Part D PDP Enrollment: Periods, LEP, and LIS Rules

Q: Who is eligible to enrol in a Medicare Part D PDP?

Eligible if entitled to Medicare Part A or enrolled in Part B and living in the plan's service area. Not eligible: PACE members, section 1876 cost-based plan members with drug coverage, Medicaid-only beneficiaries. Enrolling in a standalone PDP while in an MA-PD automatically disenrols from the MA-PD drug benefit.

Q: How is the Part D Late Enrollment Penalty calculated?

1% of the national base beneficiary premium per uncovered month after the IEP, rounded to the nearest 10 cents. The premium figure changes annually but the uncovered month count stays fixed. The penalty is permanent. LIS members are fully exempt. The plan reports uncovered months to CMS as part of the enrollment transaction.

Q: What is the difference between the IEP, AEP, and SEP?

IEP: one-time 7-month window at first Part D eligibility. AEP: October 15 to December 7 annually, coverage January 1. SEP: triggered by qualifying life events (loss of coverage, move, LIS gain, plan error). Each SEP type has its own eligibility criteria, documentation requirements, and effective date rules.

Q: Can a full LIS member change their Part D plan in October?

Yes, but only through the AEP (October 15 to December 7), not through the LIS SEP. The 2025 CMS final rule eliminated the Q4 LIS SEP. Full LIS and dual-eligible members making Q4 plan changes must use the AEP, with coverage effective January 1. Q1, Q2, and Q3 LIS SEPs remain (one change per quarter).

Q: What happens when CMS rejects an enrollment transaction?

A rejected enrollment means no valid Part D coverage exists with the plan. The TRC identifies the rejection reason (duplicate enrollment, eligibility failure, invalid effective date). The plan must resolve and resubmit in the next processing cycle. Unresolved rejections leave members without pharmacy coverage and create audit exposure.

Q: How does CMS auto-enrollment work for LIS members?

CMS auto-enrolls full LIS and dual-eligible members without Part D coverage into a benchmark PDP. Annual reassignment occurs if the current plan's premium exceeds the LIS benchmark. Transactions arrive via the TRC system and cannot be rejected by the plan. Members retain the right to opt out or choose a different plan.

Q: What changes did the CY2026 Chapter 4 LEP guidance introduce?

CMS issued CY2026 Chapter 4 guidance July 22, 2025, applying to enrollments effective January 1, 2026 or later. It provides additional clarifications and updates creditable coverage notice requirements. Model notice Exhibits were removed, plans may still use their content if all required data elements are included. CY2025 guidance applies to enrollments effective January 1 through December 31, 2025.

GUIDES: Medicare Part D PDP Enrollment

Medicare Part D PDP Enrollment: A Complete
Guide for Plan Operations Staff

Medicare Part D PDP enrollment is the operational process that determines when a beneficiary can join a standalone prescription drug plan, what that enrollment triggers for the plan, and what happens when the enrollment is processed incorrectly. Every plan operations and member services team that handles Part D enrollment must understand the full enrollment period landscape, the Late Enrollment Penalty rules that govern every new application, the LIS auto-enrollment process that CMS runs independently of the plan, and how enrollment transactions flow through the CMS systems. An enrollment processed in the wrong period, an incorrect LEP waiver, or a missed effective date creates compliance exposure and member harm that persists for the entire plan year. This guide covers every element of PDP enrollment that plan staff need to execute correctly.

