Almost every other decision in Medicare can be revisited. You can change Advantage plans, change drug plans, change your mind and go back to Original Medicare. The autumn window exists precisely so people can.
Medigap does not work that way, and the difference catches people years later, when their health has changed and they discover the door they walked past was the only one.
The window
Under federal law you get a six-month Medigap open enrollment period that starts the first month you have Medicare Part B and you are 65 or older.
Two things about that definition are worth reading carefully.
It is personal, not seasonal. It is keyed to your own Part B start date. There is no shared calendar date. Two people the same age can have entirely different windows.
Both conditions must be met. Part B and 65 or older. Someone who has Part B before 65 through disability does not start this particular clock until they reach 65.
What the window buys you
Inside it, Medicare is explicit about two protections:
- Insurance companies cannot use medical underwriting to decide whether to accept your application or deny you coverage because of pre-existing health problems.
- They cannot charge you more because of pre-existing health problems.
That is a genuinely unusual position in American health insurance, and it lasts six months.
What happens when it closes
The wording matters, so here it is plainly: after your Medigap open enrollment period ends, insurance companies do not have to sell you a Medigap policy, unless you have Medigap protections called guaranteed issue rights.
Not "they may charge more". Not "it gets harder". They do not have to sell to you at all.
Guaranteed issue rights are situations where an insurer cannot deny you a policy. They exist, and they are real, but they arise in defined circumstances — typically where coverage you had is ending through no fault of your own. They are an exception list, not a general fallback.
The confusion that costs the most
Here is the mistake worth preventing, and it is about to become seasonal.
Medicare Open Enrollment runs 15 October to 7 December. It lets you join, drop or switch a Medicare Advantage plan, join or switch a Part D drug plan, and move between Original Medicare and Advantage. Those are set out in the guide to what to check during the autumn window.
It does not give you a right to buy a Medigap policy.
So someone who chose Medicare Advantage at 65, let their Medigap window lapse, and decides three years later during the autumn window that they would rather have Original Medicare plus Medigap can make the first half of that switch and find the second half unavailable — because the Medigap insurer is entitled to look at their health.
That asymmetry is the single most consequential thing on this page. The Advantage decision is reversible. The Medigap one is much less so.
The exclusion that applies even inside the window
One qualification, so the protection is not overstated.
Where a policy is purchased during open enrollment, the issuer may impose a pre-existing condition exclusion period during the first six months the policy is in effect — but that exclusion period must be offset by prior creditable coverage.
So buying during the window stops an insurer refusing you or pricing you on your health. It does not automatically mean every condition is covered from day one. If you are coming from other coverage, that prior coverage reduces or removes the exclusion, which is why the gap between one policy ending and the next starting is worth minimising.
What this means if you are turning 65
Three practical points.
Work out your own six months. It begins the first month you have Part B and are 65 or over. Write down the end date.
Decide Medigap versus Advantage inside it, not after. Both are legitimate choices with different economics. What is not symmetrical is how easily you can change your mind. Original Medicare has no annual cap on cost sharing, which is the structural gap Medigap exists to fill — the numbers are in what Medicare actually costs.
If you take Advantage, understand what you are giving up. Not the ability to switch back to Original Medicare — that stays open. The ability to buy a Medigap policy on guaranteed terms.
Higher earners should also look at how IRMAA works, since it applies regardless of which route you take.
What to actually check
- The first month you had, or will have, Part B. That starts the clock.
- Six months from it. That is your deadline.
- Whether you are 65. Both conditions matter.
- Whether any guaranteed issue right applies to you, if the window has closed. Ask your State Health Insurance Assistance Program.
- Your prior creditable coverage, which offsets any pre-existing condition exclusion.
Everything above comes from the Medigap guidance Medicare publishes, and is background for readers in the United States rather than insurance or financial advice. Medigap policies, availability and pricing vary by state and insurer, and some states set additional rules. Check the position for your own state on Medicare.gov or with your State Health Insurance Assistance Program before deciding.
Sources
- Medicare.gov — When can I buy a Medigap policy? (opens in a new tab)
- Medicare.gov — Get ready to buy a Medigap policy (opens in a new tab)
- Medicare.gov — Medigap basics (opens in a new tab)
- Medicare.gov — Can I change my Medigap policy? (opens in a new tab)
- Medicare.gov — Buying a Medigap policy (opens in a new tab)
This article is general information for a United States audience and reflects Medigap rules published by Medicare at the date of review. It is not insurance, medical or financial advice. Medigap availability, pricing rules and additional protections vary by state, and federal rules can change. Confirm the position that applies to you on Medicare.gov or with your State Health Insurance Assistance Program before relying on any of this. Last reviewed 17 September 2026.
