This issue is being discussed in another thread about progesterone, but I think it merits its own thread.
I have ADHD and have used Adderall for the last three years and before that, it was Ritalin.
I've recently realized that Adderall has contributed to my penile shrinkage, libido problems and difficulty urinating. I recently went two weeks off the Adderall and I noticed improvement with those issues.
But I was unable to function mentally as far as work goes, so now I am back on the Adderall, but taking lower doses.
I wish I can find a decent alternative to Adderall for ADHD, but from the research I've conducted, it seems like the only effective medications are stimulants. I was hoping to replace the Adderall with Selegeline, but it didn't really help with my ADHD.
I used to think I had BPH, which is why I sometimes find it difficult to urinate, but now I believe that this is a result of my bladder wall relaxing and my sphincter closing, which is what happens during Sympathetic Stimulation.
Sympathetic Stimulation is what we know as the "fight" or take "flight" syndrome. It's basically an adrenaline rush. When you take stimulants, your body goes into Sympathetic Stimulation, causing your penis to shrink.
This is possibly nature's way of helping protect the penis if you're in a fight. Also, it's not like the blood is needed in your penis if you're in a fight.
On the other hand, Parasympathetic Stimulation causes the bladder wall to contract and the sphincter to open, allowing for a freer urine flow. More blood flows to the penis, allowing for it to hand fuller in its non-erection state as well as easier erections.
The following link goes more into detail:
Neuroscience for Kids - Autonomic Nervous System 
Libido problems with stimulants (sympathetic vs parasympathetic nervous system) 
Linear Mode
