Medicare and Employer Insurance After 65: Which Is Primary?

You can delay Medicare Part B (and Part A if you pay a premium for it) without a late penalty if you have current employer group health coverage from a company with 20 or more employees. Once that coverage ends, you get an 8-month Special Enrollment Period (SEP) to sign up for Part B. But the rules flip if your employer has fewer than 20 workers, and Part D requires its own separate check. The most common failure: assuming any employer plan qualifies and missing the Part B sign-up window, resulting in a permanent 10% premium penalty for each 12-month delay.

What this means for your next move: If you are still working at 65 and keeping employer coverage, your first verification step is employer size. Do not assume – confirm it in writing. If you have a small employer (under 20 employees), you must enroll in Part B during your Initial Enrollment Period (IEP) or you will owe a late enrollment penalty for life. If you have a large employer, you can safely delay Part B, but you must track your SEP start and end dates precisely.

When Employer Size Decides Your Part B Penalty Risk

Employer size determines whether Medicare is primary or secondary payer. The table below shows your options.

Employer size	Primary payer	Can you delay Part B?	Late penalty risk
20+ employees	Group plan	Yes, without penalty during SEP	Only if you miss the 8-month SEP
Fewer than 20 employees	Medicare	No – enroll during IEP	10% per 12-month delay (lifetime)
Retiree coverage (former employer)	Varies – Medicare is usually primary after 65	Generally must enroll during IEP	Yes, unless coverage is from current active employment
COBRA	Medicare is primary (COBRA is not current employer coverage)	Must enroll during IEP unless you have another SEP trigger	Yes – SEP starts when employment ended, not when COBRA expires

Verification step: Ask your HR or benefits department for a written statement confirming your employer’s total number of employees. If they say “under 20,” sign up for Part B during your IEP. If they say “20 or more,” keep that documentation with your Medicare records.

Failure mode to detect early: Many people rely on a spouse’s employer plan that has 15 employees. That plan does not let you delay Part B. Check your own or your spouse’s employer size before your IEP ends. If you miss the IEP because you assumed the plan qualifies, you will owe 10% of the Part B premium ($18.50/month in 2025) for every 12 months you delay, added to your premium forever.

Part D: Creditable Drug Coverage Is Not Automatic

Employer prescription drug coverage counts for Part D only if it is creditable – meaning it pays at least as much as Medicare’s standard Part D benefit. Your employer must provide a “Notice of Creditable Coverage” each year before October 15. If you do not receive one, request it in writing.

• Creditable coverage: You can delay Part D enrollment without penalty. When you lose coverage, you get a 63-day SEP to join a Part D plan.
• Non-creditable or unknown: You must enroll in a Part D plan during your IEP. If you go 63 days or more without creditable drug coverage, you owe a late enrollment penalty of 1% of the national base beneficiary premium ($36.78 in 2025) for every full month you were eligible but not enrolled. That penalty sticks for life.

Example: You go 30 months without creditable Part D coverage. Your penalty is 30% of $36.78 = $11.03 added to your Part D premium every month forever.

Practical implication: If your employer’s drug plan is not creditable, staying on it does not protect you from the Part D penalty. You must join a standalone Part D plan during your IEP – even if you keep the employer medical plan.

The Special Enrollment Period (SEP) – Know Your Deadlines

The SEP for Part B starts the month after your employment or group coverage ends – whichever happens first – and lasts 8 months. For Part D, the SEP lasts 63 days from the day coverage ends.

Common SEP triggers:

• You retire or are laid off
• Your spouse’s employment ends (if you were on their plan)
• You lose employer coverage due to downsizing
• Your COBRA coverage ends (but the SEP for Part B began when employment ended, not when COBRA ends – do not wait)

Warning: If you miss the 8-month Part B SEP, you must wait for the General Enrollment Period (January 1 – March 31 each year) and may owe a late penalty. Missing the Part D SEP means you may have to pay the penalty until you enroll, and you may have to wait for another enrollment period.

Decision Aid: 5 Quick Checks Before You Delay Medicare

Use this checklist to decide your next steps. Each item is a pass/fail check you can complete today.

1. Does my employer (or my spouse’s) have 20 or more employees?

• Yes → You can safely delay Part B
• No → You must enroll in Part B during your IEP (or face lifetime penalty)

2. Did I receive a “Notice of Creditable Coverage” for my drug plan?

• Yes → You can delay Part D enrollment
• No or unsure → Your coverage is likely not creditable – enroll in Part D during your IEP

3. Am I (or my spouse) still actively working?

• Yes → Your SEP starts when employment ends
• No → If already retired, your SEP may have started or expired – check the calendar

4. Do I have a Health Savings Account (HSA)?

• Yes → You cannot contribute to an HSA once Medicare Part A begins (even if free). Stop HSA contributions at least 6 months before applying for Medicare to avoid tax penalties.

5. Have I written down my SEP deadlines?

• Part B SEP: 8 months from the month after employment/coverage ends
• Part D SEP: 63 days from the day coverage ends
• Mark your calendar now – missing these windows costs you money for life

FAQ

Can I keep my employer coverage and also take Medicare Part A (hospital insurance) for free?

Yes. Most people get Part A premium-free (if you or your spouse paid Medicare taxes for at least 10 years). You can enroll in Part A at 65 even while working, without losing employer coverage. However, you cannot contribute to an HSA after Part A starts.

What if my employer has 20+ employees but I still want Part B?

You can enroll in Part B while keeping employer coverage. Your employer plan becomes secondary payer. You pay the Part B premium ($185/month in 2025) plus any IRMAA surcharge. This may make sense if your employer plan has high deductibles or limited coverage.

Does COBRA count as current employer coverage for delaying Medicare?

No. COBRA is continuation coverage after employment ends. Your Part B SEP starts when you stopped working, not when COBRA ends. Relying on COBRA alone can cause you to miss the SEP and incur penalties.

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Disclaimer: Medicare rules, premiums, and income thresholds change annually. The information above reflects 2025 amounts (Part B premium $185, Part D base premium $36.78). Verify your specific situation with your employer benefits administrator and check current details at Medicare.gov or call 1-800-MEDICARE. This article does not provide personalized financial or legal advice.