The short answer
How does Medicare prescription drug coverage work?
Medicare Part D helps cover outpatient prescription drugs through private plans. Each plan has a formulary, cost tiers, participating pharmacies, and possible requirements such as prior authorization or step therapy. A useful comparison checks every medication and pharmacy rather than relying only on the monthly premium.
Use this guide to understand the basics, then verify current coverage rules and costs before you enroll.
Start with your current prescriptions
A plan's formulary lists the drugs it covers. Covered drugs are often grouped into tiers that affect what you pay. Drug names, dosages, quantities, and preferred pharmacies can all change the comparison.
- Check every prescription, including the exact dosage and form.
- Review preferred and standard network pharmacies.
- Look for prior authorization, step therapy, or quantity limits.
- Consider the full year's estimated drug costs, not only the premium.
Coverage can change
Formularies and pharmacy arrangements can change. Plans must follow notice and transition rules, but you should review your coverage during each Annual Enrollment Period and whenever your medications change.
Avoid an unplanned coverage gap
Going without creditable drug coverage after you become eligible may result in a late-enrollment penalty. Ask an employer plan whether its drug coverage is creditable before delaying Part D.
Illustrative example
Why the formulary tier matters
Two plans may both list the same prescription, but one may place it on a preferred tier while another uses a higher-cost tier or requires prior authorization. The lower-premium plan is not automatically the lower-cost option. Verify the exact drug, dosage, quantity, pharmacy, and plan rules.
Ready for your next step?
Put what you learned into practice.
Start with your ZIP code to review plans available in your area, or ask for help from a licensed insurance agent.
Related questions
Frequently asked questions
What is a Medicare drug formulary?
A formulary is a plan's list of covered prescription drugs. Drugs are commonly grouped into tiers that affect cost sharing, and coverage requirements can apply.
Can a Part D formulary change?
Yes. Formularies and tiers can change, subject to Medicare rules and required notices. Review current plan documents and notices, especially before a new plan year.
Can I compare Medicare prescription drug plans?
Yes. Compare whether each medication is covered, its tier and restrictions, participating pharmacies, deductible, premium, and estimated costs across the year.
What is prior authorization?
Prior authorization means the plan may require prescriber information or approval before it covers a particular medication under the plan's terms.
Can delaying Part D cause a penalty?
A late-enrollment penalty may apply after a qualifying period without Part D or other creditable drug coverage. Confirm your coverage status and timing with Medicare.
Primary sources
- Medicare drug coverage (Part D) — Medicare.gov
- What Medicare costs — Medicare.gov