Ulcers are usually caused by the bacteria H-Pylori, but acidic foods can exacerbate the problem or cause acid reflux by themselves. Pineapple juice is extremely acidic - avoid! Stick to apple or mango juice which have much higher pH.
Note: any juice with an acid kick can or will cause some 'bubble' on tongue & also can pop up at gum--as a place that stings--as in an ulcer or little bump that really hurts. *especially if with sensitive issues. Apple juice might be a better bet; oj, pineapple, grapefruit as fresh fruit juices...are good for you but not if sickly--and many pills do not do well or are told to not use w/grapefruit with--Lipitor, is a biggy no no! I find if I grill pineapple slices, they do not effect like the fresh just cut will do!