Medicare is not a single programme but four separate ones, purchased through two mutually exclusive routes, with enrollment deadlines that carry permanent financial consequences. It is the rare area of personal finance where a purely administrative error — filing three months late — produces a bill that follows you for thirty years.
The complexity is not accidental. Original Medicare was designed in 1965, Part C in 1997 and Part D in 2003, each bolted onto the last. Understanding the structure is the only way to avoid the traps.
The four parts, plainly
| Part | Covers | Typical cost | Premium? |
|---|---|---|---|
| A — Hospital | Inpatient stays, skilled nursing, hospice | Deductible per benefit period | Free with 40 work quarters |
| B — Medical | Doctors, outpatient, labs, equipment | 20% coinsurance, no cap | Yes — standard monthly premium |
| C — Advantage | Private bundle replacing A + B, often with D | Network-based copays, has an out-of-pocket cap | Sometimes $0 above Part B |
| D — Drugs | Prescription medication | Tiered copays, annual out-of-pocket cap | Yes — varies by plan |
Original Medicare has no out-of-pocket maximum.Parts A and B alone leave you exposed to unlimited 20% coinsurance. This is the single most important structural fact about Medicare, and the reason a supplement or an Advantage plan is not optional in practice.
The two routes you must choose between
Once enrolled in A and B, you pick one of two paths. They are not combinable, and switching later is not always possible.
Original Medicare + Medigap + Part D
- Any provider nationwide that accepts Medicare — no networks, no referrals
- Predictable costs; a Plan G supplement leaves only the Part B deductible
- Travel-friendly, ideal if you split time between states
- Higher fixed monthly cost: Part B + Medigap + Part D premiums
Medicare Advantage (Part C)
- Often $0 additional premium, frequently bundles drugs, dental and vision
- Has a hard annual out-of-pocket maximum, which Original Medicare lacks
- Network restrictions, prior authorisation requirements, referrals
- Switching back to Medigap later may require medical underwriting
The asymmetry deserves emphasis. Moving from Medigap to Advantage is straightforward at any annual enrollment. Moving from Advantage back to Medigap generally requires passing underwriting, and insurers can decline you. Choosing Advantage at 65 because you are healthy can quietly become a one-way door once you are not.
Enrollment windows and what they cost to miss
- Initial Enrollment Period — seven monthsThree months before your birthday month, your birthday month, and three months after. Enrolling in the first three months means coverage starts on the first day of your birthday month.
- Special Enrollment Period — eight monthsFor those who deferred due to active employer coverage. Begins when that employment or coverage ends, whichever comes first.
- General Enrollment Period — Jan 1 to Mar 31The fallback if you missed the others. Coverage begins the month after you enrol, and late penalties apply.
- Annual Open Enrollment — Oct 15 to Dec 7For changing Advantage and Part D plans each year. Not for first-time Part B enrollment.
COBRA and retiree coverage are not qualifying coverage.This is the most expensive misunderstanding in Medicare. Only active employment-based group coverage lets you defer Part B penalty-free. If you retire at 64 and take COBRA, your Medicare clock is running the entire time. Every month counts toward a permanent penalty.
Calculate your late-enrollment exposure
The Part B penalty is 10% of the standard premium for each full 12-month period you were eligible but not enrolled. Unlike most penalties, it never expires — it is added to every monthly premium for the remainder of your life, and it grows as premiums rise.
IRMAA: the surcharge that looks backwards
Higher-income beneficiaries pay an Income-Related Monthly Adjustment Amount on top of standard Part B and Part D premiums. The mechanic that catches people is the two-year lookback: your 2026 premium is set by your 2024 tax return.
| Modified AGI (single) | Modified AGI (joint) | Part B surcharge |
|---|---|---|
| Up to ~$106,000 | Up to ~$212,000 | None — standard premium |
| ~$106k – $133k | ~$212k – $266k | Tier 1 |
| ~$133k – $167k | ~$266k – $334k | Tier 2 |
| ~$167k – $200k | ~$334k – $400k | Tier 3 |
| ~$200k – $500k | ~$400k – $750k | Tier 4 |
| Above ~$500,000 | Above ~$750,000 | Tier 5 — highest |
IRMAA is a cliff, not a slope.Exceeding a threshold by one dollar moves you into the entire next tier for both spouses, for a full year. When planning Roth conversions or realising capital gains at 63 and 64, model the bracket edge deliberately — a conversion that saves $4,000 in future tax can cost $3,000 in surcharges.
If your income has genuinely dropped because of a life-changing event — retirement, divorce, death of a spouse, loss of income property — you can appeal using Form SSA-44. Work stoppage is an accepted reason, and the appeal is routinely granted. Very few new retirees file it, and many overpay for a year as a result.
The Medigap window closes quietly
For six months beginning when your Part B coverage starts, you have a guaranteed-issue right to any Medigap policy sold in your state. Insurers cannot refuse you, cannot charge more for pre-existing conditions, and cannot impose medical underwriting.
After those six months, in most states, that protection disappears entirely. Insurers may then review your health history, charge more, or decline outright. Someone who chose Advantage at 65 and wants Medigap at 72 after a cancer diagnosis will typically find the door closed.
- Plan G is the most comprehensive available to new enrollees — it covers everything except the annual Part B deductible.
- Plan N costs less with small copays for office and emergency visits.
- High-deductible Plan G suits healthy retirees wanting catastrophic protection at a low premium.
- Compare on price alone within a letter — benefits for a given plan letter are standardised by law, so a cheaper Plan G is genuinely identical coverage.
Working past 65: the deferral rules
You may defer Part B without penalty only if you have active employer group coverage. Two conditions matter:
- Employer size decides who pays firstAt companies with 20 or more employees, the group plan pays first and Medicare is secondary — deferral is safe. Below 20 employees, Medicare typically pays first, and failing to enrol can leave you effectively uninsured for the bulk of a claim.
