With me... and a few others it is common to have alot of trouble urinating whilst under the influence of Opiates. Amphetamines basically have a reverse effect on the Central Nervous System, not to mention the cardiovascular and many other areas of the human body. So, I'm assuming that stimulants do speed up the process in which liquids pass through the body, etc...
Maybe (?) it relaxes the muscles in the penis, is this why it's so common for us males to experience "Shrinkage" after using any type of Amphetamine-like substance? Who knows?
One thing I can say for sure though, stimulants most definately make you feel the urge to shit. Especially in the mornings, followed by the good ole' "Morning-Stiffy" occurence.