63 days

Continuous Gap That Triggers the Lifetime Late Enrollment Penalty

A continuous period of 63 or more days without creditable prescription drug coverage after the end of the Initial Enrollment Period triggers the Part D Late Enrollment Penalty. The LEP is 1% of the national base beneficiary premium multiplied by the number of uncovered months. It is permanent and attaches to the beneficiary for as long as they have Part D coverage. LIS (Extra Help) members are exempt. Source: CMS: Part D Creditable Coverage and LEP

Oct 15 to Dec 7

Annual Enrollment Period: When Most Part D Plan Changes Happen

The Annual Enrollment Period runs from October 15 to December 7 every year. Coverage for AEP enrollments takes effect on January 1 of the following year. The AEP is when the majority of PDP enrollment changes occur, both new enrollments and plan-to-plan switches. Plans must accept all valid AEP applications without restriction. Source: 42 CFR 423.38(b)(1)

No Q4 LIS SEP

2025 CMS Rule: Fourth-Quarter LIS SEP Eliminated

The 2025 CMS final rule eliminated the fourth-quarter LIS/dual-eligible Special Enrollment Period. Full LIS and dual-eligible members who want to make a plan change in Q4 (October through December) must now do so through the Annual Enrollment Period, not the LIS SEP. Q1, Q2, and Q3 LIS SEPs remain available, one change per quarter. Source: GoAEP: Medicare Election Periods (2026)

Overview: The Medicare Part D PDP Enrollment Framework

Medicare Part D PDP enrollment is governed by 42 CFR Part 423 Subpart B. It determines when a Medicare beneficiary may enrol in, disenrol from, or switch a standalone prescription drug plan, and what the plan must do when an enrollment is received. Every enrollment falls into one of three period types: the Initial Enrollment Period (IEP), the Annual Enrollment Period (AEP), or a Special Enrollment Period (SEP). The period type determines the effective date, which determines when coverage begins and when the plan must begin processing the member’s claims.

Plan operations staff handle enrollment transactions that arrive from three sources: directly from the beneficiary (via phone, online, or paper application), through the CMS auto-enrollment system (for LIS members), and through the CMS systems that process PDP-to-PDP or MA-to-PDP switches. Each transaction type has different processing requirements, different effective date rules, and different error consequences. Getting the effective date wrong by one month means the plan either covers a period it should not or fails to cover a period it must, both create compliance findings and member harm.

Who handles PDP enrollment transactions

Plan enrollment operations staff who receive, validate, and process applications. Member services representatives who handle enrollment calls. Compliance teams who audit enrollment transactions against CMS eligibility files. Third-party administrators (TPAs) who process enrollment data on behalf of the plan. Every person in this chain must know which enrollment period applies, what the effective date must be, and when to reject versus escalate an application.

What plan staff must be able to do

Identify which enrollment period applies to an incoming application. Determine the correct effective date for each period type. Verify that the beneficiary meets Part D eligibility requirements. Apply the correct Late Enrollment Penalty calculation or LIS exemption. Process LIS auto-enrollment and reassignment transactions from CMS. Identify and reject invalid enrollment applications without creating a denied-enrollment finding. Document every enrollment decision for CMS audit purposes.

Prerequisites: What Plan Staff Must Know Before Processing Any Enrollment

Part D eligibility requirements

A beneficiary is eligible for Part D if they are entitled to Medicare Part A or enrolled in Medicare Part B, and they live in the plan’s service area. Beneficiaries enrolled in PACE or a section 1876 cost-based plan that offers prescription drug coverage are not eligible to enrol in a standalone PDP. Medicaid-only beneficiaries are not eligible for Part D. Every enrollment application must be validated against these eligibility conditions before processing. An enrollment processed for an ineligible beneficiary creates a compliance problem that CMS will identify during eligibility file reconciliation.

CMS eligibility data sources

CMS provides Part D plans with eligibility data through the Part D Eligibility and Enrollment Guidance and the Enrollment Database (EDB). Plans must reconcile their enrollment records against the CMS EDB monthly. Discrepancies between the plan’s enrollment records and the CMS EDB are a compliance risk, a beneficiary the plan believes is enrolled may not appear in the EDB, or vice versa. The EDB is the authoritative source for CMS purposes. Where the plan’s records and the EDB conflict, the plan must investigate and resolve the discrepancy.

