What the Annual Enrollment Period Actually Covers

Many seniors assume the Medicare Annual Enrollment Period (AEP) is only relevant if they are unhappy with their current plan. In reality, it is the primary opportunity for all Medicare beneficiaries to evaluate whether their existing coverage still fits their health and financial situation — and to act if it does not.

During AEP, you may:

  • Switch from Original Medicare (Parts A and B) to a Medicare Advantage (Part C) plan
  • Switch from Medicare Advantage back to Original Medicare
  • Move from one Medicare Advantage plan to a different one
  • Join, drop, or switch a standalone Part D prescription drug plan

To understand how Original Medicare and Medicare Advantage differ before making any change, see our side-by-side overview of Medicare Advantage vs. Original Medicare.

Mark Your Calendar in September

Your Annual Notice of Change (ANOC) arrives by late September — weeks before AEP opens. Reading it promptly gives you the most time to compare alternatives and make a deliberate decision rather than a rushed one before December 7.

Why Missing AEP Can Cost You — Even If Your Plan Continues

Staying enrolled in your current plan by default is not the same as an informed decision. Medicare plans are permitted to change their premiums, deductibles, copayments, covered drug formularies, and provider networks from one year to the next. Your plan is required to mail you an Annual Notice of Change (ANOC) each September, which details any upcoming changes.

If you do not review that notice and miss AEP, you could find yourself in January facing higher out-of-pocket drug costs, a narrowed provider network, or a formulary that no longer covers a medication you rely on — with no ability to change plans until the next AEP. These are not hypothetical inconveniences; formulary changes in particular can significantly affect seniors managing multiple chronic conditions.

Oct 15 – Dec 7

Annual Enrollment Period window each year

Established by the Centers for Medicare & Medicaid Services (CMS); all AEP changes take effect January 1.

Jan 1 – Mar 31

Medicare Advantage Open Enrollment Period

A secondary, more limited window that applies only to those already enrolled in a Medicare Advantage plan.

~50 million

Medicare Advantage enrollees in the U.S.

According to CMS data, Medicare Advantage enrollment has grown substantially, making annual plan review increasingly important as plan offerings evolve.

Missing enrollment windows is one of the most common and avoidable coverage pitfalls seniors face. For a broader look at enrollment errors that affect benefits, see common enrollment gaps and errors that can cost you coverage.

Special Enrollment Periods: When AEP Is Not Your Only Option

Missing October 15–December 7 does not necessarily mean waiting a full year. The Centers for Medicare & Medicaid Services (CMS) recognizes certain qualifying events that trigger a Special Enrollment Period (SEP), allowing plan changes outside AEP. Common qualifying events include:

  • Moving to a new address not covered by your current plan
  • Losing other creditable drug coverage involuntarily
  • Qualifying for or losing Medicaid eligibility
  • Your plan leaving the Medicare program or losing its contract

SEP windows and durations vary by circumstance, so contact Medicare directly at 1-800-MEDICARE or visit Medicare.gov to confirm whether your situation qualifies. It is worth noting that SEPs are narrower than AEP — they do not provide the same broad freedom to switch for any reason.

AEP Is Not Your Initial Enrollment Period

If you are turning 65 or becoming Medicare-eligible for the first time, a different window — your Initial Enrollment Period (IEP) — applies. The AEP is specifically for beneficiaries who are already enrolled in Medicare and want to change their plan. For guidance on first-time enrollment, see Medicare enrollment steps and timelines.

How to Use AEP Effectively

Making the most of AEP requires a few deliberate steps taken before December 7:

  1. Read your Annual Notice of Change. This document, mailed in September, outlines every material change to your plan for the coming year.
  2. Review your current medications. Use Medicare's Plan Finder tool (available at Medicare.gov) to compare how different Part D plans cover your specific drugs.
  3. Check your providers. If you are in a Medicare Advantage plan with a network, confirm your doctors and preferred facilities remain in-network for the next year.
  4. Seek free counseling. State Health Insurance Assistance Program (SHIP) counselors are trained, unbiased, and available at no cost to help you compare options.

For a structured approach to reviewing all your benefits — not just Medicare — each fall, consider working through an annual benefits review checklist.

This article is for general informational purposes only and does not constitute personalized financial, legal, or insurance advice. Medicare rules and plan details change annually. Consult Medicare.gov, a licensed insurance counselor, or your State Health Insurance Assistance Program (SHIP) for guidance specific to your situation.