannot impose the usual underwriting. It is the one moment the market is fully open regardless of medical history.
Medigap policies exist to cover the gaps in Original Medicare, the deductibles, copays, and coinsurance that Parts A and B leave to the beneficiary. Because those out-of-pocket costs are otherwise open-ended, a supplement can be the difference between predictable premiums and unlimited exposure to hospital and doctor bills, which is why what Medigap covers matters so much at the point of enrollment. The open window is when that protection is cheapest and easiest to secure.
The guarantee is narrow in time but broad in effect. A person with diabetes, heart disease, or a cancer history who applies during the window pays the same rate as a healthy applicant of the same age, something rarely possible afterward. That single feature is why advisers treat the six months as the best, and often the only, chance many retirees get to lock in comprehensive supplemental coverage on equal terms.
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What happens after the window closes
Outside the open-enrollment period, most states let insurers return to medical underwriting, and the cost of a Medigap policy can shift sharply as a result. A beneficiary who applies later can be asked about their health, charged a higher premium, or denied a policy altogether if a condition makes them too expensive to cover. The guaranteed-issue right that made the purchase automatic during the window simply disappears across most of the country.
That reversal traps people who delay for understandable reasons. Someone who joins a Medicare Advantage plan at 65, then decides years later they would rather have Original Medicare with a supplement, generally has no protected right to buy Medigap and must pass underwriting to get it. A health problem that developed in the meantime can make the switch expensive or impossible, effectively locking the person into the choice they made at first enrollment.
A handful of states impose their own rules that soften the cliff, requiring insurers to offer Medigap on a guaranteed or continuous basis, or during an annual window tied to a birthday. But those protections are the exception, and they vary widely from state to state. For most beneficiaries, the federal six-month window is the only guaranteed-issue opportunity they will ever have, and it does not reopen.
Why the timing hinges on Part B
The trigger is enrollment in Part B, not merely turning 65, which is where timing errors creep in. Someone still working at 65 with employer coverage may delay Part B without penalty, and their Medigap window does not open until they actually enroll, an interaction spelled out in Medicare’s rules on when coverage starts. Starting the supplement clock too early or too late relative to Part B can waste the guaranteed-issue right.
The interaction rewards planning around the Part B start date. A beneficiary who knows the window opens with Part B can line up a Medigap purchase for the same period, comparing plans while every insurer is still required to sell. Waiting even a few months past the six-month mark can convert a routine purchase into a medical review with an uncertain outcome, and a preexisting condition can turn that review into a denial.
The Medigap window is unusual because it is both automatic and irreversible. No application is required to receive the right; it arrives with Part B enrollment and expires six months later whether or not the beneficiary used it. There is no second open-enrollment period for Medigap the way there is for drug or Advantage plans each fall, so a missed window is missed for good in most states.
For a retiree deciding whether to add a supplement, the deadline itself is the argument for acting early. The policy bought during the window costs the same for the sick and the healthy; the one bought afterward may cost far more or be unavailable. The six months are the rare instance in Medicare where the calendar, not the health history, controls the price.
This article was produced with AI assistance and reviewed against primary sources by The Money Overview editorial team.
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