These are the Rebbe's letters to individuals about their own situations. Throughout, the Rebbe instructed people to consult an expert doctor and follow his guidance; anyone facing a medical question should do the same.
The Rebbe's guidance on stomach and digestive problems:
Ulcers: most heal through keeping a proper diet precisely and resting both mind and body, which is preferable to surgery. The medication is added to speed the healing (IK 1737, 8304).
Digestive trouble: have the doctor arrange a regimen for eating and drinking along with suitable medicines. Check tefillin and the mezuzos in the home, and wear a kosher tallis katan with checked tzitzis (IK 3770).
Eating before davening when it is for a medical reason, such as preventing a buildup of stomach acid, is explicitly permitted (IK 7070).