Medicare and private insurance work together, not against each other

When you have both Medicare and private insurance, the two plans coordinate to pay your medical bills. Medicare pays first for services it covers, then your private insurance may pay part or all of what Medicare leaves behind. The private plan is called secondary coverage, and it exists specifically to fill gaps in what Medicare alone would pay.

The coordination happens automatically once both insurers know about each other. You do not have to choose between them or file separate claims for the same visit. Your provider submits the claim to Medicare first, Medicare processes it and sends an explanation of benefits, then your private insurer receives that information and decides what it will pay on top.

This arrangement is common and legal. Millions of people over 65 hold both Medicare and a private plan — usually a Medigap policy, a Medicare Advantage plan, or retiree health coverage from a former employer.

Key Takeaways

  • Medicare always pays first when you have both plans, and your private insurance pays second based on what Medicare did not cover.
  • You must tell both insurers about each other so they can coordinate; if you do not, claims may be denied or delayed.
  • The type of private insurance you have — Medigap, Medicare Advantage, or employer coverage — changes how the coordination works and what you pay out of pocket.
  • Your provider needs the correct insurance card and member ID for both plans to submit claims in the right order.
  • If your private insurance ends, you have a limited window to switch to a different plan without penalties or waiting periods.

Tell both insurers you have coverage from the other

When you first enroll in private insurance while on Medicare, or when you enroll in Medicare while holding private coverage, you must notify both insurers. This is not optional. If the insurers do not know about each other, claims will process incorrectly, and you may end up paying bills you should not have to pay.

Contact your private insurer first and tell them you have Medicare. Give them your Medicare number, the date your coverage began, and the type of Medicare you have (Original Medicare, Medicare Advantage, or both). Write down the name of the person you spoke with and the date.

Then contact Medicare directly at 1-800-MEDICARE or log into your Medicare account at Medicare.gov. Report your private insurance — the company name, your policy number, and the date coverage started. Medicare will add this information to your file so that when claims arrive, Medicare knows to process them as primary.

If you have employer retiree coverage, your employer's benefits office should handle notifying Medicare on your behalf, but confirm this in writing. Do not assume it happened.

Understand which type of private insurance you have

The way your two plans coordinate depends on what kind of private insurance it is. The three main types work differently.

Medigap (supplemental insurance) is designed specifically to work alongside Original Medicare. It pays the coinsurance, copayments, and deductibles that Medicare does not cover. When you have Medigap, you keep your Original Medicare coverage and your Medigap plan pays second. Your out-of-pocket costs are usually lowest with Medigap because the two plans are built to work together seamlessly.

Medicare Advantage is an alternative to Original Medicare, not a supplement to it. If you have Medicare Advantage, you cannot also have Medigap. Some people hold Medicare Advantage and a separate private plan (like retiree coverage from an employer), and in that case the private plan acts as secondary coverage to Medicare Advantage. Coordination is less automatic with Medicare Advantage because each plan has its own network and rules.

Employer or union retiree coverage is health insurance you keep after you retire. When you turn 65 and enroll in Medicare, your employer plan usually becomes secondary. Some employer plans are generous and cover costs Medicare does not; others are minimal. Check your plan documents or call your benefits office to learn what your employer coverage actually pays when Medicare is primary.

Provide both insurance cards when you see a provider

When you schedule a doctor's visit, lab test, hospital stay, or any other medical service, tell the provider's office that you have both Medicare and private insurance. Bring both insurance cards to your appointment.

The provider will submit the claim to Medicare first using your Medicare number. Once Medicare processes the claim and sends back an explanation of benefits, the provider's billing office will submit the same claim to your private insurer with Medicare's explanation attached. Your private insurer then decides what additional amount it will pay.

If you do not give the provider your private insurance information, they may only bill Medicare. Your private plan will not know about the claim and will not pay its share. You could end up responsible for costs your private insurance would have covered.

Some providers' offices are slow to submit to secondary insurance. If you receive a bill from the provider more than 30 days after your visit, call the billing office and ask whether they have submitted the claim to your private insurer. If they have not, ask them to do so when ready.

Know what you owe after both plans pay

After Medicare pays and your private insurance pays, you may still owe money. What you owe depends on the type of private coverage you have and the specific service.

