HSA Guide

What is a letter of medical necessity?

Q: What is a letter of medical necessity?

A letter of medical necessity is a short note from a licensed health-care provider stating that a specific item or service is needed to treat, manage, or prevent a diagnosed medical condition. It's what can turn a dual-purpose expense, like a supplement or a gym membership, into a qualified HSA or FSA expense by documenting that it's being used as medical care rather than for general health.

Q: Who can write a letter of medical necessity?

A licensed health-care provider who is treating you, most often a physician, but depending on the item and your plan it can also be a nurse practitioner, physician assistant, dentist, or other licensed practitioner. It should be someone qualified to diagnose the condition the item is meant to treat.

Q: What should a letter of medical necessity include?

Typically your name, the diagnosed condition, the specific item or service recommended, a brief explanation of how it treats or manages the condition, how long it's needed, and the provider's name, credentials, signature, and date. Your HSA or FSA administrator may have a specific form.

Q: Do I submit the letter to the IRS?

Not with your tax return. For an HSA you keep the letter yourself with the receipt, in case the IRS asks you to substantiate the expense. For an FSA, the administrator often requires the letter up front before reimbursing you.

Q: Does a letter of medical necessity guarantee my expense is eligible?

No. A letter supports eligibility for a dual-purpose item, but it doesn't override the underlying rules. A letter cannot make a nonqualifying general wellness expense become a qualified medical expense, and your administrator or the IRS still applies the qualified-expense standard.

A letter of medical necessity is a short note from a licensed provider stating that a specific item or service is needed to treat, manage, or prevent a diagnosed medical condition. It can document when a dual-purpose expense, like a supplement or a gym membership, qualifies as an HSA or FSA expense, showing it's being used as medical care rather than for general health.

Many everyday purchases sit in a gray area: they could be for general wellness, or they could be genuine medical care, depending on why you bought them. A letter of medical necessity, sometimes shortened to LMN or LOMN, is how a provider puts that "why" on the record. It's the document that records the medical purpose supporting eligibility.

Why it matters

The IRS defines a qualified medical expense as one for the diagnosis, cure, mitigation, treatment, or prevention of disease, under Internal Revenue Code Section 213(d). Items bought for general health don't meet that standard. A letter of medical necessity supplies the missing piece: it documents that a specific item is prescribed or recommended to treat a diagnosed medical condition, rather than bought for general health. Without the letter, that same purchase usually isn't a qualified expense at all.

What a letter of medical necessity includes

There's no single official form, and your HSA or FSA administrator may provide one, but a complete letter generally covers:

Your name as the patient
The diagnosed condition the item is meant to address
The specific item or service being recommended
Any relevant dosage, frequency, or usage instructions, when applicable
How it treats or manages the condition, in a sentence or two
How long it's needed, for example a number of months
The date the recommendation begins, if different from the signature date
The provider's name, credentials, signature, and date

When you usually need one

You only need a letter for dual-purpose items, things that can be either personal or medical. Clearly medical purchases don't need one.

Expense	Letter commonly required?
Prescription medication	No, it's already medical care
Over-the-counter medicines (pain relief, allergy, cold, etc.)	No, HSA eligible without a prescription since 2020
Bandages and first-aid supplies	No
Vitamins or supplements for general health	Yes, to qualify at all
Gym membership or exercise program	Yes, if to treat a diagnosed condition
Massage therapy for a diagnosed condition	Yes, if for a diagnosed condition
Weight-loss program	Yes, if to treat a specific disease

Administrator requirements vary. Some plans may request additional documentation beyond a letter of medical necessity.

A letter doesn't turn a purely general-health purchase into a qualified one. It documents a genuine medical use that already exists. For a worked example, see are vitamins HSA eligible, where the letter is exactly what separates a qualifying supplement from a nonqualifying one.

Who can write one, and how long it lasts

A letter should come from a licensed provider who is treating you and is qualified to diagnose the condition, most often a physician, but depending on the item and your plan it can be a nurse practitioner, physician assistant, dentist, or other licensed practitioner. A letter usually covers a defined period determined by the provider or administrator, and some administrators require an updated letter for ongoing expenses. Treat it as a document that expires, not a permanent pass, and confirm the renewal rules with your plan.

Keep the letter with the receipt

A letter of medical necessity only helps if you can produce it alongside the expense it supports. For an HSA you keep the letter yourself and don't file it with your return, but you may need it years later to substantiate the expense if the IRS asks. For an FSA, the administrator often requires the letter up front before reimbursing you. Regardless of account type, retain the letter together with the supporting receipt and any other required documentation, because one without the other doesn't tell the whole story. See do you need to keep HSA receipts for how long to hold both.

Why Receipt Locker is different

A letter kept in one place and the receipt in another is how substantiation falls apart years later. Receipt Locker lets you attach the letter of medical necessity directly to the receipt it supports, so they remain linked as a single supporting record, categorized, preserved, and exportable as an independently verifiable record. When you reimburse yourself, you can show the expense, the letter that qualified it, and that the stored record's integrity can be independently verified.

Keep the letter and the receipt together

Attach each letter of medical necessity to the expense it supports, preserved for the day you reimburse yourself. Free to start, no credit card required.

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Frequently asked questions

What is a letter of medical necessity?

A short note from a licensed provider stating that a specific item or service is needed to treat, manage, or prevent a diagnosed condition. It can turn a dual-purpose expense, like a supplement or gym membership, into a qualified HSA or FSA expense.

Who can write a letter of medical necessity?

A licensed provider who is treating you and can diagnose the condition, most often a physician, and depending on the item and plan a nurse practitioner, physician assistant, dentist, or other licensed practitioner.

What should a letter of medical necessity include?

Your name, the diagnosed condition, the recommended item or service, how it treats the condition, how long it's needed, and the provider's name, credentials, signature, and date. Your administrator may have a specific form.

How long is a letter of medical necessity valid?

It is generally valid for the period specified by the provider or required by the plan. Some administrators require updated letters for ongoing expenses.

Do I submit the letter to the IRS?

Not with your tax return. For an HSA you keep it yourself in case the IRS asks. For an FSA, the administrator often requires it up front before reimbursing you.

Does a letter of medical necessity guarantee my expense is eligible?

No. It supports eligibility for a dual-purpose item but doesn't override the rules. A letter cannot make a nonqualifying general wellness expense become a qualified medical expense, and your administrator or the IRS still applies the qualified-expense standard.

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Sources: IRS Publication 502 (Medical and Dental Expenses), including its "What Are Medical Expenses?", "Capital Expenses", "Weight-Loss Program", "Nutritional Supplements", and "Health Club Dues" sections; Internal Revenue Code Section 213(d) for the definition of a medical expense; IRS Publication 969 (Health Savings Accounts) for HSA substantiation. This article is general information, not tax or medical advice. Eligibility and letter requirements can depend on your situation and your plan, so confirm with Publication 502, your HSA or FSA administrator, or a qualified professional.
Last reviewed: July 2026.