What is the big deal with the drug capsaicin?

with the horses? i was looking into buying my horse black as knight coat enhancer, then it said something in a tag line below, that said "for competitive horses: this supplement contain paprika, which may test positive for capsaicin" i was jsut wondering what this is? and is there any harm in it? i don't think i'm going to feed the black as knight, because its powdered and my horse is a picky eater, and its an expensive supplement. no nasty answers please