Creditable coverage documentation

When a new enrollee applies for a PDP and there is a potential Late Enrollment Penalty, the plan must determine whether the beneficiary had creditable prescription drug coverage during the gap period. Creditable coverage means coverage that is at least as good as the standard Part D benefit. The beneficiary is responsible for documenting their creditable coverage. The plan applies the LEP based on the information provided and the CMS guidance. The CY2026 Chapter 4 guidance issued July 22, 2025 applies to all enrollments with an effective date on or after January 1, 2026. Source: CMS: Part D Creditable Coverage and LEP

LIS status in the CMS data file

LIS (Low-Income Subsidy) status fundamentally changes the enrollment rules that apply to a beneficiary. Full LIS and dual-eligible members receive CMS auto-enrollment and reassignment. They are exempt from the Late Enrollment Penalty. They have quarterly SEP rights (Q1, Q2, Q3 only, the Q4 LIS SEP was eliminated by the 2025 CMS final rule). Plan staff must check LIS status before determining which enrollment period applies and before calculating any LEP. Applying a LEP to a full LIS member is a compliance violation.

Step-by-Step: Processing a Medicare Part D PDP Enrollment

Confirm Part D eligibility before accepting the application

What to do: Verify the beneficiary’s Medicare Beneficiary Identifier (MBI) against the CMS eligibility file. Confirm Part A entitlement or Part B enrollment. Confirm the beneficiary lives in the plan’s service area. Confirm the beneficiary is not enrolled in PACE or a cost-based plan that provides drug coverage. Do not accept the application if any eligibility condition is not met.

Why it matters: Processing an enrollment for an ineligible beneficiary creates a phantom enrollee in the plan’s system. CMS will identify the discrepancy during EDB reconciliation and the plan will be required to retroactively disenrol the member, potentially after the plan has already processed and paid claims for them. Source: 42 CFR 423.30

Identify the applicable enrollment period and effective date

What to do: Determine which enrollment period applies using the table below. The period type determines the effective date. Never back-date or forward-date effective dates outside the rules for each period. If the applicant asserts an SEP but cannot document the qualifying event, do not process the enrollment as an SEP, process it as the next available period (typically AEP).

Why it matters: An incorrect effective date means the plan either covers a period before the beneficiary’s legitimate coverage start (an overpayment and a compliance risk) or fails to cover a period the beneficiary is entitled to (a coverage denial and a member harm). Both generate findings in the ODAG audit area. Source: 42 CFR 423.38

Check LIS status and apply exemptions before any LEP calculation

What to do: Before calculating any LEP, check the beneficiary’s LIS status in the CMS data file. Full LIS (Level 1/2) and dual-eligible members are entirely exempt from the Late Enrollment Penalty, do not calculate a LEP for these members under any circumstances. Partial LIS members (Level 3/4) may have different cost-sharing but are not automatically exempt from LEP. Verify which LIS level applies before making any LEP determination. Source: CMS Chapter 4: Creditable Coverage and Part D LEP (CY2025)

Why it matters: Applying a LEP to a full LIS member is a compliance violation. It overcharges the member and requires retroactive correction. LIS status can change mid-year, a member who had LEP before gaining LIS stops accruing further LEP from the date LIS begins. If LIS is later lost, LEP resumes from where it left off, not from the beginning.

Calculate the Late Enrollment Penalty if applicable

What to do: For non-LIS members with a gap in creditable coverage of 63 or more days since the end of their IEP, calculate the LEP: 1% of the national base beneficiary premium multiplied by the number of uncovered months (rounded to the nearest 10 cents). The national base beneficiary premium changes annually, use the current year’s figure. Count uncovered months using the guidance in CMS Chapter 4 (CY2025 guidance applies to enrollments effective January 1, 2025 through December 31, 2025; CY2026 guidance applies from January 1, 2026). Report the number of uncovered months to CMS as part of the enrollment transaction.

