there are two kinds of supplement...
type 1 - the creatine, the pre workout drinks, the electrolytes, the protein shake
they do exactly what they are intended to do, give you a bit of protein boost, boost your carbs, hydrate you during workout etc.
(and there are virtually no incidents of these kind of supplements causing positive tests)
then there is
type 2 - the "natural testosterone booster", the growth hormone stimulator, the muscle gainer, the water shedder, etc etc
some of these do nothing at all, some of them work..
the ones that work are the ones with prohibited substances in them (either listed or unlisted)
MMA fighters testing positive are taking "type 2"..