A. “Artificial sweeteners can cause diarrhea in some people,” says Amy Foxx-Orenstein, D.O., a gastroenterologist at the Mayo Clinic in Rochester, Minnesota. “And carbonated diet beverages can be double whammies because they provide artificial sweetener and gas via carbonation. Caffeinated varieties may also cause intestinal irritability.” Since drinking diet soda certainly isn’t necessary for good health (in fact, replacing it with water is probably a good idea overall), try going cold turkey for a week and see if your symptoms improve. If they don’t, talk with your health-care provider.