With the same problem lifelong, I had tried Zelnorm and it did help (with some problems "of its own". Disabled and without drug coverage insurance (before the Medicare "prescription drug company benefit") I could not continue it.
Let me tell you what I've done w/o prescription drugs since.
(From another recent post of mine)
My MIRACLE aid (#1) for IBS came when Miralax came out (PEG 3350). It is not a "laxative", really ... it's a water soluable plastic that is not absorbed by the body, but which itself hangs onto water. My system tends to completely dessicate (dry out) itself "overnight (sleep)" and especially the contents of the gut. I have been known to eliminate a gallon of liquid while sleeping ... or rather trying (but missing half of it getting up). This was partially caused by iron deficiency apparently.
Miralax alone was a help when constipated, but with variable results, but MUCH better when I tried mixing HALF a dose (about 8 gm) in 10 oz. water with 1/2 teaspoonful of sugarless, orange flavored "Metamucil" and stirring for 5 minutes before drinking (once a day as needed, or twice in "bad times"). REALLY gives me some control over symptoms reliably for first time. Try this for IBS control of especially constipation and that HORRIBLE boating. Miralax is now OTC, but generic prescription is 1/2 the cost (Glycolax, etc.) In 3 years the generics will be OTC too. You may also want to try 1/3 dose in "wet" oatmeal (I have to over-wet my food) made 3:1 water/oats.
I tried this because the old standby Metamucil always made me much worse because my body just drags all the water out of it and turns it to concrete. The Miralax mixed with it, even at those low doses, seems to combat this, and the oatmeal works that way too (that is, my body needs soluable and insoluable fiber that it CAN'T dry out ... for me a nearly impossible order).
My most recent discovery is that two bowls of "wet" oatmeal per day (3:1 ratio water : oats) is very helpful ... one bowl/day wasn't. If needed (worse periods) I put a 1/3 dose of "Miralax" in the oatmeal. After 40 years of problems (and trying prescriptions and huge doses of natural and artificial fiber that never worked) I think I finally have a handle on the problem ... I need to fight the tendency for dryness in the gut with Miralax (during "trouble periods") and a lot of very specific wet fiber ("wet" oatmeal) ... one bowl per day didn't do a lot, two helps.
Of course you may have a different set of overall conditions (IBS does not seem to be "one" disorder, but a spectrum of them). However, if the tendency for extreme dryness in the gut seems to fit at all, the things I'm suggesting are certainly very benign to try. Indeed if you can keep the gut working right with lots of the right kind of fiber it seems to really help. That doesn't mean you won't have difficult times (the time for Miralax). I hope it doesn't take 40 years for you to find the right combination. We'll try to keep that from happening. Good luck.