Yeah, being plugged up sucks. I once went 14 days without poopin' while taking 2 10 mg. hydrocodones per day, so can't imagine how bad it would be with heroin or smtg. stronger.
No single thing helps everybody, so it's best to try a combination of stuff. Here's what I recommend:
*Daily exercise (abdominal crunches, side-bends and other stuff that moves your middle section.)
*Eight 8-ounce glasses of water each day, with a few tsps. full of FiberSure or Metamucil for good measure.
*Milk of Magnesia in the morning and night.
*Probiotic supplements. Taking two or three at once always made me shyte immediately.
*Cut down on refined flour, cheese and other constipating foods while adding more EFAs to the diet (flaxseed oil, fish oil, etc).
If you try all that and STILL can't go, it may be time to see a doc or quit the opiates. Constipation can be dangerous after a while, so stay on top of it.