People get gastric irritation & actual ulcers not because they are making too much acid, but because they are not making enough of the protective mucus the lining of the stomach needs....Sucralfate is essentially a mucus replacement. Endoscopy studies show it actually does fill in ulcers and coat the stomach....That's why it needs to be taken on an empty stomach a couple hours before or after a meal-- taken with food, it's more likely to be washed downstream and be ineffective.
It's essentially not absorbed into the body-- It just passes thru..... The FDA rules say that any symptom complained of by a test subject during the test period must be listed as a side effect...That's why common things that we all get from time to time-- headache, backache, nausea, diarrhea, constipation etc are listed for just about every drug on the market.
IIRC- constipation is the only complaint listed that occurs with this stuff more than with the placebo in the tests.
https://www.ncbi.nlm.nih.gov/pubmed/6896647