First, let me say this. I would be VERY careful escalating any situation to the point of drawing down on someone. Depending on your state of residence, it may not be legal to draw in defense of a third party, or to not retreat. please be very aware of local laws regarding situations like this before making any decision. with that being said:
Crack Cocaine affects the body in several ways. it causes certain chemicals to "dump" which is the reason for the high in the first place. Physicaly, the drug causes the heart to race, and blood pressure to rise, which causes adrenaline to surge. this is the reason for reports of increased strength and reduced perception of pain.
it is quite possible for a crack head to be shot and not even feel it.
If you are ever in the situation where you ever have to use your gun in self defense against someone high on crack, the best way I can tell you is to employ "zombie" rules. that is if you are going to shoot them make damn sure that you hit center mass, or a clean head shot. otherwise you may wind up losing your own life.