The Short Answer
Medigap, short for Medicare Supplement Insurance, is a private insurance policy that helps pay some of the costs Original Medicare (Parts A and B) doesn’t cover — things like deductibles, copayments, and coinsurance. It only works alongside Original Medicare, not with Medicare Advantage, and it doesn’t include drug coverage, so most Medigap holders also carry a separate Part D plan. Medigap plans are standardized by letter (like Plan G or Plan N), so a given plan letter covers the same benefits no matter which insurer sells it — only the premium differs.
In short, Medigap is a “fill the gaps” policy for Original Medicare’s out-of-pocket costs, standardized across insurers by plan letter.
How Medigap Works
- You must have Original Medicare (Parts A and B) to buy a Medigap policy — it supplements Original Medicare rather than replacing it.
- You pay a separate monthly premium to a private insurer, in addition to your Part B premium.
- The plan pays some or all of the Medicare deductibles, copays, and coinsurance left over after Medicare pays its share, depending on which lettered plan you choose.
- Plan letters are standardized, so Plan G from one insurer covers the same core benefits as Plan G from another — price and customer service are what typically differ.
Medigap vs. Medicare Advantage
- Medigap — works alongside Original Medicare, lets you see almost any provider who accepts Medicare nationwide, but doesn’t include drug or extra benefits (dental, vision) on its own.
- Medicare Advantage — replaces Original Medicare with a private plan, usually bundles drug and extra benefits, but generally uses a provider network.
- You cannot have both Medigap and Medicare Advantage at the same time — they’re two different paths for supplementing or replacing Original Medicare.
A Simple Example
Example: Someone on Original Medicare has an outpatient procedure Medicare approves at $2,000. Part B pays 80% ($1,600), leaving a $400 coinsurance bill. Without Medigap, that $400 comes out of pocket. With a Medigap Plan G policy, the plan covers that $400 coinsurance (after any applicable Part B deductible), so the person’s only ongoing cost is their monthly Medigap premium, not a surprise bill after every procedure.
When to Buy Medigap
- The best time to buy is during your Medigap open enrollment period, a 6-month window starting when you’re 65+ and enrolled in Part B, when insurers can’t deny you or charge more for health conditions.
- Buying outside that window may involve medical underwriting, meaning an insurer could charge more or deny coverage based on health history, depending on state rules.
- Switching from Medicare Advantage back to Medigap later isn’t always guaranteed without underwriting, so the decision is easier to get right the first time.
The Bottom Line
Medigap fills in Original Medicare’s deductibles, copays, and coinsurance in exchange for a monthly premium, using standardized plan letters so benefits are consistent across insurers. It pairs naturally with broad provider access but doesn’t include drug coverage on its own, and it can’t be combined with Medicare Advantage. Because the best buying window is time-limited, it’s worth comparing Medigap against Medicare Advantage before your initial enrollment period passes.
Frequently Asked Questions
What is Medigap in simple terms?
It’s private supplemental insurance that helps pay the deductibles, copays, and coinsurance left over after Original Medicare pays its share — it works alongside Original Medicare, not instead of it.
Can I have Medigap and Medicare Advantage together?
No. Medigap only works with Original Medicare. If you’re enrolled in a Medicare Advantage plan, it’s illegal for someone to sell you a Medigap policy.
Does Medigap cover prescription drugs?
No. Medigap policies sold today don’t include drug coverage, so most people who choose Medigap also enroll in a separate Part D plan.
Why do Medigap plan letters matter?
Each letter (like Plan G or Plan N) represents a standardized set of benefits required by law, so the coverage is the same no matter which company sells that letter. Comparing insurers then comes down to price and service, not benefits.
When is the best time to buy a Medigap policy?
During your 6-month Medigap open enrollment period, which starts when you’re 65 or older and first enrolled in Part B. During this window, insurers generally can’t deny coverage or charge more due to health conditions.
Is Medigap the same in every state?
Mostly, but not entirely. Standardized plan letters apply in most states, though a few states use their own different standardization systems, so it’s worth checking your state’s specific Medigap rules.
This article is for educational purposes only and is not insurance, financial, tax, or legal advice. Medicare rules, costs, and coverage change over time and vary by plan and location. Visit Medicare.gov or consult a licensed insurance agent or qualified professional before making decisions about your coverage.