[Scoundreller]

Pretty much nothing most of the time.

There are optical isomers and such that could make a difference. But even the brand names may change this up from time to time.

Then there's the whole 99% pure thing, but that 1% could be made up of highly carcinogenic nitrosamines in the microgram doses that aren't as closely tracked as they should be. But buying brand may not protect you from this, or could be worse.