- Part A is usually still worth takingIt is premium-free with 40 quarters of work history and coordinates with employer coverage. The exception is important: enrolling in Part A makes you ineligible to contribute to an HSA. If you are still funding one, delay Part A too.
- Get the coverage letter in writingWhen you retire, request written confirmation of creditable coverage dates from your employer. You will need it to prove eligibility for the Special Enrollment Period and to avoid penalties.
- Watch the six-month Part A lookbackWhen you eventually enrol, Part A coverage is backdated up to six months. HSA contributions made in that retroactive window become excess and must be corrected.
Limits, errors and borders of the standard advice
Critical reviewThis is the most heavily marketed decision in retirement. An enormous share of the information reaching people is advertising from insurers and commissioned brokers, and commission structures differ sharply between the options being compared.
"Enrol as soon as you turn eligible or you'll be penalised."
The claimLate enrolment carries permanent penalties, so sign up immediately.
Where it breaksPenalties are real and the rule has significant exceptions that catch people out in both directions. Someone still working with qualifying employer coverage may be able to delay parts of it without penalty, and enrolling anyway can create complications — including making them ineligible to continue contributing to a health savings account, which is rarely mentioned in the urgency. Conversely, some people assume any employer coverage protects them when small-employer plans may not qualify. The blanket instruction replaces a conditional rule with a simple one, and the conditions are where the money is.
The borderEnrol on time if you have no qualifying coverage — the penalties are lifelong. If you are still working, confirm in writing whether your specific plan qualifies as creditable coverage, and check the interaction with any health savings account before enrolling.
"Advantage plans are better — they include extras at no extra premium."
The claimBundled plans with low or zero premiums and added benefits are the better value.
Where it breaksThe premium is not the cost. These plans manage cost through networks, prior authorisation and referral requirements, so the real trade is money for flexibility — which matters little while healthy and enormously during serious illness, when the specialist or centre you need may be out of network. Plan terms, networks and formularies can change annually, so the plan you chose is not the plan you have in five years. The advertised extras are typically capped at modest amounts. And the marketing intensity reflects commission structures, not comparative merit.
The borderBundled plans suit people with predictable healthcare needs, comfort with network rules, local providers already in network, and a preference for a capped annual out-of-pocket maximum. Traditional coverage plus supplemental insurance suits those who travel, want any provider, or anticipate serious illness — and who can afford the higher predictable premium.
"You can switch plans every year, so the first choice doesn't matter much."
The claimAnnual open enrolment means any mistake is correctable.
Where it breaksSwitching between bundled plans is generally straightforward. Moving back to traditional coverage with supplemental insurance frequently is not, because supplemental policies can be medically underwritten outside a limited initial window — so an applicant who has developed a condition may be declined or priced out. That makes the initial decision far more consequential than "switch annually" implies, and it is the single most important asymmetry in the whole choice. It is also the fact most consistently absent from advertising, because it argues against the heavily marketed option.
The borderTreat the first election as a decision with a one-way door attached. Understand your local rules on supplemental underwriting and guaranteed-issue windows before choosing — and if your health may change, weight the initial choice accordingly.
"A free broker will find you the best plan."
The claimBrokers cost nothing and have expertise, so using one is strictly beneficial.
Where it breaksThey are paid by insurers, and not equally by all of them — commission differs by product type and by carrier, and some brokers are appointed with only a subset of available plans. So "the best plan" is selected from a menu you cannot see, with incentives you are not shown. Independent counselling services exist in many places and are genuinely unpaid, but they are far less visible than the marketing. The word "free" describes your invoice, not the absence of a financial interest.
The borderBrokers can be genuinely useful for navigating plan mechanics and comparing networks. Ask directly which carriers they represent and how they are compensated, cross-check against an official plan finder, and use an independent counselling service as a second opinion before deciding.
We name no plans, premiums, penalty amounts or enrolment windows, because these change annually and vary by location — so this page frames the decision rather than making it, and you will need current official sources. The guidance is also specific to one country's system. And our emphasis on flexibility and worst-case access reflects a risk preference: for a healthy retiree on a tight budget, the lower-premium bundled option may well be the right call, and our framing under-weights the value of money now against optionality later.
Frequently asked questions
Do I need Part B if I have veterans' benefits?
VA coverage does not count as creditable coverage for Part B, and the two systems do not coordinate — VA care must be received at VA facilities. Most advisers recommend enrolling in Part B anyway to preserve access to civilian providers and avoid the permanent penalty.
What if I am still on my spouse's employer plan?
Active employer coverage through a working spouse qualifies you for deferral, subject to the same 20-employee rule. Coverage from a retired spouse's plan does not qualify, and neither does COBRA.
Can I have both an Advantage plan and a Medigap policy?
No. It is illegal for an insurer to sell you a Medigap policy while you are enrolled in Medicare Advantage. They are alternative routes, and you must choose one.
Does Medicare cover long-term care?
Essentially no. Medicare covers limited skilled nursing after a qualifying hospital stay, capped at 100 days with heavy cost-sharing after day 20. Custodial care — help with daily living — is not covered at all. That exposure is addressed separately in our long-term care guide.
The bottom line
Diarise the three months before your 65th birthday and treat it as a hard deadline. If you are still working at a large employer, get written confirmation of creditable coverage and defer deliberately rather than by accident. Choose between Advantage and Medigap with the knowledge that only one direction stays open later. And if you are converting retirement accounts in your early sixties, model the IRMAA cliff two years forward before you press the button.
Revision log. Aug 2026: added the small-employer primary-payer warning and HSA six-month lookback. Income thresholds reviewed annually.