How Medicare Part D Formularies Work: Drug Tiers, Prior Authorization & Step Therapy Explained
Quick Answer A Medicare Part D formulary is the list of drugs your plan covers
Drugs are organized into tiers—Tier 1 costs the least, and Tier 5 costs the most. Your plan may also require prior authorization (doctor approval) or step therapy (trying a cheaper drug first) before covering certain medications. Knowing your formulary before you enroll can save you hundreds—or thousands—of dollars per year.
Many individuals select their Medicare Part D plan based solely on the monthly premium, which can lead to significant financial pitfalls. The plan with the lowest premium might place your essential medications on a high-cost tier or not cover them at all. Understanding how formularies function is crucial for selecting a Part D plan that genuinely saves you money.
What Is a Medicare Part D Formulary?
A formulary is your Part D plan’s official drug list—the specific prescription medications the plan has agreed to cover and their associated costs. Each Medicare Part D plan has its own formulary, and these formularies can vary significantly from one plan to another.
The Centers for Medicare & Medicaid Services (CMS) mandates that all Part D formularies include drugs from each “therapeutic category” (broad medical purpose). However, plans have considerable discretion in selecting which specific drugs to include and at which tier. This variability explains why the same medication can have vastly different copays or may be covered by one plan but not another.
📌 Key rule: Plans must cover at least two chemically distinct drugs in each therapeutic category, but they are not required to cover every drug. If your specific brand-name drug is absent from a plan’s formulary, you will be responsible for the full cost or must request a formulary exception.
Formularies are updated at least annually, with plans submitting changes to CMS each fall for the upcoming year. Plans can also make mid-year changes under specific limitations, which we will discuss later in this article.
The Five Drug Tiers Explained
Most Medicare Part D plans utilize a 5-tier structure, although some may employ three or four tiers. The tier classification of your drug determines your copay or coinsurance amount.
1
Preferred generics
$0–$5 copay
Metformin, lisinopril, atorvastatin
2
Non-preferred generics
$10–$20 copay
Generic drugs not on preferred list
3
Preferred brands
$40–$100 copay
Eliquis, Ozempic, Entresto
4
Non-preferred brands
$80–$150+ copay
Brand drugs not on preferred list
5
Specialty drugs
25–33% coinsurance
Cancer drugs, biologics, MS treatments
The copays listed above are typical; however, your plan’s actual amounts may vary. Always consult your plan’s Evidence of Coverage (EOC) document or the drug pricing tool on Medicare.gov before enrolling.
A note on Tier 5 specialty drugs
Specialty drugs, including biologics and chemotherapy agents, are typically categorized as Tier 5. Plans can charge up to 33% coinsurance on these medications, meaning that for a drug costing $10,000 monthly, your share could be between $2,500 and $3,300. This is why the upcoming 2026 $2,000 out-of-pocket cap is crucial for patients relying on specialty medications.
Coverage Rules: Prior Authorization, Step Therapy, and Quantity Limits
Being listed on the formulary does not guarantee automatic coverage. Plans implement three primary coverage rules that can restrict when and how a drug is covered.
Prior Authorization (PA)
Prior authorization requires your doctor to obtain approval from the plan before prescribing a specific medication. This process ensures that the drug is medically necessary, prescribed for an approved condition, and that more affordable alternatives have been considered.
PA is commonly required for brand-name drugs, specialty medications, and drugs with high potential for abuse. The approval process typically takes 1–3 business days (or 72 hours for expedited reviews). If your doctor submits the necessary documentation, approval rates are generally favorable.
⚠️ Picking up a PA-required drug without approval means you pay full retail price at the pharmacy. Ensure your doctor’s office submits the PA request before your first fill to avoid unexpected costs.
Step Therapy
Step therapy, also known as “fail first,” mandates that you try one or more lower-cost medications before your plan will cover the drug your doctor initially prescribed. The rationale is that if a generic or lower-tier drug can effectively treat your condition, the plan prefers that option first.
For example, if your doctor prescribes Eliquis (apixaban, a brand-name blood thinner), your plan may require you to first try warfarin (generic, Tier 1) for 30–60 days. If warfarin causes complications or fails to control your INR, your doctor can document this and request coverage for Eliquis.
Step therapy restrictions cannot be enforced if you are already stable on a medication when you join a plan. If you were taking Eliquis prior to enrolling in Medicare, the plan cannot require you to switch to warfarin.
Quantity Limits (QL)
Quantity limits restrict how much of a drug your plan will cover per fill or per month. For instance, there may be a limit of 60 tablets per 30-day supply or one injection pen per 28 days. These limits are based on FDA-approved dosing guidelines. If your doctor prescribes more than the quantity limit, you will need a quantity limit exception.
