As of January 1st 2011 the Government changed the laws regarding reimbursement for supplements using a flexible spending accounts (part of the new medcal rules/coverage). It is now required by law that you have to submit a physicians perscription for all over the counter items and supplement along with your receipts in order to receive your reimbursement. So unfortunately a letter of medical necessity is not going to assist you, just ask for a perscription from your doctor and you should be good.
Here is a link for the changes for 2011
hmmm... this is what I got back from my FSA when I inquired about it:
In reference to your e-mail sent regarding eligibility of Vitamin and supplements, in accordance with IRS guidelines, Vitamin and supplements are not an eligible expense on your Health Care FSA, but may be eligible if a Letter of Medical Necessity is submitted with your claim or substantiation. The physician's certification should clearly indicate that the purchase, service, or surgery is medically necessary. If the treatment is on-going, the physician should notate the applicable time period....
Per the IRS website/Link all you need is a prescription, so I would just get a 1 year script for the supplements and that should be all you need
Qualified Medical Expenses.
a medicine or drug will be a qualified
medical expense only if the medicine or drug:
1. Requires a prescription,
2. Is available without a prescription (an
over-the-counter medicine or drug) and you get a
prescription for it, or
3. Is insulin.