Medicare Part D covers most outpatient prescription drugs you pick up at a pharmacy, including brand-name and generic medications. But every Part D plan builds its own list of covered drugs, called a formulary, so the specific medications included vary from plan to plan. There are, however, federal rules that set a floor for what all plans must cover and a ceiling on what they’re allowed to exclude.
How Part D Formularies Work
Each Part D plan maintains a formulary listing the drugs it covers. Plans have flexibility in choosing which drugs to include, but they must cover at least two drugs in every therapeutic category (meaning each type of condition has at least two treatment options available). The formulary is organized into tiers that determine how much you pay out of pocket:
- Tier 1 (lowest cost): Most generic drugs
- Tier 2 (medium cost): Preferred brand-name drugs
- Tier 3 (higher cost): Non-preferred brand-name drugs
- Specialty tier (highest cost): Very high-cost drugs
Your plan’s exact tier structure may look slightly different, but the principle is the same: lower tiers mean lower copays. If a generic or biosimilar version of your medication becomes available, your plan may move the brand-name version to a higher tier, which increases your cost. You can ask your plan for an exception to get a lower copay if your prescriber believes the higher-tier drug is medically necessary for you.
Six Drug Classes Every Plan Must Cover
Federal rules require all Part D plans to cover essentially all drugs in six protected categories:
- Antidepressants: medications for depression and related conditions
- Antipsychotics: medications for schizophrenia, bipolar disorder, and other psychiatric conditions
- Anticonvulsants: medications for seizure disorders and epilepsy
- Immunosuppressants: medications that prevent organ rejection after a transplant
- Antiretrovirals: medications for HIV/AIDS
- Antineoplastics: cancer-fighting medications
For these six classes, plans can’t limit their formulary to just two options. They must include all or nearly all available drugs, with only very narrow exceptions. This protection exists because people taking these medications often can’t easily switch to a different drug without serious health consequences.
Insulin and Vaccines
Thanks to the Inflation Reduction Act, insulin costs under Part D are capped at $35 for a one-month supply of each covered insulin product, with no deductible. If you fill a three-month supply, you’ll pay no more than $105 total ($35 per month’s worth). This cap applies to everyone with Part D coverage, including those receiving Extra Help.
Part D also covers most adult vaccines that aren’t already covered under Part B. Part B handles flu shots, pneumonia vaccines, and hepatitis B vaccines for people at intermediate-to-high risk. Part D picks up the rest, including shingles vaccines, tetanus boosters, and other recommended immunizations.
Drugs That Part D Cannot Cover
Federal law excludes several categories of drugs from Part D coverage entirely, regardless of which plan you choose:
- Weight loss or weight gain drugs: medications used for anorexia, weight loss, or weight gain
- Cosmetic drugs: medications for cosmetic purposes or hair growth
- Cough and cold remedies: drugs used for symptomatic relief of cough and colds
- Over-the-counter medications: nonprescription drugs generally aren’t covered
- Benzodiazepines: such as clonazepam (Klonopin) and similar anti-anxiety medications
- Barbiturates: such as phenobarbital
The weight loss exclusion is particularly relevant right now. Popular GLP-1 medications like Wegovy and Zepbound, when prescribed specifically for weight loss, are excluded from Part D by the same law that created the program in 2003. If one of these drugs is prescribed for a different covered condition, such as diabetes or heart disease, coverage rules may differ, but the weight loss exclusion remains a hard federal barrier.
Blood glucose testing strips and lancets are also excluded from Part D because they’re classified as durable medical equipment and fall under Part B instead.
Part B vs. Part D: Where Your Drug Is Covered
Not all prescription drugs fall under Part D. Medicare Part B covers drugs that are administered in a medical setting or require special equipment. This includes injectable and IV drugs given in a doctor’s office that you wouldn’t normally give yourself at home, drugs delivered through a nebulizer or infusion pump, and certain oral cancer drugs.
Part D covers the drugs you pick up at a retail or mail-order pharmacy and take on your own, including insulin, injection supplies like syringes and needles, and self-administered medications. Some drugs can be covered under either part depending on the circumstances. Immunosuppressants after an organ transplant, for instance, are covered under Part B when tied directly to a Medicare-covered transplant but may fall under Part D in other situations. If you’re unsure which part covers a specific medication, your plan can clarify.
The New Out-of-Pocket Cap
Starting in 2025, the Inflation Reduction Act eliminated the old coverage gap (commonly called the “donut hole”) that forced people to pay a larger share of their drug costs after hitting a certain spending threshold. The Part D benefit now has three straightforward phases: a deductible, an initial coverage period where you pay 25% coinsurance, and catastrophic coverage where you pay nothing for the rest of the year.
In 2026, the annual deductible is capped at $615, and once your out-of-pocket spending on covered Part D drugs reaches $2,100, you enter catastrophic coverage and owe nothing more for covered medications through the end of the calendar year. This is a significant change from the old structure, where costs in the catastrophic phase could still add up.
Prior Authorization and Other Plan Rules
Even when a drug is on your plan’s formulary, the plan may require extra steps before it will cover the prescription. Three common rules apply:
Prior authorization means your prescriber needs approval from the plan before your prescription is filled. The plan may need documentation that the drug is medically necessary or that it’s being used for a specific condition the plan covers.
Step therapy requires you to try a less expensive drug first, typically a generic, before the plan will cover a more expensive alternative. If the cheaper option doesn’t work or causes side effects, you can then “step up” to the costlier medication.
Quantity limits restrict how much of a drug you can get in a given time period, such as 30 tablets per month, for safety or cost reasons.
You have the right to challenge any of these restrictions. Your prescriber can submit a statement to the plan explaining why the restriction isn’t appropriate for your medical situation. This can be done verbally at first, though the plan may ask for a written follow-up. The statement needs to explain why alternative drugs on the formulary would be less effective or cause adverse effects for you specifically.
How to Request Coverage for a Drug Not on Your Formulary
If your medication isn’t listed on your plan’s formulary at all, you or your prescriber can file a formulary exception request. Your prescriber must provide a supporting statement explaining that every drug the plan does cover for your condition would either be less effective or cause harmful side effects. The statement can be submitted as a letter, on the plan’s own form, or on a standard form available from CMS.
If the plan grants the exception, it will cover the drug. If it denies the request, you can appeal the decision. Plans are required to process these requests within set timeframes, and expedited reviews are available when a standard timeline could seriously harm your health.
Before enrolling in or switching Part D plans, you can search any plan’s formulary on Medicare.gov to check whether your specific medications are covered and which tier they fall under. This is the single most practical step you can take to avoid surprise costs or coverage gaps.