How to Check If Your Drug Is Covered
There are three reliable methods to check your drug’s formulary status:
1
Medicare.gov Plan Finder The official Medicare plan comparison tool at medicare.gov/plan-compare allows you to enter your drugs and see which plans cover them, at what tier, and at what estimated annual cost. This is the most reliable tool, utilizing real-time formulary data.
2
Your plan’s website or member portal If you’re already enrolled, log in to your plan’s member portal and use their formulary search tool. Enter your drug name and dosage to see your tier, any restrictions, and your estimated copay.
3
Call the plan directly Call the member services number on the back of your plan card. Ask specifically: “Is [drug name, dosage] on your formulary? What tier is it? Are there any prior authorization or step therapy requirements?” Request written confirmation from the representative.
What Happens If Your Drug Is Removed Mid-Year?
Plans can remove drugs from their formulary or move them to a higher tier during the year, but only under limited circumstances and with required notice to members.
If your drug is removed mid-year, Medicare rules mandate that your plan must:
- Notify you at least 60 days before the change takes effect
- Continue covering your drug at the old cost-sharing for at least 30 days while you transition
- Provide a Special Enrollment Period to switch plans in certain circumstances
If you are mid-treatment with a covered specialty drug, the plan generally must provide a temporary supply to allow you to complete your treatment course or transition safely to a different medication.
How to Request a Formulary Exception
If your drug is not on the formulary or is placed on a tier that makes it unaffordable, you can request a formulary exception. This formal process requires your doctor to argue that a specific drug is medically necessary for your condition.
| Request Type | What It’s For | Timeframe |
|---|---|---|
| Coverage determination | Standard request for coverage of a non-covered or restricted drug | 72 hours (standard), 24 hours (expedited) |
| Formulary exception | Request to cover a non-formulary drug or waive step therapy | 72 hours (standard), 24 hours (expedited) |
| Tier exception | Request to cover a drug at a lower tier copay | 72 hours (standard), 24 hours (expedited) |
| Appeal (Level 1) | Appeal a denied coverage determination | 7 days (standard), 72 hours (expedited) |
Your doctor must provide a “statement of medical necessity” to support the exception request. Approval rates are higher when your doctor documents why lower-tier alternatives are unsuitable for your specific medical situation.
How to Compare Formularies When Shopping for a Plan
During Medicare’s Annual Enrollment Period (October 15 – December 7), you can review and switch Part D plans. Here’s how to effectively compare formularies:
- List every drug you take — include the name, dosage, and frequency of use. Don’t forget occasional medications.
- Use Medicare.gov’s Plan Finder — enter your drug list and zip code. The tool calculates your estimated annual cost for each plan, factoring in premiums and drug cost-sharing.
- Sort by estimated annual drug cost, not premium. A plan with a $0 premium but your drug on Tier 5 with 33% coinsurance can be far more expensive than a $40/month premium plan that places your drug on Tier 3.
- Check pharmacy network — using your plan’s preferred pharmacy (often a mail-order or specific retail chain) can further reduce your copay.
- Look for quantity limits and PA requirements — the Plan Finder displays these restrictions for each drug on each plan.
Get help comparing Part D formularies
A licensed Medicare specialist can conduct a comprehensive drug cost analysis across all plans available in your zip code—at no cost to you.
Frequently Asked Questions
What is a Medicare Part D formulary?
A formulary is the list of prescription drugs your Part D plan covers. Drugs are organized into tiers—lower tiers cost less, and higher tiers cost more. Each plan’s formulary is different, which is why your costs for the same drug can vary dramatically from plan to plan.
What are the Medicare drug plan tiers?
Most plans use 5 tiers: Tier 1 (preferred generics, $0–$5), Tier 2 (non-preferred generics, $10–$20), Tier 3 (preferred brands, $40–$100), Tier 4 (non-preferred brands, $80–$150+), and Tier 5 (specialty drugs, 25–33% coinsurance).
What is prior authorization in Medicare Part D?
Prior authorization is a requirement that your doctor receive plan approval before the plan will cover a specific drug. Without prior authorization for a PA-required drug, you pay full retail price at the pharmacy.
What is step therapy in Medicare drug plans?
Step therapy requires you to try a lower-cost drug first before the plan will cover a more expensive alternative. If the first-step drug doesn’t work or causes side effects, your doctor can document the failure and request coverage for the drug they originally prescribed.
Can I appeal if my Part D plan won’t cover my drug?
Yes. You can request a coverage determination, formulary exception, or tier exception. If those are denied, you can appeal. Your doctor must provide documentation supporting why the specific drug is medically necessary. Expedited reviews are available within 24–72 hours for urgent cases.
Can I switch Part D plans if my drug is dropped from the formulary?
Yes, under certain circumstances. If your plan drops your drug or moves it to a higher tier, you may qualify for a Special Enrollment Period to switch plans mid-year. Contact Medicare at 1-800-MEDICARE or visit medicare.gov to determine your eligibility.
