His Part D Drug Costs Stop at $2,100. The Infusion in the Same Myeloma Regimen Can Keep Billing Him 20% All Year.

A 71-year-old with multiple myeloma opens two Medicare statements on the same morning. One shows that his covered Part D drug spending has reached the annual out-of-pocket limit. His oral lenalidomide prescription will cost him nothing more for the rest of the year.

The other statement is for the cancer drug administered at his oncology clinic. That one is still generating coinsurance. Same disease. Same treatment plan. Two different pieces of Medicare, and only one of them comes with the new $2,100 drug-cost ceiling.

The Pill and the Infusion Take Different Roads

Many cancer regimens combine medications patients take at home with drugs given in a doctor’s office or clinic. The self-administered prescription generally runs through Medicare Part D. In 2026, annual out-of-pocket spending on covered Part D drugs is capped at $2,100. Once the threshold is reached, the beneficiary pays no additional cost sharing for covered Part D prescriptions through December.

January starts the clock over. A cancer drug administered by a physician can instead fall under Medicare Part B. In 2026, the annual Part B deductible is $283. After that, Original Medicare generally leaves the beneficiary responsible for 20% of the Medicare-approved amount for covered Part B drugs and services. And unlike Part D, Original Medicare has no annual out-of-pocket maximum. The pill eventually hits a wall. The clinic bill can keep coming.

Twenty Percent Gets Big Quickly

Twenty percent does not sound especially alarming when the underlying service costs $150. Cancer treatment changes the scale. If the Medicare-approved amount for a Part B-covered treatment were $5,000, a 20% share would be $1,000. Another treatment could mean another charge. Hitting the Part D ceiling in March, June or September does nothing to stop the Part B coinsurance. That disconnect is easy to miss because the drugs may appear together on the oncologist’s treatment plan.

Medicare does not see them as one bill. For someone receiving Original Medicare without supplemental coverage, that distinction can turn one side of the regimen into a predictable expense while leaving the other open-ended (we cataloged this kind of coverage gap, along with IRMAA surcharges and other premium traps, in a free Medicare guide.)

Two Way Street

Medigap can change the Part B side considerably. Plan G, for example, generally covers Part B coinsurance after the beneficiary pays the annual Part B deductible. For someone receiving expensive clinic-administered drugs repeatedly, that protection can turn an unpredictable percentage into a monthly Medigap premium plus the deductible.

Timing matters, though. The federal Medigap open enrollment period lasts six months beginning the first month someone is 65 or older and enrolled in Part B. During that period, insurers cannot reject an applicant because of health. Later, medical underwriting can apply in most states unless a guaranteed-issue right or state protection intervenes.

Medicare Advantage takes a different approach. Advantage plans have an annual out-of-pocket limit for covered Part A and Part B services, so the clinic side does not remain uncapped indefinitely. But networks, prior authorization and plan-specific cost sharing matter enormously for someone already established with an oncologist or cancer center.

Before the Next Treatment, Put a Ceiling on Paper

For someone facing both Part D prescriptions and Part B cancer drugs, these three steps can make the year’s exposure much clearer:

1. Ask the oncology billing office for the Medicare-approved amount and expected patient share for the clinic-administered drug, then multiply that cost across the planned treatment schedule.
2. Check what coverage is already sitting behind Original Medicare, including Medigap, retiree coverage, Medicaid or other insurance that may absorb some or all of the 20%.
3. If the Medigap open enrollment window is still available, price that protection before it closes. If considering Medicare Advantage during an eligible enrollment period, verify the oncologist, cancer center and treatment authorization rules before switching.

The $2,100 Part D cap is meaningful relief. It just stops at the pharmacy side of the treatment plan. For a cancer patient, knowing which Medicare card pays for which drug can be worth far more than the difference between a pill and an infusion.

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