I'll tell you what finally has allowed me to be about as regular as I have ever been while dependent on opiates. It's actually 2 things, but they have worked better than any OTC meds I have tried, any change to my diet, or drinking water.
I simply have done 2 things regularly and it keeps me "regular"...well close enough, my shits are still dry/hard and I will still have times I feel the need to go but only get gas or those small pellets that aren;t worth the effort it takes to get them out. But, I am having a good bowel movement almost everyday, as long as I've kept this up. And I've had some bad constipation in the past with similar habits to what I have now.
So what I do, just these things:
1. I take 2-3 magnesium citrate pills per day. I will sometimes even taken 4 if I feel especially backed up and its been a while. To make sure the magnesium works I also take 1 or 2 vitamin D3 pills, typically I take the 2000 IU dose. I also drink a good amount of milk, which has D3 and calcium, the calcium also helps the magnesium work better. Not only is magnesium important as a remedy for constipation, but it also is vital to so many other functions of the human body and opiate abusers are almost certainly deficient in it. Taking it makes opiates work better, so that's another reason to take it.
2. I swear this is the real secret and why it's helped me so much, but a simple can of soup has helped me better than anything. Maybe it's the sodium in there, idk, but if I eat a can of Campbell Chunky or Progresso soup it seems to do wonders. It has become too frequent to be a coincidence. I started eating a can everyday for lunch and I was having a good bowel movement the next day every time. I then stopped for about a week and my constipation returned and I didn't even think about the soup. Then I got a sore throat so was eating nothing but soup and next day... voila, I was a good BM.
So that's it for me, just done those 2 things and I feel great with my regularity, better than I ever have while using opiates (even while on Suboxone at around 3-4 mg a day I have had worse contipation issues.)