Medicare Enrollment Periods: When You Can Enroll or Switch

Medicare has six enrollment windows. They overlap, they have different rules about what you are allowed to change, and the marketing you receive rarely distinguishes between them.

Here is each one, what it actually lets you do, and which of them carry consequences that outlast the window itself.

1. Initial Enrollment Period — seven months, once

Your IEP runs seven months: the three months before the month you turn 65, the month itself, and the three months after.

This is when you sign up for Part A and Part B, and when you first choose between Medicare Advantage and a Medigap policy.

Two details matter more than the rest of the mail you will receive combined.

Enrolling early changes your start date. Sign up during the three months before your birthday month and coverage begins on the first day of that month. Sign up during or after your birthday month and the start date slides — you can end up with weeks of gap you did not plan for. There is no advantage to waiting and a real cost to it.

Born on the first of a month? Medicare treats you as attaining 65 in the previous month. Every window shifts a month earlier, including the Medigap one. This catches out people who had read up on it.

The enrollment window checker does the arithmetic from your date of birth, including that rule.

2. Medigap open enrollment — six months, and it never returns

This is the one that does not appear in the glossy envelopes, and the one with permanent consequences.

It starts on the first day of the month in which you are both 65 and enrolled in Part B, and runs six months. During it, no Medicare Supplement carrier may refuse you coverage or charge you more because of your health.

After it, in Arizona, carriers may medically underwrite — and they do. A diagnosis between now and then can close a door standing wide open today.

This window does not come back. There is no annual do-over. It happens once.

That asymmetry is worth sitting with: joining a Medicare Advantage plan is never health-gated, at any age. Joining Medigap is ungated exactly once. So the two paths are not equally reversible, and the decision deserves to be made deliberately during this window rather than by default.

3. Annual Enrollment Period — 15 October to 7 December

The one everybody has heard of, because it is when the television advertising starts. Changes take effect 1 January.

During AEP you may:

• Join, switch or drop a Medicare Advantage plan
• Join, switch or drop a Part D plan
• Move from Original Medicare to Medicare Advantage, or back

What AEP does not do is guarantee you a Medigap policy. Moving from an Advantage plan to Original Medicare during AEP is straightforward; obtaining a supplement to go with it may still require underwriting if you are outside your Medigap window. People discover this in December, having assumed the two moves were a single step.

Use AEP even when you are happy. Formularies, networks, premiums and out-of-pocket maximums all reset on 1 January. The most expensive mistake I see is a plan that fit perfectly three years ago and quietly stopped fitting. Re-pricing your drug plan against your actual medication list, every autumn, is the highest-value twenty minutes in Medicare.

4. Medicare Advantage Open Enrollment — 1 January to 31 March

Narrower than AEP and frequently confused with it.

Only available if you are already on a Medicare Advantage plan on 1 January. It gives you one change:

• Switch to a different Medicare Advantage plan, or
• Return to Original Medicare and pick up a standalone Part D plan

You cannot use it to join an Advantage plan from Original Medicare, and you cannot use it to change a standalone drug plan while staying on Original Medicare. One change, then it closes.

This is the safety valve for a January surprise — a network change, a formulary shift, a plan that turned out not to fit. Worth knowing it exists before you need it.

5. General Enrollment Period — 1 January to 31 March

For people who missed their Initial Enrollment Period and have no Special Enrollment Period available.

You may enrol in Part A and/or Part B, with coverage beginning the first of the month after you sign up. A late enrollment penalty usually applies.

Before assuming you need this, check whether a Special Enrollment Period applies — if you had coverage through active employment that ended within the last eight months, you may still be in a penalty-free window.

6. Special Enrollment Periods — triggered by circumstance

These sit on top of everything above and are opened by life events rather than the calendar: losing employer coverage, moving, a plan leaving your area, qualifying for Extra Help, moving into or out of a care facility.

Each has its own length and its own start point, and most run from the event or its notification rather than from when you noticed. They are covered in detail in special enrollment periods.

