The Letter of Medical Necessity Your FSA Administrator Wants

The short answer

Most rejected FSA claims for a GLP-1 are not coverage decisions. They are substantiation failures. The administrator cannot see a diagnosis, so it cannot tell a treatment from a wellness purchase, and it declines. The fix is a one-page letter from your prescriber, and it takes a portal message to request.

If you have not yet priced the pre-tax route at all, start with GLP-1 cost without insurance: HSA/FSA and the real monthly math — that piece covers which account to use and what the pre-tax discount is actually worth. This one covers the document that unlocks it.

Why the diagnosis line is the whole game

The tax rules define a qualified expense by what the money treated, not by what it bought. IRS Publication 502 says you can include in medical expenses amounts you pay to lose weight if it is a treatment for a specific disease diagnosed by a physician (such as obesity, hypertension, or heart disease), and that you cannot include membership dues in a gym, health club, or spa¹. Separately, it says you can include amounts paid for prescribed medicines and drugs, defining a prescribed drug as one that requires a prescription by a doctor for its use by an individual¹.

Health FSAs and HSAs inherit that test: qualified medical expenses are those specified in the plan that would generally qualify for the medical and dental expenses deduction².

So an administrator looking at a charge from a telehealth brand has no way to know which side of that line it falls on. The letter tells it.

What the letter must contain

Two details cause most re-submissions. A letter that names the brand of a program rather than the medication or service, and a letter with no duration — which many administrators read as covering one purchase.

A draft to hand your prescriber

Send this in a portal message and ask them to put it on letterhead. Clinicians do this constantly; you are saving them the drafting, not asking a favor.

"To whom it may concern: [Patient name], date of birth [date], is under my care. I have diagnosed [condition, with ICD-10 code]. I am prescribing [medication or service] as medically necessary treatment for that condition, and I expect treatment to continue for [duration, or: at least twelve months, subject to review]. Please direct any questions to my office. [Clinician name, credentials, NPI, signature, date]."

Ask for it as a PDF, and ask for the duration line explicitly. If your prescriber's platform offers a "letter of medical necessity" template, that is the same document.

The diagnosis is on the label, not in your search history

It helps to know that the conditions your clinician is treating are the ones the products are approved for. Wegovy's label carries indications including reducing the risk of major adverse cardiovascular events in adults with established cardiovascular disease and either obesity or overweight, reducing excess body weight and maintaining weight reduction long term in adults with obesity or in adults with overweight in the presence of at least one weight-related comorbid condition, and noncirrhotic MASH with moderate to advanced fibrosis³.

A letter naming one of those diagnoses is describing treatment of a disease. A letter that says the patient "would like to lose weight" is describing a preference, and it is the version that gets declined.

The four rejections, and what fixes each

• Card declined at checkout. The merchant category was not recognized as medical. Pay another way and submit for reimbursement with the letter attached — same money, one extra step.
• "Additional documentation required." Send the letter plus an itemized statement showing the item, the date and the amount. A payment confirmation alone is not itemization.
• "Not an eligible expense." Usually a program or membership fee with no service breakdown. Ask the provider to itemize the clinical services separately from the subscription.
• Claim exceeds the plan year. FSAs have fixed run-out deadlines. Submit quarterly rather than annually.

If a program bills you as a subscription and will not itemize, that is worth knowing before you enroll — and it is the same trap that flat price vs dose-scaling pricing covers on the pricing side.

Timing this around your plan year

FSA elections are annual and the balance mostly does not follow you, so the calendar decides how much of this money you keep. Two habits are worth more than any optimization: request the letter at your first visit, before you need it; and set a reminder six weeks before your plan's run-out deadline to sweep any unsubmitted receipts. The annual contribution and carryover limits change every year, so confirm the current figures in IRS Publication 969 or with your administrator rather than trusting a number you read last year².

Where this leaves you

The letter is also the document that makes an out-of-network claim legible, so ask for it at the same time as a superbill for your telehealth visits. If your employer offers a lifestyle or wellness account rather than an FSA, the rules are different and mostly worse — see wellness stipends and lifestyle spending accounts. And if you are still choosing a program, our cheapest GLP-1 board is the shortlist that pre-tax dollars stretch furthest against.

The decisive takeaway

An FSA rejection is usually a missing sentence, not a denied benefit. Get a letter that names the diagnosis with its code, names the medication or service rather than the brand, states medical necessity in one sentence, and carries a duration and an NPI. Request it at your first appointment, submit quarterly, and stop losing money you already elected. Some links here earn us a referral fee, which never changes the ranking.