You're 65, you've been on Medicare for a year, and someone just offered you a good job. Or you're turning 65, you left the workforce a couple years ago, and now you're heading back. Either way, you're staring at the benefits enrollment form wondering whether your Medicare survives this. It does. But a few decisions you make in the next few months can affect what you pay in health insurance premiums for the rest of your life.
The rules are more flexible than most people expect, but there are some specific traps. The Medigap trap. The COBRA trap. The HSA trap. Each one is counterintuitive enough that plenty of people fall into them every year, often because something that sounds logical turns out to be exactly wrong.
You can keep Medicare and take your new employer coverage
The first thing to know: you don't have to choose. You can enroll in your employer's health plan and keep Medicare at the same time. The two plans coordinate benefits, meaning one pays first (primary) and the other covers some or all of what's left (secondary). Which plan is primary depends on the size of your employer, which is covered next.
This dual-coverage setup can work well in either direction. If your employer plan has a high deductible or a narrow network, having Medicare paying secondary can fill gaps. Conversely, if your employer plan is solid, it may cover most costs before Medicare gets involved. The trade-off is that you're paying premiums for both at the same time, which gets expensive quickly. The rules for people working past 65 differ depending on your specific coverage situation, but the financial math of running both plans is something you'll need to work through on your own.
One thing to flag immediately, before you get into anything else: the moment you're enrolled in any part of Medicare, you cannot contribute to a Health Savings Account. Not reduced contributions, not a partial year. Zero. If your new employer offers an HSA-eligible high-deductible plan, and you were hoping to fund an HSA, that's off the table. More on this below.
The 20-employee rule determines who pays first
The single most important variable in this whole situation is the size of your new employer. If the company has 20 or more employees, your employer plan is the primary payer and Medicare pays second. The employer plan handles claims first; Medicare steps in on whatever remains within Medicare's scope. If the company has fewer than 20 employees, it reverses: Medicare pays first and the employer plan is secondary.
This distinction matters enormously for costs. When your employer plan is primary, Medicare Part B functions as a supplement, and you may be able to get good coverage without paying for Medigap on top of everything else. When Medicare is primary at a small employer, the employer plan is filling Medicare's gaps, and some small-company plans aren't designed to do that well. Before you accept any health plan, ask HR directly which situation applies to you and how the plans are set to coordinate. Don't assume, because getting it wrong means unexpected bills.
A third scenario: if you're self-employed, or if your new coverage comes from something other than an active group health plan (retiree coverage from a previous employer, for example), Medicare becomes the primary payer regardless of company size. Retiree coverage specifically is designed to work alongside Medicare, not in front of it, so check with your benefits administrator before assuming otherwise.
Dropping Part B to save money, and what it costs you
The 2026 Part B premium is $202.90 a month for most people. If you now have solid employer coverage through a large company where the employer plan is primary, paying that $202.90 for secondary coverage may not make financial sense. You can drop Part B while you have creditable employer coverage, then re-enroll penalty-free when the job ends using the Special Enrollment Period. On paper, this is a reasonable cost-cutting move for people returning to full-time work.
The problem is Medigap. If you already have a Medicare Supplement policy, dropping Part B means you lose it. And when you later re-enroll in Part B and want Medigap back, you generally won't have a guaranteed issue right. Insurers can review your health history and decline you, or charge you significantly higher premiums based on pre-existing conditions. The guaranteed issue window, the six-month period when insurers must sell you a Medigap plan at standard rates, runs from when you first enroll in Part B. That window is almost certainly behind you if you're already 65 and on Medicare.
Some states have broader protections that give Medigap applicants more flexibility, but most don't. If you're in good health and expect to keep working for several years, dropping Part B and Medigap might still pencil out. If you have ongoing health conditions or significant healthcare costs, the math changes fast. Your state's free Medicare counseling program, called SHIP (State Health Insurance Assistance Program), can help you run through the numbers without trying to sell you anything.
Your HSA contributions stop the moment you're on Medicare
If you're enrolled in Medicare Part A, Part B, or both, you cannot make new contributions to a Health Savings Account. The IRS rule is that simple. It doesn't matter that you're enrolled in an employer's high-deductible health plan and would otherwise be eligible. Medicare enrollment overrides it. This is one of the most common surprises for people returning to work, because a new employer's HR department may not think to flag it.
Most people returning to work at 65 are already enrolled in Part A. Part A is free for nearly everyone who worked at least 10 years and paid Medicare taxes, so most people sign up when they first become eligible. That enrollment, even without Part B, ends HSA eligibility permanently as long as Medicare Part A is active. There is a workaround: you can technically disenroll from Part A, but doing so requires disenrolling from Medicare entirely, which also means repaying any Social Security benefits you've received. That is almost never a path worth taking.
One additional trap affects people who delayed Part A enrollment and are signing up for Medicare now. When you apply for Medicare after turning 65, Part A coverage can be backdated by up to six months (but no earlier than your 65th birthday). If you contributed to an HSA during those retroactive months, those contributions become excess contributions and trigger a tax penalty. If this applies to you, stop contributions immediately and talk to a tax professional about the lookback period.
You can still use the money already in your HSA. Those funds can cover Medicare premiums, deductibles, copays, and most out-of-pocket medical costs completely tax-free. The account still works; it just stops accepting new deposits.
COBRA after leaving the job does not protect you from penalties
At some point, you'll leave this job too. When that happens, the Medicare decisions you deferred become urgent again. Many people assume that taking COBRA buys them time, that being “covered” through COBRA means they can continue to delay Medicare enrollment without consequences. This assumption is wrong, and it's a common and costly mistake.
COBRA is specifically excluded from the employer coverage rules that allow penalty-free Medicare delays. When your active employment ends, an 8-month Special Enrollment Period begins, and it runs whether or not you take COBRA. Taking COBRA does not extend that window. If you enroll in Part B within that 8-month period, you're protected. If you let the SEP expire thinking your COBRA coverage was keeping you safe, you're not. You'd have to wait for the General Enrollment Period (January 1 through March 31), with coverage starting the following July, and you'd face a permanent late enrollment penalty.
The same applies to retiree health coverage from a previous employer. It may be excellent coverage, and you may feel fully insured. But Medicare does not treat it the same as active employment-based coverage, and it won't protect your SEP rights.
The late enrollment penalty is permanent
The Part B late enrollment penalty is 10% of the standard premium for every 12-month period you were eligible for Medicare but didn't enroll without qualifying for an exception. At the 2026 standard premium of $202.90, a two-year delay adds $40.58 a month to your premium. A five-year delay adds $101.45 a month. Those amounts are permanent, for as long as you have Medicare.
It compounds over time in two ways. First, the flat penalty percentage stays with you for life. Second, because the penalty is recalculated each year using the current premium rather than the premium at the time of the delay, your penalty grows in dollar terms every time CMS raises the Part B premium rate. Over a 20-year retirement, even a modest penalty turns into a significant amount of money paid for a mistake that was easily avoidable.
The penalty can be avoided entirely as long as you had continuous health coverage tied to active employment, either your own or a spouse's, throughout any period of delayed enrollment, and you use the Special Enrollment Period correctly once that active employment ends. That protection is solid and works as intended. The failure mode is losing track of when the SEP starts, or assuming something that isn't active employment coverage, like COBRA or individual marketplace plans, qualifies.
If you're not certain where you stand, call Medicare at 1-800-MEDICARE or contact your state's SHIP counselor before you leave the workforce. Getting this wrong is the kind of mistake that costs $40 to $100 a month for the next 20 years.
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