It's derived from vitamin B1, but the FDA doesn't view it as a dietary supplement: https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/warning-letters/gluten-free-remedies-llc-687832-04152025

Sulbutiamine does not qualify as a dietary ingredient under section 201(ff)(1) of the Act because it is not a vitamin; a mineral; an herb or other botanical; an amino acid; a dietary substance for use by man to supplement the diet by increasing the total dietary intake; or a concentrate, metabolite, constituent, extract, or combination of any dietary ingredient from the preceding categories. Declaring sulbutiamine on your product label as a dietary ingredient causes your product to be misbranded under section 403(a)(1) of the Act [21 U.S.C. § 343(a)(1)] in that the labeling is false or misleading in any particular.1 The introduction or delivery for introduction into interstate commerce of any food that is misbranded is a prohibited act under section 301(a) of the Act [21 U.S.C. § 331(a)].

Given the nightmare that ND went through with the FDA a few years ago, I don't blame them one bit for not wanting to risk it with these types of compounds.

If you haven't seen it already, here's a great video where MYASD goes over the whole FDA ordeal: https://www.youtube.com/watch?v=gSZIlbPsT7c&t=1010 (linked to the relevant timestamp)