Quick Answer A Medicare Part D formulary is the list of drugs your plan covers
Drugs are organized into tiers—Tier 1 costs the least, and Tier 5 costs the most. Your plan may also require prior authorization (doctor approval) or step therapy (trying a cheaper drug first) before covering certain medications. Knowing your formulary before you enroll can save you hundreds—or thousands—of dollars per year.
Many individuals select their Medicare Part D plan based solely on the monthly premium, which can lead to significant financial pitfalls. The plan with the lowest premium might place your essential medications on a high-cost tier or not cover them at all. Understanding how formularies function is crucial for selecting a Part D plan that genuinely saves you money.
What Is a Medicare Part D Formulary?
A formulary is your Part D plan’s official drug list—the specific prescription medications the plan has agreed to cover and their associated costs. Each Medicare Part D plan has its own formulary, and these formularies can vary significantly from one plan to another.
The Centers for Medicare & Medicaid Services (CMS) mandates that all Part D formularies include drugs from each “therapeutic category” (broad medical purpose). However, plans have considerable discretion in selecting which specific drugs to include and at which tier. This variability explains why the same medication can have vastly different copays or may be covered by one plan but not another.
📌 Key rule: Plans must cover at least two chemically distinct drugs in each therapeutic category, but they are not required to cover every drug. If your specific brand-name drug is absent from a plan’s formulary, you will be responsible for the full cost or must request a formulary exception.
Formularies are updated at least annually, with plans submitting changes to CMS each fall for the upcoming year. Plans can also make mid-year changes under specific limitations, which we will discuss later in this article.
The Five Drug Tiers Explained
Most Medicare Part D plans utilize a 5-tier structure, although some may employ three or four tiers. The tier classification of your drug determines your copay or coinsurance amount.
1
Preferred generics
$0–$5 copay
Metformin, lisinopril, atorvastatin
2
Non-preferred generics
$10–$20 copay
Generic drugs not on preferred list
3
Preferred brands
$40–$100 copay
Eliquis, Ozempic, Entresto
4
Non-preferred brands
$80–$150+ copay
Brand drugs not on preferred list
5
Specialty drugs
25–33% coinsurance
Cancer drugs, biologics, MS treatments
The copays listed above are typical; however, your plan’s actual amounts may vary. Always consult your plan’s Evidence of Coverage (EOC) document or the drug pricing tool on Medicare.gov before enrolling.
A note on Tier 5 specialty drugs
Specialty drugs, including biologics and chemotherapy agents, are typically categorized as Tier 5. Plans can charge up to 33% coinsurance on these medications, meaning that for a drug costing $10,000 monthly, your share could be between $2,500 and $3,300. This is why the upcoming 2026 $2,000 out-of-pocket cap is crucial for patients relying on specialty medications.
Coverage Rules: Prior Authorization, Step Therapy, and Quantity Limits
Being listed on the formulary does not guarantee automatic coverage. Plans implement three primary coverage rules that can restrict when and how a drug is covered.
Prior Authorization (PA)
Prior authorization requires your doctor to obtain approval from the plan before prescribing a specific medication. This process ensures that the drug is medically necessary, prescribed for an approved condition, and that more affordable alternatives have been considered.
PA is commonly required for brand-name drugs, specialty medications, and drugs with high potential for abuse. The approval process typically takes 1–3 business days (or 72 hours for expedited reviews). If your doctor submits the necessary documentation, approval rates are generally favorable.
⚠️ Picking up a PA-required drug without approval means you pay full retail price at the pharmacy. Ensure your doctor’s office submits the PA request before your first fill to avoid unexpected costs.
Step Therapy
Step therapy, also known as “fail first,” mandates that you try one or more lower-cost medications before your plan will cover the drug your doctor initially prescribed. The rationale is that if a generic or lower-tier drug can effectively treat your condition, the plan prefers that option first.
For example, if your doctor prescribes Eliquis (apixaban, a brand-name blood thinner), your plan may require you to first try warfarin (generic, Tier 1) for 30–60 days. If warfarin causes complications or fails to control your INR, your doctor can document this and request coverage for Eliquis.
Step therapy restrictions cannot be enforced if you are already stable on a medication when you join a plan. If you were taking Eliquis prior to enrolling in Medicare, the plan cannot require you to switch to warfarin.
Quantity Limits (QL)
Quantity limits restrict how much of a drug your plan will cover per fill or per month. For instance, there may be a limit of 60 tablets per 30-day supply or one injection pen per 28 days. These limits are based on FDA-approved dosing guidelines. If your doctor prescribes more than the quantity limit, you will need a quantity limit exception.
