How to Choose a Part D Plan: A Formulary-First Method

Part D shopping fails when people optimize the wrong variable. The plan name, carrier brand and even premium matter far less than the formulary — the drug list with tiers, restrictions and pharmacy pricing that determines your real annual cost. This file lays out the method in the order that actually works.

Step	Action	Why it matters
1	Build your exact drug list with doses	Tier placement and quantity limits drive everything
2	Enter it in Medicare.gov Plan Finder	It prices every plan in your county, total-cost view
3	Filter by preferred pharmacy	The same plan can cost hundreds more at a non-preferred chain
4	Check prior auth / step therapy flags	Restrictions can delay or block fills
5	Compare total annual cost, not premium	A $10-premium plan with bad tiers loses to $40 all day

The pharmacy variable people forget

Plans negotiate different copays per pharmacy tier — preferred retail, standard retail, preferred mail order. A maintenance generic might be $2 at preferred mail and $18 at a standard-chain counter. If you fill monthly at a specific chain, filter Plan Finder results by that pharmacy before comparing; the ranking reshuffles more often than not.

The annual re-shop discipline

Formularies, tiers and pharmacy networks all churn each January. The Plan Finder’s October–December window shows next-year data — re-run your drug list every fall against the new plans and switch during open enrollment if your total cost jumps. Auto-renewal is the default and costs the average re-shopper $200–400 a year in silent formulary drift.

FAQ

Can I change Part D plans mid-year?

Only in special cases (moving, plan termination, Medicaid, 5-star SEP). Otherwise changes happen January 1 from the Oct 15 – Dec 7 window.

Does Medicare Advantage drug coverage work the same way?

Yes — bundled MA-PD plans use the same formulary logic; run the identical Plan Finder method with your MA plans included.

What if my new plan drops my drug in January?

You may qualify for a formulary exception via your prescriber, or use the plan-transition fill rules that cover you temporarily at the start of the year.

Related reading: Part D 2026 overview · Phases explained · MA networks · Medicare FAQs