Can stomach ulcers cause diarrhea?

Not directly: Stomach or gastric ulcers are not likely to cause diarrhea, but treatments for it can (side effects). Many of the antacid meds such as zantac/pepcid, prilosec, Prevacid (lansoprazole) etc...All can do that. Also, people having ulcers may try to change their diet which may cause trouble with stool. Diarrhea-predominant irritable bowel can ulcer-like pain too, so consult doc. Good luck.