The Medigap Open Enrollment Window and Guaranteed-Issue Rights

A Medicare Supplement policy, commonly called Medigap, helps pay the out-of-pocket costs that Original Medicare leaves behind, such as deductibles and coinsurance. According to Medicare.gov, the single best time to buy one is a one-time, six-month window called Medigap open enrollment. Miss it and you can still apply later, but insurers may charge you more or turn you down based on your health. This guide explains when the window opens, why it matters so much, and the guaranteed-issue rights that can give you a second chance in specific situations. It pairs naturally with your enrollment planning, so if you have not mapped your Part B start date yet, begin with the enrollment window checker.

When the window opens and closes

Your Medigap open enrollment period begins on the first day of the month in which you are both 65 or older and enrolled in Medicare Part B, and it runs for six months. This start date is tied to your Part B effective date, which is why the two decisions are linked. Medicare.gov is explicit that this window happens once and generally does not repeat. If you delay Part B because you are still working, your Medigap window does not open until your Part B does, so the delay simply shifts the six months rather than costing you them.

Why this window is so valuable

During these six months you have what amounts to a guaranteed right to buy. Insurers must sell you any Medigap policy they offer, cannot use medical underwriting to deny you, and cannot charge you more because of past or current health problems. Outside the window, in most states, insurers can require underwriting: they can review your health history, raise your premium, impose waiting periods for pre-existing conditions, or decline you altogether. A single change in health between your 65th birthday and a later application can be the difference between an affordable policy and no policy at all.

A checklist for using the window well

• Confirm your Part B effective date first. Your six-month clock starts from it, not from your birthday, so verify it at Medicare.gov or with the Social Security Administration (SSA).
• Decide before the window opens, not after. Compare the standardized Medigap plan letters and their costs during the months before your window so you are ready to apply on day one.
• Understand that Medigap and Medicare Advantage do not mix. A Medigap policy works with Original Medicare, not with a Medicare Advantage plan. If you are choosing between those paths, read our guide on Medicare Advantage versus Original Medicare timing first.
• Buy while you are healthy. The whole point of the window is that your health cannot be used against you. Waiting until you need the coverage is exactly when underwriting can lock you out.
• Confirm state rules. A few states offer broader or additional Medigap enrollment protections. Check your state's rules at Medicare.gov.

What Medigap does, and what it does not do

It helps to be clear about what you are buying during this window. A Medigap policy fills gaps in Original Medicare, such as the coinsurance you would otherwise owe on hospital and doctor bills, so your costs become far more predictable. Medicare.gov standardizes these policies into lettered plans, so a given letter offers the same core benefits no matter which insurer sells it, which makes price the main thing that varies between companies offering the same letter. What Medigap does not include is prescription drug coverage; that comes separately through a Part D plan. It also does not work alongside a Medicare Advantage plan, and it is generally illegal for someone to sell you one if they know you have Advantage. Because the benefits within a letter are identical across insurers, the smart move during your window is to pick the letter that fits your needs and then shop purely on price and the insurer's service reputation.

Keep in mind that your premium during the window is set without regard to your health, but the way premiums rise over time still varies by insurer and by how the policy is priced. Ask how each policy's premiums are structured before you commit, and confirm the details at Medicare.gov, because the guaranteed-issue protection covers your right to buy, not the future cost of keeping the policy.

Guaranteed-issue rights: your safety net

Even outside the one-time window, federal law gives you guaranteed-issue rights in certain situations, meaning insurers must sell you a Medigap policy without underwriting. Medicare.gov lists the qualifying events. Common examples include losing other coverage through no fault of your own, a Medicare Advantage plan leaving your area, or your insurer going out of business. There is also a trial right: if you joined a Medicare Advantage plan when you first became eligible and decide within the first year to switch back to Original Medicare, you generally get a guaranteed-issue opportunity to buy Medigap. These rights are usually time-limited, often to about 63 days from the triggering event, so act quickly if one applies.

How Medigap timing fits your broader plan

Medigap is not part of your penalty-driven deadlines the way Part B and Part D are, but its window is easy to waste precisely because nothing forces you to act. The safest approach is to treat your Part B effective date as the trigger for two decisions at once: your Part B enrollment and, if you want a Supplement, your Medigap purchase. If you are still sorting out those dates, the enrollment window checker shows your Initial Enrollment Period, and our turning-65 enrollment timeline explains how the pieces line up.

A note before you rely on this

This article is general information, not medical or insurance advice. Medigap rules, guaranteed-issue events, and state-level protections can change and vary by location. Confirm your specifics at Medicare.gov and with the Social Security Administration before you buy or drop any coverage.