Both let you set aside money before tax to spend on health care, and both draw from the same IRS list of eligible expenses — but they work quite differently.

Two details matter for planning. An FSA gives you the whole year's amount on day one, which is useful if a big expense lands early — but unused funds are mostly forfeited at year-end, with only a small carryover (up to $680 for 2026) or a short grace period, never both, depending on your employer's plan (IRS, 2026 FSA limit). An HSA rolls over forever and is yours to keep, which makes it a genuine savings vehicle — you can build it up over years toward a planned procedure. To open an HSA you must be enrolled in a qualifying high-deductible health plan and not enrolled in Medicare (IRS Publication 969).

The IRS defines qualified medical expenses broadly as costs for the diagnosis, treatment or prevention of a medical condition (IRS Publication 502). For someone managing essential tremor or Parkinson's, that typically includes:

• Doctor and specialist visits — neurologist appointments, copays and deductibles.
• Prescription medications — the copays for propranolol, primidone, Parkinson's medications and others.
• Occupational and physical therapy — sessions to keep daily tasks doable.
• Out-of-pocket procedure costs — the deductible and coinsurance for deep brain stimulation or focused ultrasound, including the 20% coinsurance Medicare leaves behind.
• Prescribed medical devices — a device used to treat the tremor, when medically necessary.
• Over-the-counter medications — eligible without a prescription since the 2020 CARES Act (IRS, CARES Act).

Clearly medical costs — a doctor's visit, a prescription, standard medical equipment — need no special paperwork. The gray area is dual-purpose items: things that also have an everyday wellness use. For those, a letter of medical necessity from a licensed provider — a short statement connecting the item to your diagnosed condition and explaining why it's needed — is what makes the expense eligible (IRS Publication 502). It's a routine document; your neurologist or primary-care doctor can write one. The practical rule: if an item isn't obviously and only medical, ask your FSA or HSA administrator whether it needs a letter before you buy, and keep the letter with your receipt.

It's easy to confuse FSA/HSA spending with the itemized medical-expense deduction, but they are two different tools — and you can't use both on the same dollar:

• FSA and HSA give you an automatic, up-front tax break: the money is never taxed to begin with, and everyone with an account benefits regardless of income.
• The medical-expense deduction only helps if you itemize your taxes and your total unreimbursed medical costs exceed 7.5% of your income — and even then only the amount above that threshold counts.

Crucially, an expense you pay from an FSA or HSA has already been made with pre-tax dollars, so it cannot also be claimed as a deduction (IRS Publication 502). For most people the accounts are the simpler and more valuable route; the deduction is a fallback for large medical costs paid out of pocket. If you want the deduction side of the picture in detail, see the related guide on medical-expense deductibility below.

In practice it's straightforward. Most accounts come with a debit card you use directly at the pharmacy, clinic or for a device purchase. If you pay another way, you submit the receipt for reimbursement from the account. Either way, keep your receipts — and, for any dual-purpose item, the letter of medical necessity — because you may be asked to substantiate the expense. Good records are the whole game here: the eligibility rules are generous, and the only thing that trips people up is being unable to prove what a charge was for.