With Medigap, your out-of-pocket cost is usually zero or very small. Medigap plans are rated A through N, and the higher-letter plans cover more of Medicare's deductibles and coinsurance. Plan G and Plan N are the most common. Review your plan documents to see exactly what costs you are responsible for.

With Medicare Advantage, you may owe a copayment or coinsurance even after both plans pay, because Medicare Advantage plans have their own cost-sharing rules. You might also owe money if you see an out-of-network provider. Check your Medicare Advantage plan's summary of benefits to understand your costs for the specific service you need.

With employer retiree coverage, your out-of-pocket cost depends on what your employer plan covers. Some employer plans cover everything Medicare does not; others cover only certain costs. Call your employer's benefits office or review your plan documents to learn what you owe.

Handle changes to your private insurance

If your private insurance ends — because you lose employer coverage, stop paying premiums, or your plan is discontinued — you have a limited window to enroll in a different plan without penalties.

If you lose employer retiree coverage, you have 63 days to enroll in a Medigap plan without having to answer health questions or pay higher premiums for pre-existing conditions. This is called the Medigap open enrollment period. If you miss this window, insurers can deny you coverage or charge you more. Contact a Medigap insurer within 63 days of losing your employer coverage.

If you have Medicare Advantage and want to switch to Original Medicare plus Medigap, you can do so during the annual open enrollment period (October 15 to December 7) or if you experience a may have access to life event like loss of coverage. When you switch, you may be able to enroll in Medigap without health underwriting if you do so within 63 days of losing Medicare Advantage coverage.

If your Medigap plan is discontinued by the insurer, the insurer must offer you a replacement plan at no higher premium. You do not have to shop for a new plan; the insurer will move you automatically unless you choose a different plan.

Resolve claim problems and billing errors

Sometimes claims are denied, delayed, or processed incorrectly when you have two insurers. If you receive a bill you believe should have been paid by one of your insurers, take these steps.

First, gather the explanation of benefits from both Medicare and your private insurer. These documents show what each plan received, what they paid, and what they denied. Compare the two to see whether the claim was submitted to both insurers and whether both processed it correctly.

If Medicare denied the claim, call Medicare at 1-800-MEDICARE and ask why. If your private insurer denied the claim, call the number on your insurance card. Ask the insurer to explain the denial in writing. Do not pay the bill until you understand why it was denied.

If the provider did not submit the claim to your secondary insurer, contact the provider's billing office and request that they submit it now. Provide them with your private insurance information if they do not have it. Most insurers will pay claims submitted up to one year after the date of service.

If you believe the bill is wrong, file a complaint with your state's insurance commissioner. You can find contact information at your state's insurance department website.

Frequently Asked Questions

Can I have Medicare Advantage and Medigap at the same time?

No. Medicare does not allow you to hold both. If you have Medicare Advantage, you must drop it before enrolling in Medigap. If you have Medigap and want to switch to Medicare Advantage, you must drop Medigap first. You can switch during the annual open enrollment period without penalty.

What happens if my private insurance does not know I have Medicare?

Your private insurance may deny claims or pay as if it were primary instead of secondary, leaving you with a larger bill. Contact your private insurer when ready and provide your Medicare number and enrollment date. Ask them to reprocess any recent claims with the correct coordination of benefits.

Do I pay premiums for both Medicare and my private insurance?

Yes. You pay your Medicare Part B premium (and Part D if you have prescription drug coverage) to Medicare, and you pay your private insurance premium to your private insurer. Employer retiree coverage may be free or low-cost depending on your employer's plan. Medigap premiums vary by insurer and plan letter.

What if I see a provider who does not accept Medicare?

If a provider does not accept Medicare, your Medicare coverage will not explore, and your private insurance will be primary. You may owe the full bill or your private insurance's out-of-pocket costs. Before seeing an out-of-network provider, call your private insurer to confirm they cover that provider and what you will owe.

Can I switch from one Medigap plan to another?

Yes, you can switch Medigap plans at any time, but if you switch outside of your initial enrollment period, the new insurer may deny you or charge higher premiums based on your health. Your best opportunity to switch without health questions is during the annual open enrollment period (January 1 to March 31) or within 63 days of losing other coverage.