Article · updated November 8, 2026

Who can write a letter of medical necessity for a sauna or cold plunge?

Most people assume it has to be a physician. Often it does not. What matters is that a licensed medical practitioner who diagnoses or treats your condition, acting within the scope of their license, recommends the therapy to treat it, and that your plan accepts their credential.

The short answer

Usually yes: physicians (MD or DO), nurse practitioners (NP) and physician assistants (PA) who treat you. Administrator forms commonly list all three.^[6]^[7]
Sometimes: psychologists, chiropractors, dentists, optometrists, podiatrists and licensed naturopathic doctors, for conditions within their scope and only if your plan accepts them.
No: registered nurses, medical assistants, trainers, coaches, massage therapists, the company selling you the sauna, and you.
Who decides: for an FSA or HRA, your plan administrator. For an HSA, nobody approves in advance; you keep the letter in case the IRS asks.

What the law actually says

No statute or regulation says who must sign a letter of medical necessity. The letter is not a legal form. It is evidence for the real test: whether the purchase is medical care, meaning amounts paid primarily for the diagnosis, cure, mitigation, treatment or prevention of disease.^[1] That test is the same for HSAs, FSAs and HRAs.

The IRS describes medical expenses as payments for services “rendered by physicians, surgeons, dentists, and other medical practitioners,” and separately lists chiropractors, psychologists and osteopaths.^[2] In its 2026 guidance on wellness and general health it uses two different phrases, and the difference matters:^[3]

The diagnosis: for exercise, weight-loss and nutrition costs, the IRS asks whether they treat “a specific disease diagnosed by a physician (such as obesity, hypertension, or heart disease).”
The recommendation: for items with both medical and personal uses, the newest question (Q15) looks to “a recommendation from a medical practitioner,” together with your symptoms, diagnosis or risk factors.

The IRS has not defined “physician” for these questions, and it has not said whether a diagnosis made by a nurse practitioner or physician assistant satisfies that wording. The most common federal definition, Medicare’s, covers MDs and DOs and, for limited purposes, dentists, podiatrists, optometrists and chiropractors.^[8] Our reading: a recommendation can come from any licensed medical practitioner acting within scope, and the safest record shows the diagnosis made or confirmed by a physician or by a practitioner licensed to diagnose that condition independently in your state. When the purchase is large, such as a built-in sauna, that extra care is worth it.

FSA and HRA: your plan decides

Health FSA and HRA claims must be backed by information from a third party independent of you and your family; you cannot certify your own expense.^[5] That is why administrators publish their own letter forms and their own lists of who may sign. Two real examples:

One large administrator’s form says a “Medical Practitioner” generally includes a physician (MD/DO), physician assistant, nurse practitioner, dentist, optometrist and podiatrist, and treats a letter as effective for 12 months or to the end of the plan year.^[6]
A state employee plan accepts a medical doctor, physician assistant, nurse practitioner, licensed osteopathic doctor, chiropractor, ophthalmologist or dentist.^[7]

Some administrators tie the letter to prescriptive authority under state law, which most NPs and PAs have. Before you ask anyone to write, download your own plan’s letter form or call the administrator and ask: “Which practitioner credentials do you accept on a letter of medical necessity?” Their answer is final for your plan.

HSA: nobody approves in advance

An HSA custodian pays what you ask. If the IRS later asks whether a withdrawal was a qualified medical expense, your letter, your diagnosis in the medical record and your receipt are your evidence. A letter from the clinician who actually treats the condition carries the most weight. If it is not qualified, the withdrawal is taxed as income plus a 20% additional tax unless you are 65 or older. See the HSA and FSA rules.

Who can write it, practitioner by practitioner

Scope of practice is set by each state, so treat this as a guide and check your state and your plan.

Practitioners who may write a letter of medical necessity
Practitioner	Can diagnose?	Accepted by plans?	Best fit for sauna or cold therapy
Physician (MD or DO)	Yes, any condition.	Accepted by every plan we have seen.	Cardiologists, rheumatologists, psychiatrists, primary care. The safest signer, especially for a large or built-in purchase.
Nurse practitioner (NP)	Yes. Independently in full-practice states; with a collaborating physician in others.	Named on common administrator forms.	Often the clinician who actually manages the condition in primary care, cardiology, psychiatry and pain clinics.
Physician assistant (PA)	Yes, in collaboration with or under supervision of a physician, per state law.	Named on common administrator forms.	Same as an NP: a strong signer when they are your treating clinician.
Psychologist (PhD, PsyD)	Mental health conditions, within scope.	Varies. Not on every administrator’s list.	Depression or anxiety, where heat or cold is part of the treatment plan. A co-signature from a prescriber can help.
Chiropractor (DC)	Musculoskeletal conditions, within scope.	Varies. Some plans list them, others do not.	Low back pain and similar conditions, if your plan accepts chiropractors.
Dentist, optometrist, podiatrist	Within their field only.	Named on common forms, for conditions in their field.	Rarely relevant to a sauna or cold plunge.
Physical or occupational therapist	Treats; medical diagnosis is limited in most states.	Rarely listed as a signer.	A valuable supporting voice: ask them to write to your physician or NP, who signs.
Licensed naturopathic doctor (ND)	In states that license NDs, within that state’s scope.	Rarely listed. Ask your plan first.	Only where your state licenses NDs and your plan accepts them.

Nurse practitioners

In 2026, 27 states and Washington, D.C. grant nurse practitioners full practice authority, by the national NP association’s count: they evaluate, diagnose and treat without a supervising physician.^[9] Other states require a collaborative or supervisory agreement, but NPs there still diagnose and write orders as part of that arrangement. For many patients the NP is the clinician who manages their heart failure, depression or diabetes day to day, which makes them a natural signer.

