The single most common Part D mistake is choosing the plan with the lowest monthly premium. Premium is one of four inputs into your annual cost; the others are the deductible, per-drug tier cost-sharing, and whether your pharmacy is in the plan's preferred network. A $0-premium plan that puts your maintenance drug on tier 4 can cost thousands more than a $40-premium plan that puts it on tier 2.
Build the drug list first
Open your pill bottles rather than working from memory. For each prescription record the exact drug name (brand or generic as dispensed), strength, quantity per fill, and how often you refill. A dosage entered at 10 mg instead of 20 mg produces a cost projection that is simply wrong.
If you have a Medicare account, sign in at medicare.gov/plan-compare and your drug list carries over from last year — then edit it for anything that has changed.
Add every pharmacy you actually use
Select up to five, including mail order. Plan Finder prices each plan at each pharmacy, and the spread between preferred and standard pricing inside the same plan is often larger than the spread between two different plans.
Sort by total cost, then read the flags
Sort by Lowest drug + premium cost and open the top three results. On each plan page, check every drug for:
- Tier placement — tier 1 and 2 are generics, tier 3 preferred brand, tiers 4 and 5 non-preferred and specialty.
- PA — prior authorization required before coverage.
- ST — step therapy: you must fail a cheaper drug first.
- QL — quantity limits per fill.
- Not covered — the drug is off the formulary entirely; you would need an exception request.
A plan that is $60 cheaper per year but puts your daily medication behind prior authorization is not cheaper. It is a plan that will cost you phone calls in February.
Check the 2025 rules that changed the math
- Out-of-pocket costs on covered drugs are capped at $2,000 a year.
- The donut hole coverage gap is gone.
- Every plan must offer the Medicare Prescription Payment Plan, spreading that $2,000 across monthly installments. You elect it through the plan, not through Plan Finder.
Read the full mechanics in our Part D primer.
If you are also considering an Advantage plan
Most Medicare Advantage plans include drug coverage, so you would not buy a standalone Part D plan alongside one. Compare both paths before enrolling — see how to find a Medicare Advantage plan and the comparison checklist.
Frequently asked questions
- How do I compare Part D plans?
- Enter every prescription with dosage and refill frequency into the Medicare Plan Finder, add the pharmacies you use, then sort by lowest drug plus premium cost. That figure is your projected annual out-of-pocket total, which is the only number worth comparing across plans.
- Which Part D plan is cheapest for my drugs?
- There is no single cheapest plan — it depends entirely on your drug list and pharmacy. Two people in the same ZIP code frequently have different optimal plans. The lowest-premium plan is rarely the lowest total-cost plan for anyone taking brand-name drugs.
- What do PA, ST and QL mean on a Part D formulary?
- PA is prior authorization, meaning the plan must approve the drug before it is covered. ST is step therapy, requiring you to try a cheaper drug first. QL is a quantity limit on how much can be dispensed at once. Any drug you take daily should ideally carry none of these flags.
- Does my pharmacy change what a Part D plan costs?
- Yes, substantially. Most plans have preferred and standard pharmacy networks with different cost-sharing for the identical drug, and mail order is often cheaper for 90-day maintenance supplies. Plan Finder prices each pharmacy you select separately.
- What is the Part D out-of-pocket cap?
- Beginning in 2025, Part D out-of-pocket spending on covered drugs is capped at $2,000 per year, and the coverage gap known as the donut hole no longer exists. Every plan must also offer the Medicare Prescription Payment Plan, which spreads that cost across monthly installments.
- When can I switch Part D plans?
- During the Annual Election Period, October 15 to December 7, with coverage starting January 1. Outside that window you need a Special Enrollment Period, such as moving out of the plan's service area or losing other creditable coverage.
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