Medicare Part B vs. Part D Prior Authorization: Key Differences Clinicians Must Know (2026)

Part B vs. Part D: Which Drugs Go Where?

The distinction between Medicare Part B and Part D drug coverage is based on how the medication is administered, not its therapeutic class. Part B covers drugs that are administered by a healthcare provider in an outpatient setting — primarily injectable and infusible medications given in physician offices, hospital outpatient departments, or home health settings. Part D covers self-administered medications dispensed through a retail or specialty pharmacy.

Common Part B drugs include IV infusion biologics (infliximab, rituximab, vedolizumab), injectable osteoporosis agents administered in-office (denosumab, zoledronic acid), chemotherapy infusions, and immunoglobulin infusions. Part D covers oral medications, self-injectable drugs (adalimumab, etanercept, insulin), inhalers, and most retail pharmacy prescriptions.

Some drugs can go through either benefit depending on the setting. For example, a biologic that is available in both IV and subcutaneous formulations may be covered under Part B (IV, administered in office) or Part D (subcutaneous, self-administered at home). The route and setting of administration determine the benefit, which in turn determines the PA process, the cost-sharing structure, and the appeal pathway.

How Part B Prior Authorization Works

Medicare Part B prior authorization is relatively new and limited in scope. CMS began a voluntary Part B PA program in 2020 and has gradually expanded it. As of 2026, mandatory Part B PA applies to a defined list of categories including certain durable medical equipment, select Part B drugs identified by CMS as high-cost or frequently mis-utilized, and some diagnostic services.

The Part B PA process is administered by Medicare Administrative Contractors (MACs), not by Part D plan sponsors. Clinicians submit PA requests to their regional MAC using the MAC's specified forms or portal. Decision timelines are governed by CMS: 10 business days for standard requests and 2 business days for expedited requests. If the MAC does not respond within the timeframe, the request is deemed provisionally approved.

A critical difference from Part D is that Part B coverage is governed by National Coverage Determinations (NCDs) and Local Coverage Determinations (LCDs), not formularies. There are no tiers and no tiering exceptions. The question is simply whether the drug meets the coverage criteria for the patient's indication. Clinicians should reference the applicable LCD or NCD and demonstrate that each criterion is satisfied.

How Part D Prior Authorization Differs

Part D prior authorization is managed by the Part D plan sponsor (or its pharmacy benefit manager) and applies to any drug on the plan's formulary that has a PA restriction. Unlike Part B's limited PA scope, Part D plans can require PA for any formulary drug, and most specialty tier drugs carry a PA requirement.

Part D PA requests are submitted to the plan's pharmacy benefit manager through designated portals, fax numbers, or electronic PA platforms. CMS mandates strict decision timelines: 72 hours for standard requests and 24 hours for expedited requests. These timelines are significantly faster than Part B's 10 business days, reflecting the urgency of retail pharmacy prescriptions.

Part D plans use formularies with tier-based cost sharing, and the PA criteria are set by the plan's P&T committee within CMS guidelines. This means that two different Part D plans may have completely different PA criteria for the same drug. Clinicians treating Medicare patients enrolled in different Part D plans must tailor each PA submission to the specific plan's requirements — a one-size-fits-all approach does not work.

Appeal Timelines and Processes Compared

Part B and Part D have distinct appeal structures with different timelines. For Part B, a denied PA can be appealed through the Medicare FFS appeal process: redetermination by the MAC (60 days to file, decision within 60 days), then reconsideration by a Qualified Independent Contractor, then Administrative Law Judge hearing, Medicare Appeals Council, and federal court. The Part B appeal process is slower but follows the traditional Medicare FFS structure.

For Part D, the appeal process is faster and has five levels: redetermination by the plan (7 calendar days standard, 72 hours expedited), reconsideration by an Independent Review Entity (IRE) within 7 days standard or 72 hours expedited, ALJ hearing, Medicare Appeals Council, and federal court. The Part D appeal deadlines for filing are 60 days from the denial notice at each level.

A practical tip: for Part D expedited appeals, the clinician can request expedited processing by attesting that standard timelines could jeopardize the patient's life, health, or ability to regain maximum function. Most clinical scenarios involving specialty medications qualify for expedited processing, and clinicians should default to requesting it.

How RxCheckUp Handles Part B and Part D Differences

RxCheckUp automatically determines whether a drug falls under Part B or Part D based on the medication, route of administration, and care setting. The platform then routes the PA through the correct pathway — MAC submission for Part B, plan sponsor submission for Part D — with the appropriate forms, documentation requirements, and policy references for each.

For practices treating Medicare patients across multiple Part D plans, RxCheckUp maintains current formulary and PA criteria for every major Part D plan sponsor, ensuring that each submission is tailored to the specific plan. When denials occur, the platform identifies the correct appeal pathway and timeline, drafts appeal documentation that meets CMS requirements, and tracks deadlines to prevent missed filing windows. This eliminates the confusion that arises when staff must manually determine which Medicare benefit applies and which process to follow.