Why it matters: The LEP is permanent and the plan reports uncovered months to CMS. CMS uses this data to apply the LEP withholding from the member’s Social Security or Railroad Retirement benefit payment. An incorrect uncovered month count reported to CMS creates a permanent error in the beneficiary’s LEP record that requires CMS intervention to correct. Source: CMS Chapter 4: LEP Guidance (CY2025)

Process LIS auto-enrollment and CMS reassignment transactions

What to do: CMS auto-enrolls full LIS and dual-eligible members who have Part A and/or Part B but are not enrolled in a Part D plan, into a benchmark PDP in their area. CMS also reassigns these members annually if their current plan’s premium exceeds the benchmark. The plan receives these transactions via the CMS Transaction Reply Code (TRC) system. Plan staff must process auto-enrollment and reassignment transactions as received, they cannot reject a valid CMS-initiated transaction. Monitor the TRC system daily during the auto-enrollment processing period (typically November through January).

Why it matters: A plan that fails to process a CMS auto-enrollment transaction leaves a LIS member without coverage. That member will be unable to fill prescriptions at the LIS cost-sharing level from the coverage effective date. CMS tracks auto-enrollment transaction completion rates and flags plans that do not process them within the required timeframe. Source: 42 CFR 423.34

Submit the enrollment transaction to CMS via HPMS and confirm acceptance

What to do: Submit the completed enrollment transaction through the CMS Enrollment Database system within the required timeframe. For most direct enrollments the plan must submit the transaction within the CMS processing cycle for the applicable effective date. Monitor the transaction reply codes (TRCs) returned by CMS. A TRC of 001 means accepted. Rejection TRCs must be investigated and resolved, a rejected enrollment means the beneficiary has no CMS-recognised Part D coverage regardless of what the plan’s internal system shows. Do not consider an enrollment complete until CMS has confirmed acceptance.

Why it matters: An enrollment the plan accepted internally but that CMS rejected is not a valid enrollment. The plan is covering a member who is not in the CMS database as enrolled, this creates payment reconciliation problems, claim payment errors, and audit findings. Plans must reconcile their internal enrollment records against CMS TRCs after every submission cycle.

Send the enrollment confirmation notice to the member

What to do: Upon receiving CMS acceptance of the enrollment transaction, send the member a confirmation notice with their coverage effective date, their plan’s formulary, and their cost-sharing information. For LIS members, the confirmation must include their LIS cost-sharing tier. Plans must send this notice within 10 days of receiving the enrollment (or within 10 days of the enrollment effective date if that is earlier). The notice must include information about the Annual Notice of Change timeline and about their right to disenrol.

Why it matters: An enrolled member who does not receive a confirmation notice does not know when their coverage starts, what their cost-sharing will be, or what drugs are covered. They may arrive at the pharmacy before their effective date or after a plan change expecting different coverage. Member services complaints about coverage confusion are often traceable to missing or delayed enrollment confirmation notices. Source: 42 CFR 423.32

Part D PDP Enrollment Periods: Complete Reference

Period	When	Who May Enrol	Effective Date	LEP Risk
Initial Enrollment Period (IEP)	7-month window: 3 months before Part D eligibility, month of eligibility, 3 months after	First-time Part D eligible beneficiaries only	1st of the month of eligibility if enrolled in months 1-3; 1st of the month after enrollment if in months 4-7	None during IEP. Gap after IEP ends triggers LEP if 63+ days without creditable coverage
Annual Enrollment Period (AEP)	October 15 to December 7 each year	Any Part D eligible beneficiary; plan-to-plan switches and new enrollments	January 1 of the following year	LEP applied if applicable gap exists; plan reports uncovered months to CMS
LIS/Dual SEP (Quarterly)	Q1 (Jan-Mar), Q2 (Apr-Jun), Q3 (Jul-Sep) only, Q4 SEP eliminated by 2025 CMS rule	Full LIS (Extra Help) and full-benefit dual-eligible members; one change per quarter	1st of the month following enrollment	No LEP, LIS members are permanently exempt
Loss of Creditable Coverage SEP	Generally 63 days from the qualifying event (loss of creditable drug coverage)	Beneficiaries who involuntarily lose creditable prescription drug coverage	1st of the month following enrollment; retroactive to loss date in some cases	LEP avoided if enrolled within the SEP window without a 63-day gap
Move SEP	When a beneficiary moves out of their current plan’s service area	Beneficiaries whose permanent address is no longer in the plan’s service area	1st of the month following enrollment	LEP accrues during any gap between disenrollment and new enrollment
CMS Auto-Enrollment (LIS)	CMS-initiated; typically processed November through January for next plan year	Full LIS/dual-eligible members not enrolled in a Part D plan; CMS selects a benchmark PDP	January 1 (for annual reassignments) or 1st of the month following CMS processing	No LEP, CMS auto-enrollment explicitly exempts from penalty

