It's supposed to be "synergistic" - I don't have a history of the combo to hand. I do know that some people skip it or replace the aspirin with yohimbine (the "ECY stack"). One of the main effects apart from stimulation is appetite suppresion, and I suspect the aspirin may be part of that. Appetite suppression is a risk with any stimulant/exercise combo, because you can simply forget to eat until you bonk. Discipline is required.