Physician assistants

PAs diagnose and treat in every state, in collaboration with or under the supervision of a physician as state law requires. A PA who treats your condition can write the letter; some practices add the collaborating physician’s name, which does no harm.

Mental health clinicians

Psychiatrists are physicians. Psychiatric nurse practitioners are NPs. Psychologists diagnose mental health conditions and the IRS treats their fees as medical care,^[2] but not every FSA administrator lists them as letter signers. Licensed counselors and social workers diagnose mental health conditions in many states and are listed even less often. If your plan does not accept your therapist, ask your therapist to write to your prescriber or primary care clinician, who signs.

Chiropractors, physical therapists and others

Chiropractors appear on some plan lists and not others.^[6]^[7] Physical and occupational therapists are rarely accepted as signers, but they often know best how heat or cold fits a rehabilitation plan; their notes make a physician’s or NP’s letter stronger.

Telehealth and questionnaire letters

Telehealth is fine. A clinician you see by video is a real clinician, as long as they are licensed in your state, review your history and medications, and actually treat or evaluate your condition. What the IRS warned about in 2024 is different: a note “based merely on self-reported health information,” such as a short online questionnaire reviewed by someone you never speak to. That note “would not establish” that a personal expense is medical care.^[4] The credential on the letter cannot fix a missing clinical relationship.

Which clinician to ask

Ask the person who manages the condition you are treating. In practice:

Heart failure, coronary disease, high blood pressure: your cardiologist, cardiology NP or PA, or primary care clinician.
Fibromyalgia, arthritis, ankylosing spondylitis: your rheumatologist, rheumatology NP or PA, or primary care clinician.
Depression or anxiety: your psychiatrist, psychiatric NP or primary care clinician; a psychologist if your plan accepts one.
Chronic pain or low back pain: your pain or spine clinician, or primary care; a chiropractor if your plan accepts one.
Type 2 diabetes, obesity: your endocrinologist or primary care clinician, physician, NP or PA.

Bring the evidence for your condition and, if you like, a draft from our Start tool. Your clinician edits it, replaces it, or says no.

What makes a letter hold up

The signer’s name, credential (MD, DO, NP, PA…), license or NPI number, practice address and signature with a date.
Your diagnosis, ideally with an ICD-10 code, and that the signer treats or evaluated it.
The specific therapy (sauna, infrared sauna, cold-water immersion) and how it treats the condition.
A protocol: temperature, minutes per session, sessions per week.
That it is recommended primarily to treat the condition, not for general health.
How long it is needed and when it will be reassessed. Many plans want a new letter every 12 months.^[6]

The NPI is not a legal requirement for a letter, but many administrator forms ask for it or for a license number, and it lets them confirm the signer is licensed.

Common questions

Does it have to be my primary care doctor?

No. Any accepted practitioner who treats or evaluated the condition can write it. A specialist’s letter is often stronger.

Can I write my own letter, or can the seller provide one?

No. FSA and HRA substantiation must come from a third party independent of you,^[5] and a letter from the company that profits from the sale is not an independent clinical judgment.

My NP diagnosed me. Is that enough for the IRS?

Plans routinely accept NP letters. The IRS has not addressed NP diagnoses directly in its wellness guidance, which speaks of a disease “diagnosed by a physician.”^[3] If your NP works with a collaborating physician, or a physician has also documented the diagnosis, your records are stronger. This matters most for an HSA purchase you may need to defend later.

Does a letter make my purchase eligible?

No letter does that on its own. It documents a clinical recommendation. The purchase must also be primarily for treating the condition, and you must not have bought it anyway for personal reasons. Built-in saunas follow capital-expense rules. See the rules.

This article explains general rules. It is not tax or legal advice, and plan rules vary. Confirm with your plan administrator, and ask a tax professional about your situation. Clinicians: see our resources for clinicians.

References

26 U.S.C. §213(d): definition of medical care. Applied to HSAs by §223(d)(2). Source
IRS Publication 502 (2025), Medical and Dental Expenses: “payments for legal medical services rendered by physicians, surgeons, dentists, and other medical practitioners”; separate entries for chiropractors, psychologists, osteopaths and acupuncture. Source
IRS, Frequently asked questions about medical expenses related to nutrition, wellness and general health (updated September 23, 2026): Q8–Q10 “a specific disease diagnosed by a physician”; Q11 “even if recommended by a doctor”; Q14 “recommended by a medical practitioner as treatment for a specific medical condition diagnosed by a physician”; Q15 “a recommendation from a medical practitioner”. Source
IRS news release IR-2024-65 (March 6, 2024): a doctor’s note based on “self-reported health information” does not convert general wellness expenses into medical expenses. Source
IRS Chief Counsel Advice 202317020 (2023), applying Prop. Treas. Reg. §1.125-6(b)(3): health FSA and HRA claims must be substantiated by information from a third party independent of the employee and the employee’s spouse and dependents. Source
TASC, Letter of Medical Necessity form (State of Georgia plan, 2025): “Medical Practitioner” generally includes physician (MD/DO), physician assistant, nurse practitioner, dentist, optometrist and podiatrist; effective 12 months or to the end of the plan year. Source
Minnesota State Retirement System, HCSP Letter of Medical Necessity (2023): completed by “a medical doctor, physician assistant, nurse practitioner, osteopathic doctor (licensed), chiropractor, ophthalmologist, or dentist.” Source
42 U.S.C. §1395x(r): the Medicare definition of “physician” (MD/DO; dentists, podiatrists, optometrists and chiropractors for limited purposes). The IRS has not adopted it for §213, but it is the most common federal definition. Source
American Association of Nurse Practitioners, State Practice Environment map: states granting full practice authority (27 states plus Washington, D.C., in 2026 by AANP’s count; check the map for your state). Source