Compliance Checklist: PDP Enrollment Operations

Application Processing

Part D eligibility confirmed against CMS eligibility file before accepting application

Enrollment period identified and documented before effective date is assigned

SEP qualifying event documented before processing as SEP rather than AEP

Enrollment confirmation notice sent within 10 days of CMS acceptance

LEP and LIS

LIS status checked from CMS data file before any LEP determination is made

No LEP applied to full LIS or dual-eligible members under any circumstances

Uncovered months reported to CMS match the LEP calculation used for the member

CY2026 Chapter 4 guidance applied for all enrollments effective January 1, 2026 or later

CMS Transaction Processing and Reconciliation

Enrollment submitted to CMS within the required processing cycle for the effective date

CMS TRC 001 (accepted) confirmed before enrollment is considered complete

Rejection TRCs investigated and resolved before the next processing cycle

Internal enrollment records reconciled against CMS EDB monthly

Troubleshooting: Common PDP Enrollment Failures

Problem: Member calls saying they cannot fill prescriptions despite being “enrolled”

Root cause: The plan processed the enrollment internally but the CMS transaction was rejected or not yet submitted. The plan’s internal record shows the member as enrolled, but the pharmacy cannot verify coverage because the member does not appear in the CMS Eligibility Database. Fix: Pull the CMS TRC for the member’s enrollment. If rejected, identify the rejection reason and resubmit with corrections. If not yet submitted, submit immediately. Issue the member an emergency override at the pharmacy while the CMS transaction is pending. Document the gap and the corrective action taken.

Problem: LEP applied to a member who had LIS at the time of enrollment

Root cause: LIS status was not checked before the LEP calculation was applied. The enrollment processor relied on the member’s self-report or did not access the CMS LIS data file before determining LEP applicability. Fix: Reverse the LEP immediately. Notify CMS of the corrected uncovered month count (which should be zero). Issue the member a written correction confirming the LEP has been removed and their correct premium. Audit all enrollments from the same processing period to identify other members who may have had LEP incorrectly applied.

Problem: CMS auto-enrollment transaction not processed before the effective date

Root cause: The TRC monitoring process did not flag the auto-enrollment transaction in time, or the operations team was not monitoring the CMS transaction system during the auto-enrollment period. Fix: Retroactively process the enrollment with the correct original effective date. Issue the member a retroactive coverage confirmation and ensure the plan’s pharmacy network can verify the member’s coverage back to the effective date. For any prescriptions the member paid out of pocket during the coverage gap, process reimbursement. Escalate to management and adjust the monitoring protocol to prevent recurrence.

Problem: Member enrolled using the Q4 LIS SEP after it was eliminated

Root cause: The enrollment processor or member services representative was not aware that the 2025 CMS final rule eliminated the Q4 (October, November, December) LIS SEP. A full LIS or dual-eligible member requested a plan change in Q4 and the plan accepted it under the LIS SEP. CMS will reject this transaction. Fix: Notify the member immediately that their Q4 LIS SEP enrollment cannot be processed. Inform them that their plan change will need to be made through the AEP (October 15 to December 7), with coverage effective January 1. If they made an AEP election already, that election stands. Retrain all member services and enrollment staff on the Q4 LIS SEP elimination.

