A physician letter of necessity is a short document in your medical chart that says, in clinical language, that you have celiac disease and that strict, lifelong gluten avoidance is the prescribed treatment. It is not a tax form, it is not insurance paperwork, and you don't have to ask your doctor for anything special-sounding. You're asking for documentation of a diagnosis and a treatment plan, which is what your doctor's chart is for in the first place.
The reason it matters is that having a clear chart entry now keeps your future self out of trouble. If you ever need to substantiate the diagnosis (for school accommodations, for an employer's dietary requirements, for an insurance question, for your own records, or for the conversation your CPA or tax professional may want to have at year end), you'll have it. If you don't have it now, requesting it three years later, from a doctor you may no longer see, in a chart that may have moved between systems, is a different and harder problem.
What to ask for
Walk into your next visit (or send a portal message before the visit) and use language like this: "I'd like my chart to clearly reflect my celiac diagnosis and the treatment plan. Could you add a note that confirms the diagnosis, references the test that confirmed it, and states that strict gluten avoidance is the prescribed treatment?"
That's the entire ask. You're not asking for a favor. You're asking for the doctor's note to match what's already true.
If your diagnosis was confirmed by biopsy, ask for the biopsy date and result in the note. If it was confirmed by antibody panel (tTG-IgA, EMA, DGP), ask for the test name and result. If you were diagnosed years ago and the original records are in a different system, ask your current provider to add a confirming entry that references the historical diagnosis.
A sample paragraph for the chart
You can hand your provider this language as a starting point. They'll adjust it to fit the chart's structure and their own style.
The patient has biopsy-confirmed celiac disease (diagnosed [date], [confirming provider or institution], [test or procedure]). Strict lifelong adherence to a gluten-free diet is the prescribed and medically necessary treatment. The patient has been counseled on dietary management, label reading, and cross-contamination avoidance. A follow-up antibody panel is recommended at six to twelve months and annually thereafter.
If you weren't diagnosed by biopsy, swap "biopsy-confirmed" for the actual confirmation method (antibody-confirmed, family-history-and-clinically-confirmed, etc.). The diagnostic certainty matters; the exact wording doesn't, as long as the chart is clear.
Where this lives
The note goes in your chart, not in your hand. You may never receive a separate piece of paper called "the letter." That's fine. The chart is the record. When you (or anyone you authorize) request records from your provider, the note travels with them.
If you do want a printable letter (some people prefer one for restaurant manager conversations or for travel), ask your provider's office for "a letter confirming the celiac diagnosis and prescribed gluten-free diet." Most offices have a template. A signed letter on letterhead, with the diagnosis and the treatment plan named explicitly, is what you want.
What this letter is not
It is not a tax form. The conversation about what gluten-free spending may be reportable as a medical expense on Schedule A happens between you and a tax professional, with the chart as supporting documentation if asked. The article on what Publication 502 actually says about celiac covers the source material your CPA or tax professional may want to see.
It is not a permanent get-out-of-trouble card for accommodations. Schools, employers, and airlines all have their own processes; the chart entry is the source-of-truth that those processes draw from, but you may still need to fill out their forms.
It is not a one-time conversation. If you change providers, ask the new one to confirm the diagnosis is in your current chart at their practice, not just in records they imported. Chart imports are notoriously incomplete.
If your diagnosis is older than this
If you were diagnosed five or ten years ago and never had a clear chart note added, the time to fix it is now. Schedule a visit (or use the portal) with your current gastroenterologist or primary-care provider. Ask them to add a confirming chart entry referencing the historical diagnosis. Bring whatever paperwork you still have: the original biopsy report, antibody panel results, hospital discharge summaries, the letter from the doctor who first told you. Even fragmentary documentation is enough to support a confirming entry in a current chart.
A clean chart today is cheaper than a clean chart later. That's the entire argument for doing this in your first month, before the file gets cold.