L-glutamine is an amino acid that can aid in the healing of intestinal mucosa and permeability. Glutamine can relieve diarrhea by helping the colon reabsorb sodium and water. It can also be used to prevent and alleviate mouth sores due to certain chemotherapies and/or immunotherapies.
 
References:
Can L-Glutamine Help with IBS?
 
Intestinal Permeability in Patients With Diarrhea-Predominant Irritable Bowel Syndrome: Is There a Place for Glutamine Supplementation?
 
Oral Glutamine Reduces the Duration and Severity of Stomatitis after Cytotoxic Cancer Chemotherapy