medication
If a medication can irritate an entire stomach, think of what happens to a pouch which is about 10% the size of a normal stomach when the same medication is digested.
Some medications in combination can cause ulcers. Those are combinations to stay away from for sure.
Some medications reduce mucus production, making the pouch more likely to be damaged by either other medications or the sharp edge of a bite of food.
Some medications are timed released, and do not stay in a stomach long enough to be dissolved slowly,
I think that is the list of whys. Don't remember any others off hand.
Peter