NSAID are to be avoid at all cost with IBD in my opinion and used with parcimony by healthy people.
It has been studied and it's been pointed out as an aggravating factor for IBD, it does favorise inflammation of the digestive system in a lot of subjects and leads to drug induced IBD in rats.
Yes there is also an influence that comes from the dose and duration of the NSAID treatment. Also, there is a possibility that it is related to a form of "intolerance" that certain patient have to these agents. The point is that the number regarding NSAID and IBD are fairly high. I've encountered some sources that were pointing that NSAID would lead to relapse in 1/9 for IBD patient taking it regularly over a period of 1 year that was in a study by american gastrology association in 2000.
NSAID are known to be toxic for the guts in general either it is stomach, oesophagus, small bowel, colon, rectum. That toxicity is well and widely recognized and it leads to inflammatory conditions and permeability of the gut (and THIS is not a good thing).
NSAID are drugs like:
Without prescription: Advil, Motrin (ibuprofen), Aleve (naproxen sodium), Aspirin
Prescription meds like: Anaprox(naproxen), celebrex(celecoxib), voltaren (diclofenac), naprosyn(naproxen), etc [to name a few ]
What NSAID are not: acetaminophen (tylenol), opiates (codeine, morphine, oxycodone, etc). Those are supposed to be safer alternatives to treat pain for patients that have a fragile digestive system (I don't mean regarding the migraine, just pain management at large).
Then again, all NSAID are not equal in their risks of causing side effects resulting of their toxicity on the digestive system. They all carry risks but COX-2 selective NSAID (celebrex, vioxx, etc) are supposed to be "safer" for the digestive tract than the non-selective ones but they are dangerous for the cardiovascular system...
Once again with these treatments it is a matter of risks versus benefits but I would be tempted to say it is better to avoid them whenever it is possible in our case. I do not believe that it is a coincidence that my GP, GI and rheumatologist all suggest that I should not take these agents.