Key Takeaways

An enrollment is not complete until CMS returns TRC 001, internal processing is not enough

A plan that has accepted an enrollment internally but not yet received CMS TRC 001 acceptance has a member who cannot verify coverage at the pharmacy. The CMS Eligibility Database is the authoritative source, not the plan’s internal system. Every enrollment workflow must include a mandatory CMS TRC confirmation step before the enrollment is marked complete and before the confirmation notice is sent to the member. Enrollments with rejection TRCs must be resolved before the next processing cycle.

LIS status must be checked from the CMS data file before every LEP determination, not from the member’s self-report

A member’s self-reported LIS status is not reliable for LEP purposes. LIS status can change mid-year, can be pending verification, or can be at a level that does not exempt from LEP. The only authoritative source is the CMS LIS data file. Checking that file before any LEP calculation is the only way to ensure the plan does not apply a LEP to an exempt member or fail to apply one when required. A wrongly applied LEP generates a correction obligation, a member complaint, and a potential audit finding.

The Q4 LIS SEP no longer exists, all Q4 LIS plan changes must go through the AEP

The 2025 CMS final rule eliminated the fourth-quarter LIS Special Enrollment Period. Full LIS and dual-eligible members who want to change their Part D plan in October, November, or December must do so through the Annual Enrollment Period (October 15 to December 7) with coverage effective January 1. Q1, Q2, and Q3 LIS SEPs remain available with one change per quarter. Any plan that processes a Q4 LIS SEP will have the transaction rejected by CMS. Every member services and enrollment team member must know this rule. Member calls requesting a Q4 plan change under the LIS SEP must be redirected to the AEP immediately and not processed as an SEP.

Frequently Asked Questions

Who is eligible to enrol in a Medicare Part D PDP?

A beneficiary is eligible for a standalone Part D PDP if they are entitled to Medicare Part A or enrolled in Medicare Part B, and they live in the plan’s service area. Beneficiaries enrolled in PACE or a section 1876 cost-based plan that offers drug coverage are not eligible. Medicaid-only beneficiaries are not eligible. Beneficiaries who are already enrolled in a Medicare Advantage Prescription Drug plan (MA-PD) cannot simultaneously enrol in a standalone PDP, enrolling in a PDP while enrolled in an MA-PD automatically disenrols them from the MA-PD’s drug benefit. Source: 42 CFR 423.30

How is the Part D Late Enrollment Penalty calculated?

The LEP is 1% of the national base beneficiary premium for each month the beneficiary went without creditable prescription drug coverage after the end of their IEP, rounded to the nearest 10 cents. The national base beneficiary premium changes annually, plans must use the current year’s figure. The penalty is permanent and adjusts each year as the premium changes, but the number of uncovered months stays fixed. LIS members are completely exempt. The LEP is withheld from the beneficiary’s Social Security or Railroad Retirement payment. The plan reports the number of uncovered months to CMS as part of the enrollment transaction. Source: CMS Chapter 4: LEP Guidance (CY2025)

What is the difference between the IEP, AEP, and SEP?

The Initial Enrollment Period (IEP) is a one-time 7-month window when a beneficiary first becomes eligible for Part D. Missing the IEP without creditable coverage starts the LEP accumulation. The Annual Enrollment Period (AEP) runs October 15 to December 7 every year and is the main window for plan changes and new enrollments, with coverage effective January 1. Special Enrollment Periods (SEPs) are triggered by qualifying life events, loss of creditable coverage, a move out of the service area, gaining LIS status, or a plan error, and allow enrollment outside the AEP. Each SEP type has its own eligibility criteria, documentation requirements, and effective date rules. Source: 42 CFR 423.38

Can a full LIS member change their Part D plan in October?

