when I had a serious opiate habit I experienced some difficulties with my bowels, but not nearly as much as I hear from the horror stories around Bluelight. like taking a piss on opiates, taking a shit can take some practice and some time. however, it can be done.
there are OTC meds that can help a lot. the chocolate ex-lax has been one med that worked specifically well. your diet is also important to your bowels. make sure you eat a lot of fiber every day. also, I noticed that when I drink a lot of coffee in the morning, it helps to loosen up my stomach. if you are not a coffee person then dairy products, such as milk or ice cream, also help out a lot. it is important to release your bowels at least once a day. if not, you can get hemorroids (sp?) and those are seriously not fun for an opiate addict.
if you have a serious, heavy habit, then I have a bit of a tip for you. if you wake up a little dope sick every day (like most opiate addicts do) and you go out and score your opiates of choice, right before you administer the opiates (whether snorting the line of OCs or prepping the shot of heroin) doesn't your stomach get sent in turmoil? mine would feel like it instantly wanted to get rid of all your bowels? what I would do if I had problems shitting was right before I would administer my dose of preferred opiate, since my stomach would want to get rid of all that nasty stuff in there, I would take a nice long crap right before dosing.
I know it sounds rather icky, but trust me it works...