What Is Medicare Supplement Insurance?
Medicare Supplement policies — commonly called Medigap — are sold by private insurers to work alongside Original Medicare (Part A and Part B). They help pay the costs Original Medicare leaves you owing: coinsurance, copays, and deductibles.
Medigap doesn't replace Original Medicare. You keep Part A and Part B exactly as they are. The Medigap policy just picks up some or all of your share of the bill.
Medigap Plans Are Standardized
Outside of Massachusetts, Minnesota, and Wisconsin (which use their own standardized formats), Medigap plans come in lettered versions: A, B, D, G, K, L, M, N, and a few others. The same letter covers the exact same benefits no matter which insurance company sells it, so the only real differences between insurers are price and customer service.
Plans C and F, which used to cover the Part B deductible, are no longer available to anyone who became newly eligible for Medicare on or after January 1, 2020. If you were already eligible before that date, you may still be able to keep or buy one. For most people newly eligible today, Plan G is the most comprehensive option available, with Plan N as a popular lower-premium alternative that adds small copays for some office visits and ER trips.
What Medigap Does Not Cover
- Prescription drugs: you'll need a separate Part D plan
- Dental, vision, and hearing aids, same as Original Medicare
- Anything Original Medicare doesn't cover in the first place
No Networks, No Referrals
With a Medigap policy, you can see any provider in the country who accepts Medicare. No network restrictions, no referrals needed. This matters most if you travel frequently, split time between Oregon, Washington, or another state, or simply want the freedom to choose any doctor without checking a network directory first.
The Most Important Thing to Know: Timing
You get a one-time, 6-month Medigap Open Enrollment Period that starts the month you're both 65 or older and enrolled in Part B. During this window, you can buy any Medigap policy sold in your state at the best available rate, regardless of your health history. Insurers can't deny you or charge you more for pre-existing conditions.
Miss that window, and in most states insurers can medically underwrite you, meaning they're allowed to charge more or deny coverage based on your health. Oregon and Washington both go beyond the federal minimum. Oregon's birthday rule gives existing policyholders a window that starts 30 days before their birthday and ends 30 days after, to switch to an equal-or-lesser Medigap plan without medical underwriting. Washington lets people already on Plan B through N switch to another Plan B through N at any time without a health questionnaire. Plan A can only be swapped for another Plan A. See the Oregon & Washington rules guide for the full picture. These rules change, so we'll confirm exactly what applies to your situation before you decide anything.
Medigap vs. Medicare Advantage
- Premium: Medigap has a monthly premium on top of Part B; Medicare Advantage often has no added premium.
- Networks: Medigap has none, any provider nationwide; Medicare Advantage limits you to a plan network.
- Cost predictability: Medigap is the most predictable, with few surprise bills; Medicare Advantage has variable copays up to an annual cap.
- Extra benefits: Medigap doesn't include dental, vision, or hearing; Medicare Advantage often does.
For the full breakdown, see the Medicare Advantage guide.