A Letter of Medical Necessity, or LMN, is a written statement from a licensed healthcare provider explaining that a specific item or service is needed to treat a specific medical condition. It's the magic key that turns a long list of borderline products and services — gym memberships, supplements, air purifiers, special diets, and more — into HSA-eligible expenses.
Used correctly, an LMN can unlock thousands of dollars of qualified medical spending. Used incorrectly, it does nothing and can land you in trouble in an audit. Here's what you need to know.
What an LMN Actually Does
The IRS lets you use HSA funds for an expense if it's "primarily to alleviate or prevent a physical or mental defect or illness." For obvious medical expenses (a doctor's visit, a prescription), eligibility is automatic. For dual-use items — things that could be either medical or general health — you need additional evidence that the item is being used for a medical condition. That evidence is the LMN.
When You Need One
You need an LMN for items the IRS classifies as "eligible with a Letter of Medical Necessity." Common examples:
- Gym memberships and fitness equipment when prescribed for obesity, diabetes, hypertension, etc.
- Vitamins and nutritional supplements for diagnosed deficiencies or specific conditions
- Air purifiers, humidifiers, dehumidifiers for severe allergies, asthma, or respiratory illness
- Massage therapy and acupressure for chronic pain or specific orthopedic conditions
- Special foods or diets beyond normal dietary needs (e.g., gluten-free for celiac disease, where the cost is the difference vs. regular food)
- Weight-loss programs when prescribed for a specific condition
- Hearing aid batteries and accessories in some cases
- Childcare during medical treatment in narrow cases
You don't need an LMN for things that are obviously medical (prescriptions, dental work, therapy with a licensed provider, OTC medications under the CARES Act, etc.). When in doubt, search the item in our eligible items directory — it'll tell you whether an LMN is required.
What the Letter Must Include
An LMN should be on letterhead from a licensed healthcare provider (MD, DO, PA, NP, sometimes a licensed psychologist or other practitioner depending on the issue) and include:
- Patient information: name and date of birth
- Specific diagnosis: the medical condition being treated, ideally with the ICD-10 code
- Specific item or service: exactly what the patient needs (e.g., "gym membership at any facility" rather than "exercise")
- Medical necessity statement: a sentence explicitly stating that this item or service is medically necessary to treat the diagnosed condition
- Duration: how long the treatment is recommended (typically up to 12 months — most LMNs need to be renewed annually)
- Provider signature, credentials, license number, and date
A Sample Template
You can hand this to your provider. Most are happy to write one if it's appropriate.
[Provider Letterhead]
Date: [date]
Re: Letter of Medical Necessity for [Patient Name], DOB [date of birth]
To Whom It May Concern,
[Patient Name] has been diagnosed with [condition, with ICD-10 code]. As part of the treatment plan for this condition, I am prescribing [specific item or service]. This [item/service] is medically necessary to treat, alleviate, or prevent the worsening of [Patient Name]'s diagnosed condition.
This recommendation is effective for the next [12 months / specified duration] beginning [start date]. Please do not hesitate to contact my office for further information.
Sincerely,
[Provider Signature]
[Provider Name, Credentials]
[License Number] · [Phone]
How to Actually Get One
Three approaches, in order of likelihood to succeed:
- Ask at your next regular appointment. Bring up the issue while you're already there — most providers can produce an LMN in five minutes via their EHR system.
- Send a portal message. Many providers will write an LMN remotely if you have an active diagnosis on file. Include the template above.
- Use a telehealth LMN service. Companies like Truemed, Sika Health, and others connect you with a licensed provider for the express purpose of evaluating and (if appropriate) issuing an LMN. Useful when your regular provider doesn't write them.
What an LMN Doesn't Do
- It doesn't pre-approve anything with the IRS. The IRS doesn't review LMNs in advance — they only matter if you're audited.
- It doesn't make any expense eligible. Cosmetic surgery, for example, isn't eligible even with an LMN unless it corrects a deformity from injury or congenital abnormality.
- It doesn't last forever. Most LMNs need annual renewal. The condition has to still exist; the recommendation has to still be current.
Documentation: Save the Letter With the Receipt
An LMN by itself isn't proof of an HSA-qualified expense — you need the LMN plus the receipt for the item and proof of payment. File them together in your receipt system. If you're auditing five years from now, having the matching LMN attached to the receipt makes the whole thing trivial to defend. See our guides to HSA receipt storage and receipt organization.
Common Examples of LMN Wins
- Type 2 diabetes patient + gym membership = $500–$1,500/yr eligible
- Severe asthma + HEPA air purifier = $200–$600 eligible
- Chronic back pain + therapeutic massage = $1,000+/yr eligible
- Vitamin D deficiency (lab-confirmed) + supplements = ~$50–$200/yr eligible
- ADHD + noise-canceling headphones for focus = ~$300 eligible (with provider documentation)
None of these expenses are eligible without an LMN. With one, they all are.
The Bottom Line
If you have a diagnosed medical condition and are spending real money on something that helps you manage it, ask your provider whether they'd write an LMN. The 5-minute conversation can unlock thousands of dollars of HSA-eligible spending you didn't know you had. Combine that with a strong shoebox strategy and you'll never leave qualified dollars on the table.