Federal Employee Health Benefits (FEHB) coverage doesn’t end when you become eligible for Medicare. You can keep both, and how they work together depends on whether you enroll in Medicare Part B. That single decision reshapes which plan pays first, how much you pay out of pocket, and what your overall coverage looks like in retirement.
Which Plan Pays First
When you have two health insurance plans, one is the “primary payer” and the other is “secondary.” The primary plan pays your claims first, up to its coverage limits, then sends the remaining balance to the secondary plan. For federal retirees, the order depends entirely on your Medicare Part B enrollment status.
If you enroll in Medicare Part B, Medicare becomes your primary payer. Your FEHB plan shifts to secondary and may pick up the cost-sharing that Medicare leaves behind: deductibles, copayments, and coinsurance. Together, the two plans can significantly reduce or eliminate your out-of-pocket costs for covered services.
If you do not enroll in Part B, your FEHB plan stays primary and continues providing the same coverage it did while you were working. Nothing changes about how your claims are processed. Your FEHB plan simply keeps operating as your sole health coverage.
Medicare Part A: The Easy Decision
Most federal retirees enroll in Medicare Part A (hospital coverage) as soon as they’re eligible at age 65. The reason is straightforward: if you or your spouse paid Medicare taxes for at least 10 years, Part A is premium-free. It adds a layer of hospital coverage at no cost, and your FEHB plan can coordinate with it to cover any remaining hospital expenses.
One important catch applies if you’re enrolled in a high-deductible health plan with a Health Savings Account. Once you enroll in any part of Medicare, including Part A, you can no longer contribute to your HSA. You can still withdraw existing funds in the account, but new contributions must stop. If you’re still working past 65 and relying on HSA contributions, you may want to delay Part A enrollment.
The Part B Decision
Medicare Part B covers doctor visits, outpatient care, preventive services, and medical equipment. Unlike Part A, it comes with a monthly premium (starting at $185 per month in 2025, higher for people with greater income). This is where the decision gets more nuanced for federal retirees.
The case for enrolling in Part B is strongest if you want two layers of coverage working together. With Medicare paying first and FEHB picking up remaining costs, you can end up with very low out-of-pocket expenses on medical bills. Some FEHB plans cover nearly all of Medicare’s cost-sharing, which means your only real expense for covered services becomes the Part B premium itself plus your FEHB premium.
The case against Part B comes down to cost. You must continue paying your full FEHB premiums regardless of whether you enroll in Medicare. Adding Part B means paying both premiums every month. If your FEHB plan already provides solid coverage and you’re healthy, some retirees decide the extra monthly expense isn’t worth it.
Timing matters here. If you skip Part B when you first become eligible and change your mind later, you’ll face a late enrollment penalty: your Part B premium increases by 10% for every full 12-month period you could have had Part B but didn’t. You’ll also have to wait for the General Enrollment Period (January through March each year) to sign up, with coverage not starting until July. That gap and penalty are permanent.
Switching FEHB Plans at Medicare Eligibility
Becoming eligible for Medicare counts as a qualifying life event under FEHB rules. Starting 30 days before your Medicare eligibility date, you can switch to any available FEHB plan or option. This is a one-time opportunity outside the normal Open Season window, and it’s worth using strategically. Some FEHB plans are designed to coordinate especially well with Medicare, offering lower premiums because they expect Medicare to handle primary coverage. Reviewing your plan options at this point can save you significant money.
Prescription Drug Coverage
Medicare Part D covers prescription drugs, but most federal retirees don’t need to enroll in a standalone Part D plan. FEHB prescription drug coverage is considered “creditable,” meaning it’s comparable to or better than the standard Part D plan. As long as you maintain FEHB coverage, you won’t face a late enrollment penalty for Part D if you decide to pick it up later.
A growing number of FEHB carriers now offer Medicare Advantage plans with built-in prescription drug coverage (known as MA-PD plans) specifically for enrollees who have both Medicare Part A and Part B. These plans are designed to replace your traditional FEHB coverage with a Medicare Advantage structure while still operating within the FEHB program. If your carrier offers one, it may simplify your coverage into a single plan that handles both medical and drug benefits.
Special Rules for Postal Service Retirees
The Postal Service Health Benefits (PSHB) program, which launched in 2025, has fundamentally different rules from standard FEHB. If you’re a postal retiree, Medicare Part B enrollment is mandatory to maintain your PSHB coverage. There is no option to keep PSHB without Part B unless you qualify for a specific exception, such as receiving workers’ compensation benefits with a determination that you’re unable to return to duty.
The consequences of skipping Part B under PSHB are severe. If you choose not to enroll in Part B and don’t qualify for an exception, you lose eligibility for PSHB coverage in retirement. Even if you later enroll in Part B, you won’t be able to get back into PSHB. This rule also applies to Medicare-eligible family members on your plan. If your spouse becomes eligible for Medicare, doesn’t enroll in Part B, and doesn’t qualify for an exception, your spouse will be dropped from your PSHB family coverage.
What FEHB Won’t Cover
A common question among retirees is whether FEHB plans help pay for Medicare premiums. They do not. For active federal employees, FEHB is primary coverage, so plans don’t reimburse Part B premiums or waive costs related to Medicare. For retirees, the same holds: your FEHB plan won’t offset the cost of your Part B premium, even though the two plans coordinate on claims.
This means your total insurance cost in retirement is your FEHB premium plus your Part B premium (if you enroll), with no discount on either for having both. The financial benefit comes on the back end, through lower out-of-pocket costs when you actually use medical services, not through reduced premiums.