Medigap Explained: How Medicare Supplement Plans Work With Original Medicare

If you’re on Original Medicare (Part A and Part B), you already know it doesn’t pay 100% of your medical costs. Deductibles, copays, and coinsurance can add up quickly. Medigap, also called Medicare Supplement Insurance, is designed to help cover those gaps so your out‑of‑pocket costs are more predictable.

What is Medigap?

Medigap is private insurance that works alongside Original Medicare, not instead of it. You must be enrolled in Medicare Part A and Part B to buy a Medigap policy.

Key points about Medigap:

• It helps pay Medicare cost-sharing: Part A and B deductibles, coinsurance, and copayments, depending on the plan.
• Policies are standardized in most states and labeled by letters: Plan A, B, C, D, F, G, K, L, M, and N.
• Each lettered plan offers a specific set of benefits, and those benefits are the same across insurers offering that plan type.
• You pay a monthly premium to the Medigap insurer, in addition to your Part B premium.

Medigap only works with Original Medicare. It does not work with Medicare Advantage (Part C) plans.

How Medigap Works With Medicare in Practice

When you get care:

1. Medicare pays first for covered services.
2. Your Medigap plan pays second, covering some or all of the remaining approved charges, based on your plan’s benefits.
3. You’re responsible only for any costs not covered by Medicare or Medigap.

For example, under Original Medicare you usually pay 20% coinsurance for most Part B services after meeting the Part B deductible. A Medigap plan that covers Part B coinsurance would pay that 20%, so you owe little or nothing for that service.

Some Medigap plans also cover:

• Part A hospital deductible
• Skilled nursing facility coinsurance
• Part A hospice coinsurance
• Limited foreign travel emergency care

What Medigap Does Not Cover

Medigap policies generally do not cover:

• Prescription drugs (you usually need a separate Part D plan)
• Dental, vision, or hearing services beyond what Medicare allows
• Long-term custodial care, like most nursing home stays
• Non‑Medicare services or providers who don’t accept Medicare

If Medicare doesn’t cover a service at all, Medigap typically won’t either.

Enrollment Timing and Eligibility Basics

The easiest time to get Medigap is during your Medigap Open Enrollment Period:

• It starts the month you’re both 65 or older and enrolled in Part B.
• It lasts six months.
• During this period, you generally can’t be turned down or charged more due to health conditions.

After that window, in many cases you may face medical underwriting, meaning an insurer can review your health history and could deny coverage or charge more, depending on your state’s rules and any special protections you may have.

Medigap vs. Medicare Advantage

You must choose one path:

• Original Medicare + Medigap + (usually) Part D, or
• Medicare Advantage (Part C), which replaces Original Medicare and often includes drug coverage and extra benefits.

You cannot use Medigap with a Medicare Advantage plan, and it’s illegal for an insurer to sell you Medigap if you’re enrolled in Medicare Advantage, except in limited switch-back situations.

The core idea: Medigap doesn’t give you new types of coverage; it makes your existing Medicare coverage more predictable and less costly at the point of care. If you want to stay with Original Medicare but worry about deductibles and coinsurance, comparing Medigap plan letters and premiums can help you find a balance between monthly cost and financial protection when you need care.