How to Check If Your Drug Is Covered
There are three reliable methods to check your drug’s formulary status:
1
Medicare.gov Plan Finder The official Medicare plan comparison tool at medicare.gov/plan-compare allows you to enter your drugs and see which plans cover them, at what tier, and at what estimated annual cost. This is the most reliable tool, utilizing real-time formulary data.
2
Your plan’s website or member portal If you’re already enrolled, log in to your plan’s member portal and use their formulary search tool. Enter your drug name and dosage to see your tier, any restrictions, and your estimated copay.
3
Call the plan directly Call the member services number on the back of your plan card. Ask specifically: “Is [drug name, dosage] on your formulary? What tier is it? Are there any prior authorization or step therapy requirements?” Request written confirmation from the representative.
What Happens If Your Drug Is Removed Mid-Year?
Plans can remove drugs from their formulary or move them to a higher tier during the year, but only under limited circumstances and with required notice to members.
If your drug is removed mid-year, Medicare rules mandate that your plan must:
- Notify you at least 60 days before the change takes effect
- Continue covering your drug at the old cost-sharing for at least 30 days while you transition
- Provide a Special Enrollment Period to switch plans in certain circumstances
If you are mid-treatment with a covered specialty drug, the plan generally must provide a temporary supply to allow you to complete your treatment course or transition safely to a different medication.
How to Request a Formulary Exception
If your drug is not on the formulary or is placed on a tier that makes it unaffordable, you can request a formulary exception. This formal process requires your doctor to argue that a specific drug is medically necessary for your condition.
| Request Type | What It’s For | Timeframe |
|---|---|---|
| Coverage determination | Standard request for coverage of a non-covered or restricted drug | 72 hours (standard), 24 hours (expedited) |
| Formulary exception | Request to cover a non-formulary drug or waive step therapy | 72 hours (standard), 24 hours (expedited) |
| Tier exception | Request to cover a drug at a lower tier copay | 72 hours (standard), 24 hours (expedited) |
| Appeal (Level 1) | Appeal a denied coverage determination | 7 days (standard), 72 hours (expedited) |
Your doctor must provide a “statement of medical necessity” to support the exception request. Approval rates are higher when your doctor documents why lower-tier alternatives are unsuitable for your specific medical situation.
How to Compare Formularies When Shopping for a Plan
During Medicare’s Annual Enrollment Period (October 15 – December 7), you can review and switch Part D plans. Here’s how to effectively compare formularies:
- List every drug you take — include the name, dosage, and frequency of use. Don’t forget occasional medications.
- Use Medicare.gov’s Plan Finder — enter your drug list and zip code. The tool calculates your estimated annual cost for each plan, factoring in premiums and drug cost-sharing.
- Sort by estimated annual drug cost, not premium. A plan with a $0 premium but your drug on Tier 5 with 33% coinsurance can be far more expensive than a $40/month premium plan that places your drug on Tier 3.
- Check pharmacy network — using your plan’s preferred pharmacy (often a mail-order or specific retail chain) can further reduce your copay.
- Look for quantity limits and PA requirements — the Plan Finder displays these restrictions for each drug on each plan.
Get help comparing Part D formularies
A licensed Medicare specialist can conduct a comprehensive drug cost analysis across all plans available in your zip code—at no cost to you.
Frequently Asked Questions
What is a Medicare Part D formulary?
A formulary is the list of prescription drugs your Part D plan covers. Drugs are organized into tiers—lower tiers cost less, and higher tiers cost more. Each plan’s formulary is different, which is why your costs for the same drug can vary dramatically from plan to plan.
What are the Medicare drug plan tiers?
Most plans use 5 tiers: Tier 1 (preferred generics, $0–$5), Tier 2 (non-preferred generics, $10–$20), Tier 3 (preferred brands, $40–$100), Tier 4 (non-preferred brands, $80–$150+), and Tier 5 (specialty drugs, 25–33% coinsurance).
What is prior authorization in Medicare Part D?
Prior authorization is a requirement that your doctor receive plan approval before the plan will cover a specific drug. Without prior authorization for a PA-required drug, you pay full retail price at the pharmacy.
What is step therapy in Medicare drug plans?
Step therapy requires you to try a lower-cost drug first before the plan will cover a more expensive alternative. If the first-step drug doesn’t work or causes side effects, your doctor can document the failure and request coverage for the drug they originally prescribed.
Can I appeal if my Part D plan won’t cover my drug?
Yes. You can request a coverage determination, formulary exception, or tier exception. If those are denied, you can appeal. Your doctor must provide documentation supporting why the specific drug is medically necessary. Expedited reviews are available within 24–72 hours for urgent cases.
Can I switch Part D plans if my drug is dropped from the formulary?
Yes, under certain circumstances. If your plan drops your drug or moves it to a higher tier, you may qualify for a Special Enrollment Period to switch plans mid-year. Contact Medicare at 1-800-MEDICARE or visit medicare.gov to determine your eligibility.
