Is mouth sores a side effect of ibs?

Not Usually: But they could be a sign of a type of inflammatory bowel disease- different from IBS which is "irritable bowel disease". The two are treated differently as well. Check with your doctor so he/she can order the proper tests/ referrals to make the correct diagnosis.

Cause same: The cause for inflammation of gut mucosa affects mucosal tissue in and around mouth - usually a sign of depleted b vitamins esp riboflavin which is needed to repair mucosa. Pellagra b3 deficiency can co-exsist. To heal use b vit + glutamine to calm inflammation.