How to Compare Medicare Part D Drug Plans: Formularies, Tiers, and Pharmacy Costs

Medicare drug coverage is highly personal. Two people in the same county can look at the same plan and get very different results because their prescriptions, dosages, refill patterns, and preferred pharmacies are different.

Before comparing Part D plans or Medicare Advantage plans with drug coverage, write down each medication name, dosage, how often you take it, and whether you use a local pharmacy, mail order, or both. If you take insulin, inhalers, injectables, or brand-name medications, the details can matter even more.

A plan’s list of covered drugs is called a formulary. Medicare.gov explains that each Medicare drug plan has its own formulary, and all plans must cover a wide range of drugs that people with Medicare take. Plans also must include most drugs in certain protected classes, such as drugs used to treat cancer, HIV/AIDS, depression, psychosis, seizures, and organ-transplant immunosuppression.

That does not mean every plan covers every medication the same way. A drug may be covered by one plan, covered with restrictions by another, or not listed on a third plan’s formulary. If a drug is not on the formulary, you may pay the full price unless you qualify for an exception or your prescriber identifies an appropriate covered alternative.

Many Medicare drug plans place covered drugs into tiers. Medicare.gov says lower tiers generally cost less than higher tiers, but each plan can structure its tiers differently. One plan may put a generic drug on a very low-cost tier while another plan treats a similar prescription differently.

Common tier examples include preferred generics, other generics, preferred brand-name drugs, non-preferred brand-name drugs, and specialty-tier drugs. If one of your medications lands on a higher tier, ask whether there is a lower-tier alternative your prescriber considers appropriate. If not, you or your prescriber may be able to request a tiering exception when the plan rules allow it.

The pharmacy you use can affect what you pay. Some plans have preferred pharmacies, standard pharmacies, mail-order options, or different cost sharing depending on where you fill the prescription. A plan that looks inexpensive at one pharmacy may not be the lowest-cost option at another.

When comparing plans, run the numbers with the pharmacy you actually use. If you are willing to switch pharmacies or use mail order, compare those options too — but do not assume the advertised premium tells the whole story.

Medicare drug plans may use coverage rules for certain medications. Medicare.gov lists common rules such as prior authorization, step therapy, quantity limits, medication safety checks, drug management programs, and Medication Therapy Management programs for complex health needs.

Prior authorization means the plan requires approval before it covers the drug. Step therapy may require you to try a lower-cost drug first before the plan covers a more expensive option. Quantity limits restrict how much of a drug the plan covers over a certain period. These rules are not automatically bad, but they can create friction if you need a specific medication on a specific timeline.

For 2026, Medicare.gov says no Medicare drug plan may have a deductible higher than $615. After any deductible, you generally pay part of the cost during the initial coverage stage until out-of-pocket spending on covered Part D drugs reaches $2,100 in 2026. After that, catastrophic coverage begins and you do not have to pay out of pocket for covered Part D drugs for the rest of the calendar year.

Those numbers can help you compare financial risk, but they do not replace plan-specific math. Premiums, deductibles, drug tiers, pharmacy pricing, and coverage rules all affect your actual cost. People who qualify for Extra Help, Medicaid, a Medicare Savings Program, a State Pharmaceutical Assistance Program, or other assistance may have different cost responsibilities.

There are two common ways to get Medicare drug coverage: a standalone Part D plan that works with Original Medicare, or drug coverage included in a Medicare Advantage plan. Medicare.gov notes that most people in a Medicare Advantage HMO or PPO who join a separate Part D plan will be disenrolled from their Medicare Advantage plan and returned to Original Medicare.

That is why you should not add a standalone drug plan casually if you already have Medicare Advantage. Confirm how your current coverage works before making a change, especially during Open Enrollment or when comparing plans for the next year.

For each plan, check whether all of your prescriptions are on the formulary, what tier each drug is on, whether prior authorization or step therapy applies, and what your estimated annual drug cost looks like at your preferred pharmacy. Then compare premium, deductible, pharmacy options, mail-order pricing, and whether your medications could be affected by plan rules.

If a medication is expensive, newly prescribed, or medically hard to substitute, ask your prescriber or a licensed Medicare agent to help verify coverage details before you enroll. Medicare Choose can help organize plan comparisons, but final enrollment choices should be verified with official plan materials, Medicare.gov, or licensed help.

Medicare.gov: “Choose how you get drug coverage,” “What do drug plans cover?”, “How do drug plans work?”, “Drug plan rules,” and “How much does Medicare drug coverage cost?” These official pages explain Part D coverage options, formularies, protected drug classes, tiers, exceptions, prior authorization, step therapy, quantity limits, pharmacy/cost considerations, and 2026 Part D cost stages.