The calendar, in one place

Window	Dates	Who it is for	What you may change
Initial Enrollment	7 months around your 65th	Everyone turning 65	Enrol in Part A and B; choose your path
Medigap open enrollment	6 months from Part B start at 65	Everyone turning 65	Buy any Medigap policy, no health questions
Annual Enrollment	15 Oct – 7 Dec	Everyone	Advantage, Part D, or switch paths
Medicare Advantage OEP	1 Jan – 31 Mar	Advantage members only	One switch, or back to Original
General Enrollment	1 Jan – 31 Mar	Those who missed their IEP	Enrol in Part A and/or B
Special Enrollment	Varies by event	Those with a qualifying event	Depends on the event

If you qualified before 65

Medicare is not only for people turning 65. Two routes bring people in earlier, and the windows work differently.

Disability. After 24 months of Social Security Disability Insurance payments, Medicare begins automatically — you do not apply. Your Initial Enrollment Period runs seven months around that 25th month.

ALS or end-stage renal disease. ALS brings Medicare the month disability benefits begin, with no 24-month wait. ESRD has its own timing rules tied to the start of dialysis or a transplant.

The important wrinkle: Medigap protections under 65 are a matter of state law, not federal. Federal rules do not guarantee anyone under 65 the right to buy a supplement, and states differ enormously in what they require. If you are on Medicare under 65 in Arizona, the supplement question needs a specific answer for your circumstances rather than the general one above.

And there is a second Medigap window most people never hear about: when you turn 65 while already on Medicare through disability, you get a fresh six-month Medigap open enrollment period at 65, with full federal protections. That is a genuine second chance, and it is missed constantly because nobody expects it.

The mistakes I see most

Five, in rough order of how expensive they are:

1. Assuming AEP will get you a Medigap policy. It will not. Moving off an Advantage plan is easy; getting a supplement to replace it may require underwriting.
2. Treating COBRA as employer coverage. It is not active employment. The Part B clock runs anyway.
3. Waiting until the last month of the IEP. Coverage start slides, and you decide under time pressure.
4. Skipping AEP because nothing changed for you. The plan changed even if you did not.
5. Confusing the Medicare Advantage OEP with AEP. The January window is narrower and only open to Advantage members.

Still working at 65?

The rules shift, and in your favour — with conditions.

If you or your spouse are covered by an employer plan through active employment, you can generally delay Part B without penalty and get an eight-month Special Enrollment Period when that employment ends.

Three cautions, all of which cost real money when missed:

• COBRA does not count. It is not active employment. The clock runs anyway.
• Retiree coverage does not count either, for the same reason.
• Under 20 employees usually means Medicare becomes the primary payer at 65 whether you enrolled or not — so delaying can leave you substantially uninsured rather than merely penalised.

Confirm with your benefits administrator, in writing, before relying on any of it.

What to do, and when

Six months before 65: work out which structure you want. Not which plan — which structure. Running the plan type finder is a reasonable starting point.

Three months before: enrol. Early, not late, so coverage starts on time and you are choosing without deadline pressure.

During your first six months on Part B: settle the Medigap question, because that window is the only one here that never returns.

Every October: re-price your drug plan against your current medication list. Every year. The plan changes even when your health does not.

Any January that brings a surprise: check whether the Medicare Advantage Open Enrollment Period gives you a way out.

The short version

• IEP — seven months at 65. Enrol early; coverage timing depends on it.
• Medigap OEP — six months, once, never repeats. The consequential one.
• AEP — 15 Oct to 7 Dec, open to all, changes effective 1 January.
• MA OEP — 1 Jan to 31 Mar, Advantage members only, one change.
• GEP — 1 Jan to 31 Mar, the way back in if you missed your IEP.
• SEPs — triggered by events, with their own clocks.

If you are unsure which window you are actually in, the checker will tell you from your date of birth — and if your situation has something unusual in it, that is exactly the sort of thing worth a short conversation rather than a guess.