How Medigap (Supplement) Plans Work — Medicare

If you have Original Medicare, you've probably heard you should consider a "Medigap" or "supplement" plan — and it's solid advice for many people, because Original Medicare leaves real gaps in what it pays, and those gaps can add up. Medigap exists to fill them. Here's how supplement plans actually work and who they make sense for.

The problem Medigap solves

Original Medicare (Parts A and B) covers a lot, but it doesn't cover everything, and it generally has no cap on your out-of-pocket costs on its own. You're left with deductibles, coinsurance, and other cost-sharing — and in a serious health year, that cost-sharing can climb high, because there's no built-in ceiling. Medigap is private insurance designed specifically to "supplement" Original Medicare by covering some of those gaps — the very cost-sharing that Original Medicare leaves to you.

How it works

• It works alongside Original Medicare. Medigap doesn't replace Original Medicare — it pairs with it. You keep Original Medicare as your primary coverage, and the Medigap policy helps pay your share of the costs Medicare leaves.
• It covers cost-sharing gaps. Depending on the policy, Medigap helps with things like coinsurance, copayments, and deductibles under Original Medicare — bringing predictability to what would otherwise be open-ended costs.
• Plans are standardized. Medigap policies generally come in standardized types identified by letters, so a given plan type offers the same core benefits no matter which company sells it. This makes them comparable — you're largely comparing price and the company for the same standardized coverage.
• You pay a monthly premium for it. Medigap adds a premium on top of your Part B premium, in exchange for covering the gaps. The trade is predictability and protection for that added cost.

Important things to know

• Medigap is for Original Medicare, not Medicare Advantage. You generally can't use a Medigap policy with a Medicare Advantage plan — Medigap pairs with Original Medicare. This is part of the bigger Original-vs-Advantage decision.
• Medigap doesn't include drug coverage. It covers Original Medicare cost-sharing, not prescriptions — for drugs, you'd add a separate Part D plan.
• When you buy matters. There are timing windows for enrolling in Medigap with the strongest protections — particularly when you first become eligible. Buying during the right window can matter significantly for your ability to get a policy and at what terms. This is worth understanding before you decide, because the easiest time to get Medigap is often early.

Who it's for

Medigap tends to fit people who choose Original Medicare and want predictability and protection against open-ended cost-sharing — who'd rather pay a steady premium than face potentially large, uncapped costs in a bad health year, and who value Original Medicare's provider freedom. It's a core part of the traditional Medicare approach: Original Medicare for the coverage and freedom, Part D for drugs, and Medigap to cap and smooth the costs.

This isn't medical or legal advice, and enrollment timing rules matter — it's how Medigap supplement plans work and who they're for.