How Medicare Part D Works
Medicare Part D is the prescription drug benefit available to all Medicare enrollees. It is offered exclusively through private insurance companies approved by CMS — there is no government-run Part D plan. You can get Part D coverage in two ways: a standalone Prescription Drug Plan (PDP) paired with Original Medicare, or a Medicare Advantage Prescription Drug plan (MAPD) that bundles medical and drug coverage together.
Every Part D plan must meet minimum coverage standards set by CMS, but plans vary widely in premiums, formularies, pharmacy networks, and cost-sharing. Choosing the right plan based on the specific drugs you take can save hundreds or even thousands of dollars per year.
2026 Part D Cost Structure
| Coverage Phase | Your Responsibility |
|---|---|
| Deductible (up to $590) | 100% of drug costs until deductible met |
| Initial coverage | Copays or coinsurance per tier (typically 25%) |
| Coverage gap (donut hole) | Effectively eliminated — manufacturer discounts + plan coverage keep your share at 25% |
| Catastrophic coverage ($2,000 OOP cap) | $0 — Medicare covers 100% above the cap |
The $2,000 annual out-of-pocket cap, introduced by the Inflation Reduction Act, continues into 2026. Once your true out-of-pocket spending hits that threshold, you pay nothing for the rest of the year. Many plans also offer a monthly payment option (the Medicare Prescription Payment Plan) so you can spread costs evenly across the year.
Understanding Drug Tiers
Part D plans organize covered drugs into tiers, each with different cost-sharing:
- Tier 1 — Preferred generics (lowest copay, often $0–$10)
- Tier 2 — Non-preferred generics ($10–$25)
- Tier 3 — Preferred brand-name drugs ($30–$50)
- Tier 4 — Non-preferred brand-name drugs (25–50% coinsurance)
- Tier 5 — Specialty drugs (25–33% coinsurance, often $100+)
Always check whether your medications are on a plan's formulary and at what tier before enrolling. A drug on Tier 2 in one plan may be Tier 4 in another — costing you hundreds more annually.
Formulary Restrictions to Watch
Prior authorization
The plan requires your doctor to get approval before the drug is covered. This is common for expensive biologics and specialty medications used for conditions like cancer treatment or autoimmune disorders.
Step therapy
You must try a lower-cost drug first and show it did not work before the plan covers the preferred medication. This is frequent with blood pressure medications, antidepressants, and pain management drugs.
Quantity limits
The plan caps how many pills or doses you can get per fill. Common with opioids but also applied to medications for diabetes and heart conditions.
How to Choose the Right Part D Plan
The single best tool is the Medicare Plan Finder at Medicare.gov. Enter your zip code and the exact drugs you take (including dosages), and the tool ranks plans by estimated total annual cost. Focus on:
- Total estimated annual cost — not just the monthly premium
- Your pharmacy — preferred pharmacies have lower copays
- Formulary coverage — all your drugs should be listed without burdensome restrictions
- Star rating — 4+ star plans tend to offer better service and fewer claim issues
Part D Late Enrollment Penalty
If you go 63 or more consecutive days without creditable drug coverage after your Initial Enrollment Period, you will pay a permanent penalty when you finally enroll. The penalty equals 1% of the national base beneficiary premium (approximately $36.78 in 2026) multiplied by the number of uncovered months. After 2 years without coverage, that is roughly $8.83/month added to your premium for life.
Creditable coverage includes employer drug plans, VA benefits, TRICARE, and Federal Employee Health Benefits (FEHB). If you have creditable coverage, keep the annual notice your insurer sends — you may need it as proof.
Insulin and Vaccine Cost Caps
Under the Inflation Reduction Act, insulin copays are capped at $35 per month across all Part D plans. Additionally, all adult vaccines recommended by the Advisory Committee on Immunization Practices (ACIP) — including shingles, Tdap, and hepatitis B — are covered at $0 cost-sharing under Part D. These provisions represent major savings, especially for diabetic beneficiaries who previously paid hundreds for insulin. Check your state-specific costs for average prescription pricing.
Switching Plans Annually
Part D plans change formularies, premiums, and pharmacies every year. Even if your plan worked well this year, review it during the Annual Enrollment Period (October 15 through December 7). CMS data shows that beneficiaries who compare plans annually save an average of $500 or more compared to those who auto-renew without checking.