Six Months of Guaranteed Issue, Then Your Medical History
Medicare has one deadline whose consequences are permanent and almost never explained at the time. For six months beginning when your Part B starts, any insurer must sell you any Medigap policy they offer, at their standard price, whatever your medical history. Once that window closes, in most states they can ask about your health, charge more, or decline you outright — and the decision you made at 65 becomes very hard to revisit.
- The window:: Six months from the first day of the month you are both 65 or over and enrolled in Part B. It does not repeat and cannot be extended.
- What it guarantees:: Any policy the insurer sells in your area, at their standard rate, regardless of pre-existing conditions.
- What follows it:: In most states insurers may apply medical underwriting to a later application — meaning a higher premium or a refusal.
- The hidden cost of Advantage:: Choosing Medicare Advantage at 65 uses the window. Switching to Original Medicare later usually means applying for Medigap with underwriting.
Where the AI summary above gets this wrong
"You can switch between Medicare Advantage and Original Medicare during open enrollment every year."
That's surface-true. Here's what it misses:
- Switching plans and buying Medigap are different questions — You can indeed move between Advantage and Original Medicare each year. What does not come back is the guaranteed right to buy a Medigap policy. Moving to Original Medicare without one leaves you exposed to unlimited coinsurance, and the Medigap application is usually underwritten.
- The six-month window is the real deadline — It is described as a purchase opportunity and it is closer to a one-time insurability right. Annual enrollment periods repeat; this does not, and after it your medical history becomes a factor in most states for the rest of your life.
- A few states are different, and the rest are not — A small number of states require guaranteed issue on an ongoing or annual basis. Advice written from one of those states does not travel, and someone reading it elsewhere can conclude they have a right they do not have.
01 What Medigap does, and why the timing matters
Original Medicare leaves gaps: the Part A deductible per benefit period, the Part B deductible, and 20% coinsurance on most Part B services with no annual out-of-pocket maximum. That last point is the one that matters — 20% of an unbounded number is an unbounded number.
A Medigap policy covers some or all of those gaps. The plans are standardised by letter, so a Plan G from one insurer covers exactly what a Plan G from another covers. What differs is the premium, which varies substantially between insurers for identical coverage — worth shopping precisely because the product is identical.
Because a Medigap policy is the thing standing between you and unlimited coinsurance, the ability to buy one is not a convenience. It is the difference between a bounded and an unbounded exposure, which is why losing the guaranteed right to it is more serious than it sounds.
Source: Medigap basics
02 The six months, and what they guarantee
The Medigap open enrollment period runs for six months, starting on the first day of the month in which you are both 65 or older and enrolled in Part B. It happens once. There is no annual repeat and no general extension.
Inside it you have guaranteed issue rights. An insurer must sell you any Medigap policy they offer in your area, cannot charge more because of your health, and cannot refuse you for a pre-existing condition. They may in limited circumstances delay coverage of a pre-existing condition for a period, but they must issue the policy.
Outside it, most states permit medical underwriting on a new application. The insurer can ask about your health, price accordingly, or decline. A separate set of guaranteed issue rights exists for specific situations — an employer plan ending, an Advantage plan leaving your area — but they are situational rather than general, and they do not cover simply changing your mind.
Shows: what applying outside the open enrollment window can cost if underwriting produces a higher premium rather than a refusal. Ignores: the possibility of being declined altogether, premium increases over time, and the wide variation between insurers pricing the identical standardised plan.
Source: Ready to buy a Medigap policy
03 The Advantage decision that spends the window
This is where the deadline does its quiet damage. Choosing Medicare Advantage at 65 is presented as reversible, and in one sense it is: you can return to Original Medicare in a later annual enrollment period.
What does not return is the guaranteed right to buy Medigap. Your six months ran while you were on the Advantage plan, and applying afterwards means underwriting in most states. Someone who chose Advantage at 65 for its lower premium, developed a condition at 71, and then wanted the wider provider access of Original Medicare can find the Medigap policy that would make that workable is priced out of reach or unavailable.
There is a narrow protection: a trial right generally allows someone who joined Advantage when first eligible to switch back within the first year with guaranteed issue. Beyond that, the choice hardens. That asymmetry belongs in the Medicare decision itself, because the two options are not equally reversible even though they are presented as though they were.
Source: Changing Medigap policies
If I could put one sentence in front of everyone approaching 65, it would be that the Medigap window is an insurability right and not a shopping opportunity. Everything else in Medicare repeats annually, which trains people to think decisions are revisable, and this one is not. The Advantage plan with the lower premium may well be the right choice — it often is — but it should be made knowing that the door back to Original Medicare with a Medigap policy narrows considerably once those six months pass, and that your health at 71 is not something you can predict at 65.
FAQ
When is my Medigap open enrollment period?
The six months beginning on the first day of the month in which you are both 65 or older and enrolled in Part B. It occurs once, does not repeat annually, and cannot generally be extended.
What happens if I apply for Medigap after the window closes?
In most states the insurer may apply medical underwriting — asking about your health, charging a higher premium, or declining you. Separate guaranteed issue rights exist for specific situations such as employer coverage ending, but not for simply changing your mind.
Can I switch from Medicare Advantage to Original Medicare later?
You can change plans in a later enrollment period, but the guaranteed right to buy a Medigap policy does not come back. A trial right generally allows switching back within the first year of joining Advantage when first eligible; after that a Medigap application is usually underwritten.
Sources
Regulator references
- Medigap basics · Centers for Medicare & Medicaid Services · 2026What a Medigap policy covers and how the standardised plan letters work.Last verified: 2026-09-07
- Ready to buy a Medigap policy · Centers for Medicare & Medicaid Services · 2026The open enrollment window and the guaranteed issue rights attached to it.Last verified: 2026-09-07
- Changing Medigap policies · Centers for Medicare & Medicaid Services · 2026What happens when you try to switch after the window has closed.Last verified: 2026-09-07
Calculator unit tests · the assertions this page's worked example is checked against, and their last result
Changelog
- 2026-09-07 — initial publish (new format)
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Jordan Reeves
Founder of Talk Through Wealth. A software engineer for over a decade before turning to retirement planning, Jordan built the projection engine after watching family members get fragmented, country-by-country advice that never reconciled. He writes about retirement the way the engine computes it: month-by-month, lifetime-long, and skeptical of any rule of thumb that hasn't been run through the math.