system
Given the following post, respond as if you're commenting on Reddit:
human
How does nicotine, amphetamines, and coffee act on sympathetic nervous system yet still make me shit my brains out?

I understand these substances work via the sympathetic nervous system which would cause all the affects except one:  Why must I be near a bathroom when these kick in.  I take adderall and when it kicks in, I have to shit so hard.  Doesn't the parasympathetic nervous system control SLUDD (salivation, lacrimation, urination, Digestion, and Defecation).  I would think that these substances that act on the sympathetic and cause the opposite.  How do they make me excrete this ungodly smelling floaters?
gpt
This is a fairly complex question, especially if we look at each drug individually.  That being said, there are two important concepts.  First, there is more overlap and redundancy in the autonomic nervous system then the simple "sympathetic does this, parasympathetic does the opposite."  Secondly, although those drugs may all have some sympathetic nervous system component, they are also "sloppy" in that they have other effects as well.

The bottom line though is that all three drugs you listed affect smooth (and striated) muscle tone.  Activation of the sympathetic nervous system might cause an eventual decrease in movement of material and secretion of fluid into the bowel, but there's another component to defecation; holding your poop in.  For most people at most times, there is always a little poo hanging out in your distal colon and rectum.  Normally, your anal sphincter muscles are happily contracted at a balance point of not-too-contracted and not-too-open.  If said sphincters get too tense or too loose a defecation reflex is started.  

So, depending on the stimulant, your sphincter gets too tight or too loose and you get the urge to expel whatever it is that's hanging out in your pooper.