Part D Prescription Drug Plans
Part D helps cover the cost of prescription drugs. Understanding how formularies, tiers, and preferred pharmacies work can significantly affect what you pay.
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What Is Medicare Part D?
Medicare Part D provides coverage for outpatient prescription drugs. Part D coverage is offered through private insurance plans approved by Medicare - either as a standalone Prescription Drug Plan (PDP) used alongside Original Medicare or Medigap, or as drug coverage bundled into a Medicare Advantage plan (MAPD).
Each Part D plan has its own list of covered drugs (formulary), its own cost structure (tiers), and its own network of pharmacies. This means what you pay - and whether your drugs are covered - depends entirely on the specific plan you choose.
Part D plans can change their formularies, pharmacy networks, and cost-sharing annually. Reviewing your plan each year during the Annual Enrollment Period (AEP) helps ensure your coverage still matches your prescriptions.
Formularies and Drug Tiers
A formulary is the plan’s list of covered drugs. Drugs on the formulary are divided into tiers - and the tier your drug falls in determines your cost.
Preferred Generics
Lowest cost-sharing. Generic drugs preferred by the plan. Most people pay a small copay for Tier 1 drugs.
Non-Preferred Generics / Preferred Brands
Moderate cost-sharing. May include non-preferred generics or selected brand name drugs.
Non-Preferred Brands
Higher cost-sharing than Tier 2. Brand-name drugs that are not on the plan’s preferred list.
Specialty Drugs
Highest cost-sharing. High-cost or specialized medications - often requiring prior authorization. Cost-sharing may be a percentage of the drug cost rather than a flat copay.
The same drug may be placed on different tiers in different Part D plans. Before enrolling, verify your specific medications on each plan’s formulary - not just whether the drug is covered, but at which tier and through which pharmacies.
Preferred Pharmacies
Part D plans typically have pharmacy networks - and within those networks, preferred pharmacies where cost-sharing is lower. Using a non-preferred pharmacy may mean paying more, even for the same drug.
Preferred vs Standard Pharmacies
Some plans offer lower copays at specific preferred pharmacies (often major retail chains or mail-order pharmacies). Using a standard in-network pharmacy may still be covered, but your cost-sharing could be higher.
Mail-Order Pharmacies
Many Part D plans offer lower cost-sharing when using mail-order pharmacy services for 90-day supplies of maintenance medications. Mail-order availability and pricing vary by plan.
Out-of-Network Pharmacies
Most Part D plans do not cover drugs filled at out-of-network pharmacies, except in specific emergency or out-of-area situations. Confirm pharmacy network details before relying on a specific pharmacy.
The Part D Late Enrollment Penalty
If you do not enroll in a Part D plan when you are first eligible - and do not have other creditable prescription drug coverage - you may pay a late enrollment penalty for as long as you have Medicare drug coverage.
How the Penalty Works
For every month you were eligible but not enrolled in a qualifying Part D plan or other creditable drug coverage, a penalty accumulates. This penalty is added to your monthly Part D premium and is permanent - it stays as long as you have Part D coverage.
What Counts as Creditable Coverage
Coverage from an employer, union, TRICARE, VA, or other source may qualify as creditable prescription coverage - meaning it is at least as good as Medicare Part D. If you have this coverage, you may be able to delay Part D without penalty. You should receive a letter annually confirming whether your coverage is creditable.
Even if you take few medications today, enrolling in a Part D plan during your Initial Enrollment Period protects you from future penalties and provides coverage if your medication needs change.
The Coverage Gap Overview
Part D plans have a coverage structure with multiple phases. Understanding this structure helps you anticipate your costs, especially if you take expensive medications.
Deductible Phase
At the start of the year, you may pay a deductible before your plan begins sharing drug costs. Not all plans have a deductible, and amounts vary. Some plans waive deductibles for certain tiers.
Initial Coverage Phase
After your deductible is met, you and your plan share costs based on tiers (copays or coinsurance) until your total drug spending reaches a threshold set by Medicare each year.
Coverage Gap (historically called the 'Donut Hole')
After the initial coverage threshold is reached, the cost-sharing structure changes. Legislation has reduced the gap significantly in recent years, and for 2025 and beyond, beneficiaries generally pay no more than 25% for covered drugs during this phase. Rules and thresholds can change annually.
Catastrophic Coverage Phase
After reaching the out-of-pocket threshold, the plan pays a higher portion of drug costs. This threshold and the cost-sharing structure in this phase are set by Medicare annually.
Get Help Comparing Part D Plans
A licensed agent can review your specific prescriptions and compare available Part D plans in your area - looking at formulary coverage, tier placement, and total annual cost estimates.
Benefits vary by plan, county, and eligibility.
Compare Part D Plans for Your Medications
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