Can the plan just say it's "not medically necessary" with no further explanation?

The denial notice has to explain the basis for the decision and tell you how to appeal. If it doesn't point to a specific rule or criteria, that's itself worth raising in your appeal — you're entitled to know what standard was applied.

What if I don't have all the evidence listed above?

Include what you have. An appeal with partial evidence and a clear explanation of the rule still gets a real review — it doesn't need to be complete to be worth filing.

Medicare Part D denied a drug the plan says belongs under Part B instead? What the rule says and how to appeal.

Checked against the current rules on September 25, 2026.

A denial for a drug the plan says belongs under Part B instead isn't the end of the story — Medicare Part D plans have to follow specific coverage rules, and a denial that doesn't square with those rules is exactly the kind of appeal that tends to win.

Here's what the rule actually requires, what evidence usually turns a denial around, and the deadline you're working against.

What the rule requires

Coverage for a drug the plan says belongs under Part B instead under a Medicare Part D plan is governed byDrugs Covered Under Medicare Part A or B andPart D Sponsor Due Diligence in Prior Authorization of Part A or B. A Medicare Part D plan generally cannot apply a stricter test than what these rules require.

Questions that decide it

Before you appeal, these are the facts that usually decide whether a denial like this holds up:

Was the drug given in a doctor's office or an infusion center, rather than picked up at a pharmacy?
Is the drug administered by injection or infusion by a provider, rather than self-administered?
Did the plan's denial say this should be billed to a different part of Medicare?

What a winning appeal has to show

An appeal that wins usually includes:

Records of exactly how and where the drug was administered — The Part B/Part D line depends on the drug's own administration method and setting (self-administered vs. physician-administered, home vs. office) — the plan's own due-diligence obligation under §20.2.2 is to get this right before denying. (The facility provides this.)Drugs Covered Under Medicare Part A or B
Any denial or non-coverage decision already received from Part A or Part B for the same drug — If Part A/B already declined to cover it, that's direct evidence Part D coverage should be evaluated, not assumed away. (You or your family provide this.)Drugs Covered Under Medicare Part A or B

Your deadline to appeal

No deadline to make this first request — it's your initial ask, not yet an appeal. The 65-day appeal clock only starts once the plan denies it.42 CFR 423.566

The plan itself then has its own clock to decide. 72 hours for a standard drug-benefit request (24 hours if expedited). For an exception request, the clock instead runs from receipt of your prescriber's supporting statement — 72 hours standard / 24 hours expedited from that point, or from the end of a 14-day wait if the plan never receives one. Payment requests: 14 calendar days (42 CFR § 423.568, § 423.570, § 423.572).

What to ask your doctor to address

If you're asking the treating physician for a supporting letter, it should speak to:

Exactly how, where, and by whom the drug is administered.Drugs Covered Under Medicare Part A or B

Questions people ask

Can the plan just say it's "not medically necessary" with no further explanation?: The denial notice has to explain the basis for the decision and tell you how to appeal. If it doesn't point to a specific rule or criteria, that's itself worth raising in your appeal — you're entitled to know what standard was applied.
What if I don't have all the evidence listed above?: Include what you have. An appeal with partial evidence and a clear explanation of the rule still gets a real review — it doesn't need to be complete to be worth filing.

Sources — last checked September 25, 2026

42 CFR 423.566as of September 23, 2026
Drugs Covered Under Medicare Part A or Bas of September 25, 2026
Part D Sponsor Due Diligence in Prior Authorization of Part A or Bas of September 25, 2026

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By CaseWhy Appeals editorial. See how this content is produced. Not legal advice. Not affiliated with or endorsed by Medicare, CMS, or any health plan. A product of CaseWhy LLC.