Medicare Coordination of Benefits: Who Pays First and How to Update Your Records

Reviewed October 2, 2026. Medicare coordination of benefits (COB) is the set of rules that decides which insurance pays first when you have Medicare plus other coverage, such as an employer plan, COBRA, retiree coverage, Medicaid, TRICARE or workers’ compensation. The “primary” payer pays first up to its limits, and the “secondary” payer may pick up some of the rest. To update what Medicare has on file about your other coverage, call the Benefits Coordination & Recovery Center (BCRC) at 1-855-798-2627 (TTY 1-855-797-2627).

Quick answer: If you’re 65 or older and covered by a current employer’s group plan, the employer plan pays first when the employer has 20 or more employees; with fewer than 20, Medicare generally pays first. Medicare pays first over COBRA and retiree coverage. Medicaid always pays after Medicare for services Medicare covers.

Who pays first: Medicare coordination of benefits rules

These summaries follow Medicare.gov’s “Who pays first?” guidance and the official booklet How Medicare Works with Other Insurance. Special situations, such as multi-employer plans, can change the result, so confirm with the plan’s benefits administrator.

How Medicare coordination of benefits works

When Medicare is primary, your doctor bills Medicare first. Medicare pays its share, then the claim can cross over to your secondary insurance, which may pay some or all of what’s left. When another plan is primary, the provider bills that plan first and Medicare may pay toward the remaining costs it covers.

Problems usually start when Medicare’s records don’t match reality: for example, Medicare thinks you still have an employer plan after you retired, or a provider doesn’t know about your other insurance. Claims can then be denied or paid by the wrong insurer, and one of them may later ask for money back.

Medicare.gov’s advice: “Tell your doctor and other health care providers if you have coverage in addition to Medicare. This will help them send your bills to the correct payer and avoid delays.”

How to update your Medicare coordination of benefits

1. Call the BCRC at 1-855-798-2627 (TTY 1-855-797-2627). Have your Medicare number ready (see how to find your Medicare number), plus your other plan’s name, policy number, the policyholder’s name and the start or end date of that coverage.
2. Tell the BCRC what changed: you retired, your employer coverage ended, you started COBRA, or you gained Medicaid, TRICARE or a new job-based plan.
3. Tell your providers and pharmacy so they bill the right insurer.
4. Tell your other plan about your Medicare coverage and its effective dates. Some employer and retiree plans also send a questionnaire to update their records.
5. Follow up on denied claims. Once your record is corrected, ask your provider to resubmit any claims that were denied because of the wrong primary payer.

For claims tied to an accident, lawsuit or workers’ compensation, Medicare may have paid “conditionally” and later seek repayment. That recovery process is handled separately by CMS’s Medicare Secondary Payer recovery contractors; the BCRC can tell you where your case stands.

Common COB situations

You’re 65 and still working

If your employer has 20 or more employees, the employer plan pays first and many people delay Part B without penalty. When the job or coverage ends, you get an 8-month Special Enrollment Period for Part B. Read Working past 65: Medicare, employer coverage, COBRA and HSA rules and can I drop my employer’s health insurance for Medicare?

You’re on COBRA

Medicare pays first. If you’re eligible for Medicare and only have COBRA, the COBRA plan may pay very little, so don’t skip Part B because you have COBRA. COBRA doesn’t count as current employment coverage for the Part B Special Enrollment Period.

You have retiree coverage

Medicare pays first and the retiree plan pays second. Many retiree plans require you to enroll in Part A and Part B. See Medicare and group health insurance.

You have Medicaid too

Medicare pays first, and Medicaid may cover premiums, cost-sharing and services Medicare doesn’t. See dual eligibility in Medicare and Medicaid.

You have TRICARE or VA benefits

TRICARE pays first while you’re on active duty; with TRICARE For Life, Medicare pays first. VA and Medicare don’t pay for the same service. See the TRICARE to Medicare transition.

Frequently asked questions

What is the Medicare coordination of benefits phone number?

The Benefits Coordination & Recovery Center is at 1-855-798-2627, TTY 1-855-797-2627.

Is Medicare primary or secondary to my employer plan?

At 65 or older with an employer of 20 or more employees, the employer plan is primary. With fewer than 20 employees, Medicare is generally primary.

Is Medicare primary over COBRA?

Yes, for people who are 65 or older or otherwise eligible for Medicare.

Can I update my coordination of benefits online?

Medicare.gov directs beneficiaries to call the BCRC to report other coverage. Your providers and plans can also report changes.

Why was my claim denied for “other insurance primary”?

Medicare’s records show another insurer as primary. Call the BCRC to confirm or correct the record, then ask the provider to rebill.

This article is general information, not legal or individual insurance advice. Medicare365 is not affiliated with or endorsed by the U.S. government or the federal Medicare program.

Official sources

• Medicare.gov: Who pays first?
• Medicare.gov: How Medicare Works with Other Insurance (publication 02179)
• Medicare.gov: Medicare’s Coordination of Benefits – Getting Started
• CMS: Coordination of Benefits