Two Neighbors Will Start Leqembi, Listed at $28,270 a Year. One With Medigap Plan G Will Owe $0 in Coinsurance After the Part B Deductible, While the Other Will Owe 20% of Medicare-Approved Infusion Costs

Two hypothetical 72-year-old neighbors have been diagnosed with early Alzheimer’s disease. Their neurologists recommend Leqembi, a treatment from Biogen (NASDAQ:BIIB | BIIB Price Prediction) and Eisai that lists for about $28,270 a year.

Both have Original Medicare, but only one has Medigap Plan G. After the Part B deductible, her policy picks up the coinsurance. Her neighbor, who never bought supplemental coverage, faces 20% of Medicare’s approved cost for every infusion. Neither woman knows exactly what her treatment will cost until the bills arrive.

Three Conditions Medicare Sets Before It Pays

The Centers for Medicare & Medicaid Services (CMS) covers Leqembi for eligible patients with mild cognitive impairment or mild Alzheimer’s dementia and confirmed amyloid plaque. Their treating physician must also participate in an approved registry that tracks patients’ treatment and outcomes.

The registry rule is the one families miss. If her neurologist does not report to a qualifying registry, Medicare does not cover the drug. Confirm that before anyone books the first infusion.

What 20% Coinsurance Means at a $28,270 List Price

With Original Medicare, she first pays the $283 Part B deductible. After that she owes 20% coinsurance of the Medicare-approved amount for every infusion. Leqembi’s IV list price is about $28,270 annually for a patient weighing 165 pounds. Twenty percent would be $5,654, but that isn’t necessarily what Medicare will charge her. Medicare pays its own approved amount for Part B drugs, which can differ from the list price, and the infusion center bills separately for giving the drug.

Original Medicare has no yearly out-of-pocket cap. Social Security estimated the average retired worker would receive $2,071 a month in January 2026. The illustrative $5,654 would equal about 2.7 months of that check, although her actual coinsurance depends on Medicare’s approved charges.

Same Drug, Three Very Different Bills

A patient who already has Plan G is in the strongest position. After the deductible, Plan G generally covers her Leqembi coinsurance. She still pays her regular premiums. A patient with Original Medicare and no supplement carries the most risk.

The catch for that patient is that buying Plan G now is hard. Her six-month Medigap open enrollment window closed years ago. In most states, insurers can review her health history, and an Alzheimer’s diagnosis could make Plan G difficult to obtain. New York and Connecticut give broader guaranteed-issue rights, so check your own state’s rules.

Medicare Advantage puts a ceiling on her yearly spending, but she has to get approval first. Insurance companies publish drug coverage policies for Leqembi, and UnitedHealthcare says its policy may also be applied to Medicare Advantage plans. The cap excludes Part D drugs. Out-of-network costs depend on the plan and may have a separate spending limit. A plan also helps only if its network includes a neurologist who participates in a registry and an infusion center that gives the drug.

For a patient who cannot get Medigap, Medicare Advantage may offer some protection through its annual out-of-pocket limit. But she needs to check whether the plan covers Leqembi and includes her neurologist and infusion center before switching.

Costs Beyond the Infusion

Leqembi can cause brain swelling, known as ARIA. To check for it, patients need a baseline MRI and additional scans before the 3rd, 5th, 7th and 14th infusions. The FDA added the 3rd-infusion scan in August 2025. Under Original Medicare, each scan, each specialist visit and the earlier amyloid test (a PET scan or spinal fluid test) carries its own 20% coinsurance.

Prices differ from one facility to the next, so no single total applies to every patient. If a scan shows swelling, she may need extra MRIs and delayed doses, and those cost money too. These are the sort of surcharges and coverage gaps we cataloged in a free guide to Medicare’s hidden bills, from IRMAA on up.

Steps to Take Before December 7

1. Request a written estimate for her own treatment. Ask the infusion center and the neurologist’s billing office to price the drug, the fee for giving it and each scheduled MRI at Medicare-approved amounts.
2. For Medicare Advantage, call the plan now. Confirm that her neurologist and infusion center are in network, ask for the in-network out-of-pocket maximum and get the prior authorization approval in writing before the first dose.
3. For Original Medicare with no supplement, call your state’s SHIP office this week. SHIP offers free Medicare counseling. Ask whether your state gives guaranteed-issue Medigap rights. If it does not, compare Medicare Advantage plans that include her registry neurologist before enrollment closes December 7. Changes take effect January 1, 2027.

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