If you're approaching Medicare eligibility or already enrolled, one of the most consequential decisions you'll face is when to buy a Medicare Supplement — commonly called Medigap — policy. Unlike Medicare Advantage, which has predictable annual enrollment windows, Medigap operates under a different set of rules that heavily favor people who act at the right time. Get the timing right and you can lock in comprehensive coverage at standard rates regardless of your health. Get it wrong and you may face medical underwriting, higher premiums, or outright rejection depending on where you live.
The single most important enrollment window for Medigap is called the Medigap Open Enrollment Period. This is a federally guaranteed six-month window that begins on the first day of the month in which you are both 65 years old and enrolled in Medicare Part B. During this window, any insurance company selling Medigap in your state must sell you any plan they offer at the same price they charge a healthy person your age. They cannot ask about your medical history, charge you more because of a pre-existing condition, or make you wait for coverage to begin for conditions you already have. This is the gold standard of Medigap access, and it only happens once in your life under federal rules — you do not get a second federally guaranteed open enrollment period if you let this one pass.
The timing of when you enroll in Part B matters enormously here, and it's a detail many people miss. If you're still working at 65 and covered by an employer group health plan, you may delay Part B enrollment without penalty. But the moment you retire and enroll in Part B — whether that's at 65, 67, or 70 — your six-month Medigap Open Enrollment Period begins. So if you retire at 68 and enroll in Part B that month, your protected window starts then, not when you turned 65. This is actually good news for people who delayed Part B, because it means your protected window is still ahead of you. The key is to apply for Medigap within those six months of your Part B effective date.
Outside of that open enrollment window, federal law does not require insurers to sell you a Medigap policy. In most states, if you apply for Medigap after your open enrollment period has closed, insurers can use medical underwriting. That means they review your health history and can legally charge you a higher premium, exclude coverage for certain conditions for up to six months, or deny your application entirely. Common conditions that trigger underwriting concerns include heart disease, diabetes, COPD, cancer history, and kidney disease — conditions that are extremely common among people in their late 60s and 70s. This is why acting during your protected window is so financially significant.
There are, however, specific situations called Guaranteed Issue Rights — sometimes called Special Enrollment Periods for Medigap — where federal law requires insurers to sell you a policy without medical underwriting even outside your open enrollment window. These rights are triggered by qualifying life events. For example, if you had a Medicare Advantage plan and that plan is leaving your area or losing its Medicare contract, you have a guaranteed right to buy certain Medigap plans. If you tried a Medicare Advantage plan for the first time and want to switch back to Original Medicare within the first 12 months — a period sometimes called the Medicare Advantage trial right — you can buy a Medigap Plan A, B, C, F, K, or L without underwriting. Similarly, if you lost Medigap coverage because your insurer went bankrupt or committed fraud, you have guaranteed issue rights. These windows are typically 63 days from the triggering event, so acting quickly is essential.
State law adds another important layer to this picture. Thirteen states have enacted what's commonly called a birthday rule or similar guaranteed issue protections that go beyond federal minimums. In California, Idaho, Illinois, Kentucky, Louisiana, Maine, Maryland, Missouri, Nevada, New Jersey, New York, Oklahoma, and Oregon, beneficiaries have additional protected windows to switch Medigap plans — often annually around their birthday — without medical underwriting. New York and Connecticut go furthest, requiring insurers to sell Medigap to any Medicare beneficiary at any time of year at community-rated prices, meaning your age and health history cannot affect your premium. If you live in one of these states, your options for switching or enrolling in Medigap are significantly broader than what federal law alone provides. If you're unsure of your state's rules, your State Health Insurance Assistance Program (SHIP) counselor can walk you through them at no cost — find your local SHIP at shiphelp.org.
It's also worth understanding what Medigap actually covers before committing to a plan, because the premiums vary significantly by plan type. In 2025 and 2026, the most comprehensive standardized plans are Plan G and Plan N. Plan G covers everything Medicare covers except the Part B deductible, which is $257 in 2025. Plan N also skips the Part B deductible and adds small copays — up to $20 for office visits and $50 for emergency room visits that don't result in inpatient admission. Plan F, which covers the Part B deductible, is no longer available to people who became eligible for Medicare on or after January 1, 2020, though people already enrolled in Plan F can keep it. Monthly premiums for Plan G vary widely by insurer, age, and location — ranging from roughly $100 to over $300 per month in many markets — which is why comparing multiple insurers matters even within your protected enrollment window.
One practical strategy worth knowing: even during your open enrollment period, you are not locked into the first Medigap policy you find. You can compare plans from multiple insurers because the benefits within each lettered plan (Plan G from one company versus Plan G from another) are standardized by federal law — the coverage is identical, only the price and the company differ. Use Medicare's Plan Finder tool at medicare.gov or call 1-800-MEDICARE to compare premiums in your ZIP code. Some states also have their own comparison tools through the state insurance commissioner's office. Shopping around during your protected window can potentially save you $500 to $1,500 or more per year in premiums for identical coverage.
Finally, if you believe you missed your Medigap open enrollment window and don't have a qualifying life event, don't assume you're out of options entirely. Some insurers do sell Medigap policies through medical underwriting to people in good health, and depending on your health status you may still qualify at a reasonable rate. Additionally, if you're under 65 and on Medicare due to disability, your rights vary significantly by state — some states require insurers to offer at least one Medigap plan to under-65 beneficiaries, while others do not. Checking with your state insurance department or a licensed Medicare insurance broker who works with multiple carriers can help you understand what's realistically available to you given your specific situation and health history.