Yes, but only through the Annual Enrollment Period, not through the LIS SEP. The 2025 CMS final rule eliminated the fourth-quarter LIS SEP, full LIS and dual-eligible members no longer have a standalone SEP to make plan changes in October, November, or December. They must use the AEP (October 15 to December 7) for Q4 plan changes, with coverage effective January 1. Q1 (January through March), Q2 (April through June), and Q3 (July through September) LIS SEPs remain available with one change allowed per quarter. Source: GoAEP: Medicare Election Periods (2026)

What happens when CMS rejects an enrollment transaction?

A CMS-rejected enrollment means the beneficiary has no valid Part D coverage with the plan, regardless of what the plan’s internal system shows. The TRC returned by CMS identifies the rejection reason. Common rejection reasons include: duplicate enrollment (the beneficiary is already enrolled in another plan for that period), eligibility failure (the beneficiary does not meet Part D eligibility requirements), and invalid effective date (the date does not match the rules for the applicable enrollment period). The plan must resolve the rejection and resubmit within the next processing cycle. A rejected enrollment that is not resolved promptly leaves a member without pharmacy coverage and generates an audit exposure.

How does CMS auto-enrollment work for LIS members?

CMS auto-enrolls full LIS and dual-eligible members who have Part A and/or Part B but are not enrolled in a Part D plan into a benchmark PDP in their area. CMS also reassigns these members annually if their current plan’s premium exceeds the LIS benchmark premium. CMS-initiated auto-enrollment and reassignment transactions are sent to the plan via the TRC system. The plan cannot reject a valid CMS auto-enrollment transaction. Members who are auto-enrolled or reassigned retain the right to opt out or choose a different plan. CMS processes auto-enrollment transactions primarily in November and December for coverage effective January 1, and on a monthly basis for newly LIS-eligible members. Source: 42 CFR 423.34

What changes did the CY2026 Chapter 4 LEP guidance introduce?

CMS issued the CY2026 Chapter 4 guidance on July 22, 2025, with changes applying to all enrollments with an effective date on or after January 1, 2026. The update provides additional information, clarifies policy, and updates a requirement for creditable coverage notices. Notably, model notices (Exhibits) and references to those Exhibits were removed from Chapter 4, plans may continue to use the content and language from existing Exhibits if all required data elements are included in the plan’s notifications. Plans must apply the CY2026 guidance for any enrollment effective January 1, 2026 or later, while the CY2025 guidance applies to all enrollments effective January 1, 2025 through December 31, 2025. Source: CMS: Part D Creditable Coverage and LEP

Sources

Government and Regulatory Sources

42 CFR 423.30, Eligibility to Enrol in a Part D Plan: source for Part D eligibility requirements including Part A/Part B entitlement and the PACE/cost-plan exclusions.
42 CFR 423.34, Coordination of Enrollment with States: source for CMS auto-enrollment and reassignment of LIS and dual-eligible beneficiaries into benchmark PDPs.
42 CFR 423.38, Enrollment Periods: source for the AEP dates (October 15 to December 7), IEP structure, and SEP categories.
42 CFR 423.32, Requirements for Enrollment: source for the enrollment confirmation notice requirement.
CMS: Part D Creditable Coverage and Late Enrollment Penalty: the primary CMS resource page for LEP guidance; source for the CY2026 Chapter 4 guidance issued July 22, 2025 and the CY2025 guidance issued August 5, 2024.
CMS: CY2025 Creditable Coverage and Part D LEP Guidance (Chapter 4): source for the LEP formula (1% of national base beneficiary premium per uncovered month), LIS exemption from LEP, counting methodology for uncovered months, and effective dates for the CY2025 guidance.

Industry Sources

GoAEP: Medicare Election Periods Explained (2026): source for the Q4 LIS/dual SEP elimination by the 2025 CMS final rule, the quarterly SEP structure (Q1/Q2/Q3 only), and the IEP effective date rules for Part D.
Get Medicare Solutions: Medicare Part D Key Dates and Penalties (2026): source for the AEP dates, the 2026 one-time special open enrollment period (January 1 to March 31, 2026), and the SEP qualifying events including relocation, loss of creditable coverage, and LIS eligibility.